Calculation Formulas
Stock Briggs & Stratton 5 hp. specifications:
Deck height: 6.2835"
Rod length: 3.8750"
Compression height: 1.1900"
Rod bore (crank): 1.0010"
Rod bore (wrist pin): .4910"
Cylinder bore: 2.562"
Stroke: 2.4370
Measuring Stroke
Stroke is measured from the center line of the crank bearing journal to the center line of the rod journal multiplied by 2.
Measuring Deck Height
Deck height is measured from the center line of the crankshaft bore to the deck of the block.
Measuring Compression Height
Compression height is measured from the center line of the wrist pin bore to the top of the piston.
Calculating Cubic Inch Displacement (cid)
To calculate cubic inch displacement (cid):multiply bore x bore x stroke x .7854example: 2.562 x 2.562 x 2.437 x .7854 = 12.5633 cid
Rod Length & Compression Height - Made Simple
Calculating Rod Length
To calculate rod length:subtract deck height - (stroke divided by 2) - compression height.example: 6.2863 - (2.437 / 2) - 1.190 = 3.875 rod length
Calculating Compression Height
To calculate compression height:subtract deck height - (stroke divided by 2) - rod length.example: 6.2835 - (2.437 / 2) - 3.875 = 1.190 compression height
Wednesday, November 11, 1998
Does It Matter? A common sense approach to engine building.
Historically, human nature has taught us two basic things about the question "Does It Matter ?"
1 - If a problem exists, and we have the capability to correct it, we tell the world "It Does Matter".
2 - If we don't have any interest or desire to correct a problem, we either keep our mouths shut or simply say "It just doesn't matter".
PREFACE
This article could have been greatly expanded into a full length novel but would have gotten boring with illustration on top of illustration.What's written here is not fiction, and mixed with a bucket full of common sense and imagination, you will run faster and longer.If you disagree with our opinions, let us know. We can learn too.If you have any questions or problems, send us an e-mail or call 1-800-521-3560. We'll be here if you need us.
DOES IT MATTER
If the base of a block is not perfectly flat and true ?If the surface of a motor mount that accepts the base of a block is not true and flat ?Through no fault of the chassis builder, it’s virtually impossible to have the mounting rails for the motor mount perfectly aligned.Does the weight of the driver flex or move the motor mount rails on the chassis ?Do the motor mount rails move and flex during a race ?
In other words, can you take a perfectly good motor, bolt it to a motor mount and then to the chassis and totally mess it up ? And the answer is - YES !
CONSIDER THE FOLLOWING TEST RESULTS
We took a block and precision align bored it perfectly round and straight from top to bottom and bolted it down to a rigid, perfectly flat surface plate. Under one corner we put a .050 shim. This shim could now represent a distortion caused by any one of or combination of the problems mentioned above.Next, we took a good tenth (.0001) reading dial bore gauge to examine the bore. Looking at the top of the block, and considering between the two valve seats would represent 6 o’clock, we took 2 readings. One aimed at 11 o’clock and one aimed at 1 o’clock. Starting at the top of the bore we moved down a ½ inch at a time. As we moved down the once perfectly round bore, it started taking the shape of an egg.The distortion was .002.
This test was performed on a Kool bore motor bored .030 over.The crankshaft bores had also moved and were slightly tighter in the DU bushing.A dual bearing block is more forgiving when this happens but either way it is a problem.The deck of the block also moved and became distorted.
Here’s a shocker:Before we did the test, we bolted a side cover on the block without a gasket and could see day light between the two surfaces. Then we took feeler gauges and it was easy to find where a .002 gauge would fit but we also got a .004 gauge to fit in one place.
Have you ever wondered why sometimes you have problems with oil leaks and blown side cover gaskets ?
These are some problems that can happen because of this:
• Scuffing or galling pistons.
• Abnormal wear in the cylinder wall.
• Rings not seating.
• Loss of compression.
• Loss of horsepower.
• Increased blow by in the crankcase.
• Oil consumption.
• Oil contaminating the fuel charge.
• Engine life short lived.
• Blown head gaskets.
• Side cover gaskets leaking.
• Blown side cover gaskets.
• Blocks cracking because of stress.
The question is: DOES ANY OF THIS MATTER ?
The answer is: ABSOLUTELY...
OTHER THINGS TO THINK ABOUT (and remember)
Briggs blocks are not equal and will vary from block to block.Kool bore blocks will distort more than IC blocks.Big overbores will distort more than stock bores.Pre-Raptor motors, with thinner castings, will distort more than the current castings.Large overbores for sleeves and then boring the sleeve for large pistons will distort even more.Over a period of years we have heard the following question asked several times a week from our customers:"I built two identical motors, 1 runs super and the other is a real dog, what’s wrong" ?or "I dyno tested this motor and it was great, then put it on the chassis and it wouldn’t fall out of a tree by itself. Why" ?
I wonder if anything we have discussed so far would shed any light on this subject ?
SOLVING THE PROBLEM
To solve one of the problems, the base of the block must be surfaced perfectly flat. The side cover must be on the block when it is surfaced. If you ever change side covers you must check the new one and make sure it does not stick down below the base surface of the block. If it does grind this material away.
The two drain plugs should be installed in the block TIGHT before it is surfaced. The reason here is that the drain plugs swell the block and create a knot on the base surface. Either before or after surfacing the base, with the drain plugs installed, take a die grinder and remove about .050 of material. The area to work on is 1 inch to the right and the left, and into the center of the block from the drain plug.
One last important thing:You must change your habits because you never thought about protecting the base of the block before.A special fixture (part number 7718) for milling or grinding the base of the block is available from ARC. This fixture also aligns the crankshaft parallel with the base of the block and aligns the existing bore 90 degrees with the base front to rear and is also excellent for use prior to boring a block on a vertical mill.
THE CYLINDER WALL • DOES IT MATTER ?
Ask yourself this question. Would you go to a race and add some oil to your fuel and loosen your spark plug just a little so you would have less compression? Obviously not. But you might be accomplishing the same thing and here’s how.Remember this rule of thumb. For every .001 wear on the diameter of the rings, and or the cylinder wall, the end gap of your rings grow by .003.
The following are some common examples of imperfect cylinder bores:
THE BARREL BORE
This cylinder measures the correct size at the top and the bottom of the bore but it is .004 bigger in the middle.You set the end gap of the rings at the top of the cylinder correctly but the end gap of the rings open up an additional .012 when they get to the middle of the bore.
THE UPSIDE DOWN FUNNEL BORE
This cylinder measures the correct size at the top of the bore but is .006 bigger at the bottom.You set the end gap of your rings at the top of the cylinder correctly but when the piston gets to the bottom of the stroke, the end gap opens up an additional .012.These examples may or may not be exaggerated, but its all relevant.
THE MORAL OF THIS STORY
The end gap of the rings and the correctness of the bore is a very controllable loss of compression to the crankcase and oil is being forced past the rings to dilute the fuel charge. ITS JUST HORSEPOWER BEING WASTED. Now stir this problem in with the first problems of distortion and you might as well call in the dogs.
Two more examples of imperfect bores:
THE FUNNEL BORE
The cylinder measures the correct size at the top but is .006 smaller at the bottom.
THE HOUR GLASS BORE
This cylinder measures the correct size at the top and bottom of the bore but is .004 smaller in the middle.You set the end gap of your rings at the top of the bore correctly but as the piston gets to the middle of the bore we have what is known as a CRUSHED END GAP. The cylinder wall galls, heat builds up and rings lock onto the piston. Need I say more.
WHAT IS GOOD ENOUGH IN A CYLINDER BORE
Perfection in any area is highly improbable but, as a rule of thumb, it should be within (.0005) ½ of a thousands. Holding the tolerance to less than this is certainly possible, so never give up trying at perfection.You should own a very good dial bore gauge that reads at least in the ½ thousandths (.0005).
When a motor is ready to compete in a race, the cylinder wall should be as perfect as possible. The bore should be round, straight from top to bottom and aligned exactly 90 degrees form the crankshaft. The cylinder wall should be as slick as possible with the rings already seated to the cylinder wall. The ring end gap should be at a minimum of .004 to .005" and the clearance between the piston and the cylinder wall (depending on preference) can be between .004 and .008.
ENGINE BREAK-IN • AN ELECTRIC RUN-IN STAND vs. LIVE RUNNING
There are many methods and theories on breaking in that new motor. The end result is that all the parts, especially the rings and cylinder wall, must wear a small amount to become compatible. WE MUST SEAL THE CYLINDER. Anything that is moving inside the engine is going to wear to some degree. During this break in period, all the metallic debris is being splashed back on these new parts causing more wear.
The big difference between these two methods:An engine running under power is under far more stress because of the pressures created by combustion. Therefore the metallic debris is under more pressure as it circulates over the moving parts. Fuel contamination of the oil is another problem. Fumes and noise are certainly a negative. Depending on motor design, engine RPM is sometimes hard to control. Ring end gap must be set at .004 to .005 because of the combustion heat. When the rings and cylinder wall have seated the end gap on the rings will now be .008 to.009.
An engine running on an electric motor run in stand is not under the stress of compression because we are running without a spark plug. At approximately 900 RPM we have a much more friendly environment to allow these parts to seat in. We are developing heat in the motor and it can run for several hours unattended with no noise or fumes. You can stop and change oil conveniently. The motor can even run without the valves, camshaft or lifters.The biggest benefit: Your engine can run with the ring end gap set at .001" which allows the rings and cylinder wall to wear and seat with a final end gap of about .004 to .005".
As you can see, by now, we believe that ring end gap is very important. Some of you are going to take issue with us and say that it should be .007 to .008 or more.The only thing that you have to worry about when you have the end gap at .004 is, MAKE SURE YOUR MOTOR IS AT OPERATING TEMPERATURE BEFORE YOU RACE OR LOAD THE MOTOR. Ring end gap is a controllable leak of compression.
Lets stop here and discuss a very important process.
PLATEAU HONING – WHAT IS IT?
After you finish hone your block to size with a nice cross hatch pattern and could look at it under a magnifying glass, you would see little peaks and valleys.These sharp peaks are going to be scraped of very quickly when the motor is first run. All of this metallic debris is going to be circulated through the engine.
Plateau honing is nothing more than wrapping a fine piece of wet or dry sand paper around your hone and with very light pressure make 2 passes from top to bottom. We now have Plateaus and valleys.
This is really important if you are using cast iron rings. These rings are porous and softer than chrome rings and the fractured material coming off the cylinder wall will become imbedded in the ring. When this happens it becomes increasingly difficult to seal the cylinder. Chrome rings are not effected by this material.In any event, plateau honing is well worth the time and in fact should be done on every motor.
Another subject worth mentioning:
JUST WHAT IS CLEAN?
Most parts will come clean enough in a good solvent bath. THE DEFINITION OF CLEAN FOR A CYLINDER WALL IS, A BUCKET OF HOT SOAPY WATER AND A SCRUB BRUSH. Then when you think you have it clean take a white rag dampened in solvent and wipe the bore. Now, is it clean?
THE SUBJECT OF RING SEATING
As we have discussed earlier, seating happens when the cylinder wall and the ring wears enough to seal the bore. If you inspect the bore after this has happened you will find that the cross hatch pattern has partially worn away and the bore is a lot slicker and the rings are polished the full 360 degrees.
CAST IRON RINGS vs. CHROME RINGS
Cast iron rings are easier to seat, are porous and will retain oil.Your final honing should be done with 320 grit stones and plateau honed with 600 grit sandpaper.Chrome rings are harder to seat and the surface finish will not retain oil.Your final honing should be done with 280 grit stones and plateau honed with 600 grit sandpaper. It may take a little longer run in time to seat them also.
FIGHTING FRICTION
The rings have already done a good job of reducing friction, but we can take this a little further. This, however, is another one of those subjects that there are many opinions on and we are probably going to step in you know what. But here goes…
As we stated earlier, chrome rings will not retain oil on their surface but cast iron rings will. So it is imperative that the cylinder wall has a texture that will hold oil.
In theory, after the rings are seated there should never be a metal to metal contact between the rings and the cylinder wall. It takes a thin film of oil between the cylinder wall and the rings to maintain the seal and keep the two parts from wearing. If this doesn’t happen, you can call in the dogs, the hunt is over.
A SECOND HONING PROCESS
Take a piece of 600 grit sandpaper and wrap it around your hone and with very light pressure, hone the block. This should only take a couple of minutes. The cylinder wall should remain bright and shinny. If you use to much pressure, or spend to much time in the bore it will start to look dark and you are now burnishing it. That finish will hurt oil retention.THIS FINAL PROCESS CAN ONLY BE DONE AFTER THE RINGS ARE SEATED.
IMPORTANT: WHEN YOU ARE BREAKING IN THE MOTOR NEVER USE YOUR RACING OIL. USE YOUR FAVORITE OIL THAT YOU PUT IN YOUR CAR OR TRUCK.
BREAKING-IN A MOTOR
We are going to use an electric motor run in stand, a leak down tester and a crankshaft locking bar.First, assemble all the internal parts in the motor, set the proper valve lash and install a breather plate. Do not install the cylinder head at this point.Fill the motor with your favorite engine oil.Secure the motor on an electric motor run in stand, bring the piston to top dead center and install the crankshaft locking bar.Install the cylinder head. Install the Leak Down Tester.
What we are going to do is a leak down test before we ever run the motor and document the results. Now as we go through the break in we can test periodically and measure our progress.
Remove the leak down tester and the crankshaft locking bar.Do not install the spark plug.Set the timer on the run in stand for 1 hour.
Now, make another leak down test and document your results.Run for another ½ hour, test and document your results.At some point, there will be no improvement and its time to stop.
The documentation will be a handy reference for future testing.
Now its time to disassemble the motor and do our final honing for friction. After that is done the parts need to go through the cleaning process and be reassembled for the final run in test.
Repeat the previous run in test (it should only take about half the time).
Document the results for future reference.
LEAK DOWN TEST RESULTS
The following is a good rule of thumb:
8 % - Something is really wrong
5 % - Just OK
4 % - Good
3 % - Very good
2 % - Excellent
1 % - Unbelievable
0 % - Almost impossible
This break in process can be done without the leak testing and for that matter without the cam and lifters. Your first run should be about 3 hours and the second about 1 ½ hours.
A BORING & HONING STRESS PLATE • DOES IT MATTER?
This subject is probably one if the most misunderstood areas we can discuss, and some will take us to task for our theory and opinions but here goes…Most people think a stress plate bolted to the top of a block pulls and distorts the cylinder bore, this is not quite true. However there is one exception were it possibly can, and this is on a very wavy and untrue deck surface. Were not going to build a motor with this kind of problem anyway, so it really doesn’t matter.
I KNOW, YOU’RE READY TO ARGUE, BUT I DID GET YOUR ATTENTION.NOW LET ME EXPLAIN WHAT REALLY DOES MATTER.
When a head bolt penetrates the threaded area of the block nothing happens until it is tightened down to the proper torque specifications. What happens then is a knot or swelling occurs in the bore of the block at the exact depth of the head bolt. And that’s all there is to it. Simple right, not quite.
Equal bolt penetration and torque on the bolts moving from the stress plate to the cylinder head should be a good match and this is the key.Quite naturally, all blocks are not equal and the amount of swelling into the bore is different. Kool bores, IC blocks, old style blocks, overbores and sleeved blocks are all going to react differently.Clean, undamaged bolt holes and bolts are also important.If you don’t have a stress plate, you can use a short piece of tubing and head bolts to accomplish almost the same swelling, but a stress plate is the best.
USING A STRESS PLATE DOES MATTER !
One last thing on stress plates. There are a number of people who won’t grind the valve seats without a stress plate on the block. My opinion is that it does no good.
THE SUBJECT OF CYLINDER HEAD STUDS
This is another subject that is controversial and I’m sure there will be some disagreement on our opinion.I do not like studs and can see no benefit from using them. Plus, they can cause a lot of problems.While there are many illustrations I could give you, the following is a classic one.You have a block ready to build and, for whatever reason, you decide to use studs.The block has been bored and honed properly - and it is perfect.
Now, you install a set of studs and tighten them down. If you stop just when they run out of threads, and they are nice and snug, you probably have already pushed a knot in the top of the bore. This lump only goes down about .125 but it is different than the one caused by the head bolts. This one is caused by the tapered conclusion of the threads ending on the stud.
Lets assume the piston fits and this goes unnoticed. The head is installed, and the proper torque is applied, but the stud decides to turn a little more and push’s more metal into the cylinder wall. No need to go further because this ball game is over.I could give many more illustrations, but I hope you got the message.
IF YOU FEEL YOU MUST USE STUDS, FOLLOW THIS PROCEDURE
Before you do any boring or honing:
• Clean the threads in the block and the studs with alcohol and let dry.
• Screw the studs in with a good coat of Lock-Tite. (it can take up to 24 hours for the Lock-Tite to cure)
• Now, using a stress plate, do your boring and honing.
VALVE GUIDES • DO THEY MATTER?
The valve guides that come in a new Briggs block are loose and sloppy and one thing is for sure, the valve will never stick. It is not uncommon to find them misaligned with the lifter bore. This can cause abnormal wear and valve seating problems. As far as using them for a racing engine they certainly come up short.
A full length brass guide is better, such as stacking 2 Briggs guides on top of each other. However the full length phosphorus bronze guide from ARC is the best.The ARC Bronze guide can be run with .0015 clearance on the exhaust, .001 on the intake and out last several sets of valve stems.A must with either brass or bronze guides is a smooth finish on the valve stem.
There are many people who feel a loose valve guide is the best because it reduces friction, but an ARC Bronze guide properly fit at the above tolerances will have little or no friction.Remember: Loose valve guides will contribute to oil contamination in the fuel charge which reduces horsepower.
The biggest problem with a loose valve guide is the poor little valve trying to find the seat at high RPM. Think about this: at 6000 rpm a valve must find the seat and seal it perfectly 50 times a second. Seems to me it needs all the help it can get.
ARC has complete valve guide installation equipment available that’s quick and easy, and also corrects any mis-alignment between the lifter bore and the valve stem.
SOME WORDS OF WISDOM AND CAUTIONS
Leak down testing is a very handy tool to use for determining if you have a problem with a motor or if it is still race worthy.
The compression ratio of an average stock Briggs motor is about 6.02 to 1, it would be very hard to increase it much with out killing flow and flame travel.
If cranking compression (with an electric starter) is 150 psi, on a great running motor, the compression at 6000 rpm will only be about 20 psi (or less).
Without a sealed cylinder, compression will just disappear at high rpm and guess what, the motor slows down.
If your motor is not running up front, and you make a header change, gear change, carburetor change and nothing seems to help. Do a leak down test and you’ll probably find the problem.
When you drive a new valve seat into your block, you will distort the top of the bore.
The average racer spends much more time tweaking and worrying with a camshaft than he does with sealing and maintaining the cylinder. The cylinder should come first and foremost.
A ball hone should only be used for freshening up or re-ringing a motor.
Be careful of "trick of the week" parts that claim to perform magic. They just don’t work.Just remember Kart Racing can make you moderately wealthy, if you start out filthy stinking rich
1 - If a problem exists, and we have the capability to correct it, we tell the world "It Does Matter".
2 - If we don't have any interest or desire to correct a problem, we either keep our mouths shut or simply say "It just doesn't matter".
PREFACE
This article could have been greatly expanded into a full length novel but would have gotten boring with illustration on top of illustration.What's written here is not fiction, and mixed with a bucket full of common sense and imagination, you will run faster and longer.If you disagree with our opinions, let us know. We can learn too.If you have any questions or problems, send us an e-mail or call 1-800-521-3560. We'll be here if you need us.
DOES IT MATTER
If the base of a block is not perfectly flat and true ?If the surface of a motor mount that accepts the base of a block is not true and flat ?Through no fault of the chassis builder, it’s virtually impossible to have the mounting rails for the motor mount perfectly aligned.Does the weight of the driver flex or move the motor mount rails on the chassis ?Do the motor mount rails move and flex during a race ?
In other words, can you take a perfectly good motor, bolt it to a motor mount and then to the chassis and totally mess it up ? And the answer is - YES !
CONSIDER THE FOLLOWING TEST RESULTS
We took a block and precision align bored it perfectly round and straight from top to bottom and bolted it down to a rigid, perfectly flat surface plate. Under one corner we put a .050 shim. This shim could now represent a distortion caused by any one of or combination of the problems mentioned above.Next, we took a good tenth (.0001) reading dial bore gauge to examine the bore. Looking at the top of the block, and considering between the two valve seats would represent 6 o’clock, we took 2 readings. One aimed at 11 o’clock and one aimed at 1 o’clock. Starting at the top of the bore we moved down a ½ inch at a time. As we moved down the once perfectly round bore, it started taking the shape of an egg.The distortion was .002.
This test was performed on a Kool bore motor bored .030 over.The crankshaft bores had also moved and were slightly tighter in the DU bushing.A dual bearing block is more forgiving when this happens but either way it is a problem.The deck of the block also moved and became distorted.
Here’s a shocker:Before we did the test, we bolted a side cover on the block without a gasket and could see day light between the two surfaces. Then we took feeler gauges and it was easy to find where a .002 gauge would fit but we also got a .004 gauge to fit in one place.
Have you ever wondered why sometimes you have problems with oil leaks and blown side cover gaskets ?
These are some problems that can happen because of this:
• Scuffing or galling pistons.
• Abnormal wear in the cylinder wall.
• Rings not seating.
• Loss of compression.
• Loss of horsepower.
• Increased blow by in the crankcase.
• Oil consumption.
• Oil contaminating the fuel charge.
• Engine life short lived.
• Blown head gaskets.
• Side cover gaskets leaking.
• Blown side cover gaskets.
• Blocks cracking because of stress.
The question is: DOES ANY OF THIS MATTER ?
The answer is: ABSOLUTELY...
OTHER THINGS TO THINK ABOUT (and remember)
Briggs blocks are not equal and will vary from block to block.Kool bore blocks will distort more than IC blocks.Big overbores will distort more than stock bores.Pre-Raptor motors, with thinner castings, will distort more than the current castings.Large overbores for sleeves and then boring the sleeve for large pistons will distort even more.Over a period of years we have heard the following question asked several times a week from our customers:"I built two identical motors, 1 runs super and the other is a real dog, what’s wrong" ?or "I dyno tested this motor and it was great, then put it on the chassis and it wouldn’t fall out of a tree by itself. Why" ?
I wonder if anything we have discussed so far would shed any light on this subject ?
SOLVING THE PROBLEM
To solve one of the problems, the base of the block must be surfaced perfectly flat. The side cover must be on the block when it is surfaced. If you ever change side covers you must check the new one and make sure it does not stick down below the base surface of the block. If it does grind this material away.
The two drain plugs should be installed in the block TIGHT before it is surfaced. The reason here is that the drain plugs swell the block and create a knot on the base surface. Either before or after surfacing the base, with the drain plugs installed, take a die grinder and remove about .050 of material. The area to work on is 1 inch to the right and the left, and into the center of the block from the drain plug.
One last important thing:You must change your habits because you never thought about protecting the base of the block before.A special fixture (part number 7718) for milling or grinding the base of the block is available from ARC. This fixture also aligns the crankshaft parallel with the base of the block and aligns the existing bore 90 degrees with the base front to rear and is also excellent for use prior to boring a block on a vertical mill.
THE CYLINDER WALL • DOES IT MATTER ?
Ask yourself this question. Would you go to a race and add some oil to your fuel and loosen your spark plug just a little so you would have less compression? Obviously not. But you might be accomplishing the same thing and here’s how.Remember this rule of thumb. For every .001 wear on the diameter of the rings, and or the cylinder wall, the end gap of your rings grow by .003.
The following are some common examples of imperfect cylinder bores:
THE BARREL BORE
This cylinder measures the correct size at the top and the bottom of the bore but it is .004 bigger in the middle.You set the end gap of the rings at the top of the cylinder correctly but the end gap of the rings open up an additional .012 when they get to the middle of the bore.
THE UPSIDE DOWN FUNNEL BORE
This cylinder measures the correct size at the top of the bore but is .006 bigger at the bottom.You set the end gap of your rings at the top of the cylinder correctly but when the piston gets to the bottom of the stroke, the end gap opens up an additional .012.These examples may or may not be exaggerated, but its all relevant.
THE MORAL OF THIS STORY
The end gap of the rings and the correctness of the bore is a very controllable loss of compression to the crankcase and oil is being forced past the rings to dilute the fuel charge. ITS JUST HORSEPOWER BEING WASTED. Now stir this problem in with the first problems of distortion and you might as well call in the dogs.
Two more examples of imperfect bores:
THE FUNNEL BORE
The cylinder measures the correct size at the top but is .006 smaller at the bottom.
THE HOUR GLASS BORE
This cylinder measures the correct size at the top and bottom of the bore but is .004 smaller in the middle.You set the end gap of your rings at the top of the bore correctly but as the piston gets to the middle of the bore we have what is known as a CRUSHED END GAP. The cylinder wall galls, heat builds up and rings lock onto the piston. Need I say more.
WHAT IS GOOD ENOUGH IN A CYLINDER BORE
Perfection in any area is highly improbable but, as a rule of thumb, it should be within (.0005) ½ of a thousands. Holding the tolerance to less than this is certainly possible, so never give up trying at perfection.You should own a very good dial bore gauge that reads at least in the ½ thousandths (.0005).
When a motor is ready to compete in a race, the cylinder wall should be as perfect as possible. The bore should be round, straight from top to bottom and aligned exactly 90 degrees form the crankshaft. The cylinder wall should be as slick as possible with the rings already seated to the cylinder wall. The ring end gap should be at a minimum of .004 to .005" and the clearance between the piston and the cylinder wall (depending on preference) can be between .004 and .008.
ENGINE BREAK-IN • AN ELECTRIC RUN-IN STAND vs. LIVE RUNNING
There are many methods and theories on breaking in that new motor. The end result is that all the parts, especially the rings and cylinder wall, must wear a small amount to become compatible. WE MUST SEAL THE CYLINDER. Anything that is moving inside the engine is going to wear to some degree. During this break in period, all the metallic debris is being splashed back on these new parts causing more wear.
The big difference between these two methods:An engine running under power is under far more stress because of the pressures created by combustion. Therefore the metallic debris is under more pressure as it circulates over the moving parts. Fuel contamination of the oil is another problem. Fumes and noise are certainly a negative. Depending on motor design, engine RPM is sometimes hard to control. Ring end gap must be set at .004 to .005 because of the combustion heat. When the rings and cylinder wall have seated the end gap on the rings will now be .008 to.009.
An engine running on an electric motor run in stand is not under the stress of compression because we are running without a spark plug. At approximately 900 RPM we have a much more friendly environment to allow these parts to seat in. We are developing heat in the motor and it can run for several hours unattended with no noise or fumes. You can stop and change oil conveniently. The motor can even run without the valves, camshaft or lifters.The biggest benefit: Your engine can run with the ring end gap set at .001" which allows the rings and cylinder wall to wear and seat with a final end gap of about .004 to .005".
As you can see, by now, we believe that ring end gap is very important. Some of you are going to take issue with us and say that it should be .007 to .008 or more.The only thing that you have to worry about when you have the end gap at .004 is, MAKE SURE YOUR MOTOR IS AT OPERATING TEMPERATURE BEFORE YOU RACE OR LOAD THE MOTOR. Ring end gap is a controllable leak of compression.
Lets stop here and discuss a very important process.
PLATEAU HONING – WHAT IS IT?
After you finish hone your block to size with a nice cross hatch pattern and could look at it under a magnifying glass, you would see little peaks and valleys.These sharp peaks are going to be scraped of very quickly when the motor is first run. All of this metallic debris is going to be circulated through the engine.
Plateau honing is nothing more than wrapping a fine piece of wet or dry sand paper around your hone and with very light pressure make 2 passes from top to bottom. We now have Plateaus and valleys.
This is really important if you are using cast iron rings. These rings are porous and softer than chrome rings and the fractured material coming off the cylinder wall will become imbedded in the ring. When this happens it becomes increasingly difficult to seal the cylinder. Chrome rings are not effected by this material.In any event, plateau honing is well worth the time and in fact should be done on every motor.
Another subject worth mentioning:
JUST WHAT IS CLEAN?
Most parts will come clean enough in a good solvent bath. THE DEFINITION OF CLEAN FOR A CYLINDER WALL IS, A BUCKET OF HOT SOAPY WATER AND A SCRUB BRUSH. Then when you think you have it clean take a white rag dampened in solvent and wipe the bore. Now, is it clean?
THE SUBJECT OF RING SEATING
As we have discussed earlier, seating happens when the cylinder wall and the ring wears enough to seal the bore. If you inspect the bore after this has happened you will find that the cross hatch pattern has partially worn away and the bore is a lot slicker and the rings are polished the full 360 degrees.
CAST IRON RINGS vs. CHROME RINGS
Cast iron rings are easier to seat, are porous and will retain oil.Your final honing should be done with 320 grit stones and plateau honed with 600 grit sandpaper.Chrome rings are harder to seat and the surface finish will not retain oil.Your final honing should be done with 280 grit stones and plateau honed with 600 grit sandpaper. It may take a little longer run in time to seat them also.
FIGHTING FRICTION
The rings have already done a good job of reducing friction, but we can take this a little further. This, however, is another one of those subjects that there are many opinions on and we are probably going to step in you know what. But here goes…
As we stated earlier, chrome rings will not retain oil on their surface but cast iron rings will. So it is imperative that the cylinder wall has a texture that will hold oil.
In theory, after the rings are seated there should never be a metal to metal contact between the rings and the cylinder wall. It takes a thin film of oil between the cylinder wall and the rings to maintain the seal and keep the two parts from wearing. If this doesn’t happen, you can call in the dogs, the hunt is over.
A SECOND HONING PROCESS
Take a piece of 600 grit sandpaper and wrap it around your hone and with very light pressure, hone the block. This should only take a couple of minutes. The cylinder wall should remain bright and shinny. If you use to much pressure, or spend to much time in the bore it will start to look dark and you are now burnishing it. That finish will hurt oil retention.THIS FINAL PROCESS CAN ONLY BE DONE AFTER THE RINGS ARE SEATED.
IMPORTANT: WHEN YOU ARE BREAKING IN THE MOTOR NEVER USE YOUR RACING OIL. USE YOUR FAVORITE OIL THAT YOU PUT IN YOUR CAR OR TRUCK.
BREAKING-IN A MOTOR
We are going to use an electric motor run in stand, a leak down tester and a crankshaft locking bar.First, assemble all the internal parts in the motor, set the proper valve lash and install a breather plate. Do not install the cylinder head at this point.Fill the motor with your favorite engine oil.Secure the motor on an electric motor run in stand, bring the piston to top dead center and install the crankshaft locking bar.Install the cylinder head. Install the Leak Down Tester.
What we are going to do is a leak down test before we ever run the motor and document the results. Now as we go through the break in we can test periodically and measure our progress.
Remove the leak down tester and the crankshaft locking bar.Do not install the spark plug.Set the timer on the run in stand for 1 hour.
Now, make another leak down test and document your results.Run for another ½ hour, test and document your results.At some point, there will be no improvement and its time to stop.
The documentation will be a handy reference for future testing.
Now its time to disassemble the motor and do our final honing for friction. After that is done the parts need to go through the cleaning process and be reassembled for the final run in test.
Repeat the previous run in test (it should only take about half the time).
Document the results for future reference.
LEAK DOWN TEST RESULTS
The following is a good rule of thumb:
8 % - Something is really wrong
5 % - Just OK
4 % - Good
3 % - Very good
2 % - Excellent
1 % - Unbelievable
0 % - Almost impossible
This break in process can be done without the leak testing and for that matter without the cam and lifters. Your first run should be about 3 hours and the second about 1 ½ hours.
A BORING & HONING STRESS PLATE • DOES IT MATTER?
This subject is probably one if the most misunderstood areas we can discuss, and some will take us to task for our theory and opinions but here goes…Most people think a stress plate bolted to the top of a block pulls and distorts the cylinder bore, this is not quite true. However there is one exception were it possibly can, and this is on a very wavy and untrue deck surface. Were not going to build a motor with this kind of problem anyway, so it really doesn’t matter.
I KNOW, YOU’RE READY TO ARGUE, BUT I DID GET YOUR ATTENTION.NOW LET ME EXPLAIN WHAT REALLY DOES MATTER.
When a head bolt penetrates the threaded area of the block nothing happens until it is tightened down to the proper torque specifications. What happens then is a knot or swelling occurs in the bore of the block at the exact depth of the head bolt. And that’s all there is to it. Simple right, not quite.
Equal bolt penetration and torque on the bolts moving from the stress plate to the cylinder head should be a good match and this is the key.Quite naturally, all blocks are not equal and the amount of swelling into the bore is different. Kool bores, IC blocks, old style blocks, overbores and sleeved blocks are all going to react differently.Clean, undamaged bolt holes and bolts are also important.If you don’t have a stress plate, you can use a short piece of tubing and head bolts to accomplish almost the same swelling, but a stress plate is the best.
USING A STRESS PLATE DOES MATTER !
One last thing on stress plates. There are a number of people who won’t grind the valve seats without a stress plate on the block. My opinion is that it does no good.
THE SUBJECT OF CYLINDER HEAD STUDS
This is another subject that is controversial and I’m sure there will be some disagreement on our opinion.I do not like studs and can see no benefit from using them. Plus, they can cause a lot of problems.While there are many illustrations I could give you, the following is a classic one.You have a block ready to build and, for whatever reason, you decide to use studs.The block has been bored and honed properly - and it is perfect.
Now, you install a set of studs and tighten them down. If you stop just when they run out of threads, and they are nice and snug, you probably have already pushed a knot in the top of the bore. This lump only goes down about .125 but it is different than the one caused by the head bolts. This one is caused by the tapered conclusion of the threads ending on the stud.
Lets assume the piston fits and this goes unnoticed. The head is installed, and the proper torque is applied, but the stud decides to turn a little more and push’s more metal into the cylinder wall. No need to go further because this ball game is over.I could give many more illustrations, but I hope you got the message.
IF YOU FEEL YOU MUST USE STUDS, FOLLOW THIS PROCEDURE
Before you do any boring or honing:
• Clean the threads in the block and the studs with alcohol and let dry.
• Screw the studs in with a good coat of Lock-Tite. (it can take up to 24 hours for the Lock-Tite to cure)
• Now, using a stress plate, do your boring and honing.
VALVE GUIDES • DO THEY MATTER?
The valve guides that come in a new Briggs block are loose and sloppy and one thing is for sure, the valve will never stick. It is not uncommon to find them misaligned with the lifter bore. This can cause abnormal wear and valve seating problems. As far as using them for a racing engine they certainly come up short.
A full length brass guide is better, such as stacking 2 Briggs guides on top of each other. However the full length phosphorus bronze guide from ARC is the best.The ARC Bronze guide can be run with .0015 clearance on the exhaust, .001 on the intake and out last several sets of valve stems.A must with either brass or bronze guides is a smooth finish on the valve stem.
There are many people who feel a loose valve guide is the best because it reduces friction, but an ARC Bronze guide properly fit at the above tolerances will have little or no friction.Remember: Loose valve guides will contribute to oil contamination in the fuel charge which reduces horsepower.
The biggest problem with a loose valve guide is the poor little valve trying to find the seat at high RPM. Think about this: at 6000 rpm a valve must find the seat and seal it perfectly 50 times a second. Seems to me it needs all the help it can get.
ARC has complete valve guide installation equipment available that’s quick and easy, and also corrects any mis-alignment between the lifter bore and the valve stem.
SOME WORDS OF WISDOM AND CAUTIONS
Leak down testing is a very handy tool to use for determining if you have a problem with a motor or if it is still race worthy.
The compression ratio of an average stock Briggs motor is about 6.02 to 1, it would be very hard to increase it much with out killing flow and flame travel.
If cranking compression (with an electric starter) is 150 psi, on a great running motor, the compression at 6000 rpm will only be about 20 psi (or less).
Without a sealed cylinder, compression will just disappear at high rpm and guess what, the motor slows down.
If your motor is not running up front, and you make a header change, gear change, carburetor change and nothing seems to help. Do a leak down test and you’ll probably find the problem.
When you drive a new valve seat into your block, you will distort the top of the bore.
The average racer spends much more time tweaking and worrying with a camshaft than he does with sealing and maintaining the cylinder. The cylinder should come first and foremost.
A ball hone should only be used for freshening up or re-ringing a motor.
Be careful of "trick of the week" parts that claim to perform magic. They just don’t work.Just remember Kart Racing can make you moderately wealthy, if you start out filthy stinking rich
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