STROKER ROD DESIGN

By: Carl Amundsen Date: March 20, 2000

Evidently a lot of people have read and paid attention to our editorial " Oil Clearance is Not a Myth But a Calculation ". We appreciate all the calls and hope it has helped save some engines.

A constant question that keeps coming up is about the bore size on the stroker rod for the .875 (7/8) journal crankshaft.

Why are the rod bores on some brands of stroker rods as much as .004" out of round, right out of the box?

These rods all have angled or what is termed "splayed rod bolts". This is a necessary design because of the amount of stroke in the crankshaft and the need for clearance at the camshaft and inside the block.

The problem occurs when the parting surfaces at the rod and cap are not 90 degrees to the rod bolts. On the illustration below the right rod bolt enters at 15 degrees and the left rod bolt enters at 7 1/2 degrees. The parting surfaces of the rod and cap are at 75 degrees and 82 1/2 degrees respectively to the rod bolt.

It is easy to see from the illustration that the seat area under the rod bolt and the seat area of the cap will be at an angle to the parting surfaces of the rod assembly. This is where the problem begins.

During the manufacturing process the rod and cap are assembled together and the bolts are tightened to a specific torque value. The final machining of the bore now takes place, and comes out on size and round. The next thing that happens is that the rod bolts are removed and new ones are installed. Here is your problem. Because of the adverse angle at which the bolts enter the seam of the rod, they pull the bore out of round. It will always come out egg shaped, and the more you play with the torque values the worse it gets. This is not a one out of ten problem this will happen 100 times out of 100 times. It can be bad enough to lock the motor up. Bet on it. THE PROBLEM IS IN THE DESIGN.

To add insult to injury, some rods are designed with the rod bolts exposed to the bore. This just reduces the integrity of the bore and weakens the seam area. If that's not enough, you must grind some clearance into the back side of the bearings to accommodate the rod bolts.

THE FOLLOWING IS AN ILLUSTRATION OF THE ARC DESIGNED STROKER ROD, NOTICE THAT THE BOLTS AND THEIR PARTING EDGES ARE PERPENDICULAR TO ONE ANOTHER. THIS DESIGN ELIMINATES THE PROBLEMS CAUSED BY THE ANGLED BOLT TO PARTING EDGE SITUATION. OUR WAY STILL ISN'T PERFECT, BUT IT IS A HUGE STEP IN THE RIGHT DIRECTION.

OIL CLEARANCE IS NOT A MYTH IT IS A CALCULATION

By: Carl Amundsen Date:2/8/2000

Over the years I have had many opportunities to discuss oil clearance with engine builders. Some are building for their own use, some are small shops while some are large. In any event when the question is posed "HOW DO YOU KNOW WHAT YOUR OIL CLEARANCE IS?" the responses vary. Some can give a good mathematical response, some don't have a clue. The common denominator here is that everyone knows they have oil clearance.
A common phenomenon that occurs when a motor fails is that no one looks in the mirror for answers. The vast majority of the time it is blamed on an engine part failure. The connecting rod too many times is the favorite fugitive.
A LESSON IN MEASUREMENTS
A very dangerous but commonly used method of measuring is a dial caliper. The average dial caliper is super for measuring the thickness, inside diameter, and outside of an empty toilet paper roll. A SHOCKING STATEMENT , NO DOUBT! If you look through some catalogs you will find a dial caliper for $40.00 that has an advertised accuracy of +/- .008 . That is an error factor of 16 thousandths. Pay $200.00 for a digital caliper with advertised accuracy of +/- .0015, the margin of error is now 3 thousandths. An error factor of 1 thousandths can cost you a motor. Calipers are great tools for some things but not for the task at hand. WE MUST BE ABLE TO READ IN TENTHS OF THOUSANDTHS(.0001)
ON WITH THE LESSON
If you were to take a good 0 to 1 inch micrometer and measure the thickness of a page in the ARC catalog, you will find it to be five thousandths (.005) thick. This is very important to remember as you read on. Now feel the paper with your fingers and imagine somehow splitting one of these pages 50 times. One single sheet would now be 1 tenth of 1 thousandth of an inch thick (.0001). I hope I have your attention, because this is getting down so that you can't even feel the thickness. Just incase I am going to be confusing you with decimals please remember the following:
1.0000 = 1 inch0.1000 = 100 thousandths 0.0100 = 10 thousandths0.0010 = 1 thousandth0.0001 = 1/10 thousandth

It makes no difference who you buy your parts from, nor does it matter what brand they are, ARC included,they need to be checked and double checked. NOBODY IS PERFECT.
For example let's build a stock stroke Briggs racing engine. The following is the measurement specifications on the typical parts to be used:
Stock Briggs crank rod journal size......................... .998 +/- .0015Aftermarket Connecting Rod................................... 1.150 +/- .005Aftermarket Rod Bearing thickness........................ .075 +/- .003
ALL THE PARTS WE ARE USING ARE WITHIN THE MANUFACTURERS TOLERANCES.
You are going to use a medium weight oil and you are shooting for .0025
The Connecting Rod is on the small side................. 1.1495The Rod Bearing is on the big side.......................... .0753The Crankshaft is on the big side.............................. .9985
Let's put this motor together and go racing.
The Connecting Rod is small by .0005...................... 1.1495The Thickness of the Rod Bearingare big by .003(2 bearings) =.1506............................. - .1506Net Rod Bore size with Bearingsinstalled................................................................... . 9989The Crank Rod Journal is big by .0005..................... - .9985The Calculated oil Clearance = ............................... .0004We were shooting for ............................................... .0025
With a little luck this motor will crank up and run, as long as the motor is running at no load and a low RPM it may be OK for a while. The minute you go racing the lack of oil flow between the bearing and the crank journal will cause heat build up. The bearing will seize on the crank journal, break the rod and just make a mess of everything. BAD PARTS RIGHT ? WRONG !
Now let's build another motor and go the opposite way, still trying to achieve an oil clearance of .0025
The Connecting Rod is on the big side........................ 1.1505The Rod Bearing is on the small side.......................... .0747The Crankshaft is on the small side.............................. .9965
The Connecting Rod is big by .0005............................ 1.1505The Thickness of the Rod Bearingare small by .003(2 bearings) =.1494............................. - .1494Net Rod Bore size with Bearingsinstalled...................................................................... 1.0011The Crank Rod Journal is small by .0015..................... - .9965The Calculated oil Clearance = .................................. .0046We were shooting for .................................................. .0025
This motor is going to run, but what is going to happen here is: The bearing is going to get pounded at the top and bottom of the rod bore, because there is an air gap between the two surfaces. The oil is not thick enough to prevent this from happening. This is going to convert the rod bore into the shape of an egg standing on end. The crankshaft will now start to loose it's round shape and wear.
EVERY MINUTE IT RUNS, THESE PARTS WILL INCREASE THE OIL CLEARANCE UNTIL IT EXPLODES. HOW LONG WILL IT LAST? I CAN'T SAY EXACTLY, BUT NOT TO LONG!
The crying shame here is that everyone will point their fingers at the connecting rod, bearings and/or crankshaft as the culprit.
THE MOST OVERLOOKED AREA IN ENGINE BUILDING IS AS FOLLOWS:You have a motor that has run for many months , but you notice you're getting a little blow-by and it could probably use a set of rings. It needs freshening up so we tear it down. Every thing looks great so we touch the bore with a hone and put a new set of rings in. Back to the races. If it ain't broke don't fix it. STOP! Under the most ideal conditions engine parts will change with use. Bearings can look good but will wear, connecting rod bores will change shapes and crank journals will wear and rod bolts will stretch and fatigue. It is just as important now as ever to check the dimensions of the parts. Don't go to sleep.
Some may think there is some deep mystery behind the term oil clearance. The reason is, everyone has a different opinion as to what it should be. As a rule of thumb, it can be anywhere between .0015 and .0035 and be in the ball park. If the clearance is on the low side, use thin oil, heavy oil will not work. If clearance is on the high side , use a heavier oil, thin oil will not work. How can you make a judgment on the oil to use unless you know what your oil clearance is? THERE IS NO MAGIC, YOU CANNOT KNOW WHAT YOUR OIL CLEARANCE IS UNLESS YOU MEASURE THE PARTS THAT AFFECT IT.
DEFINITION OF WHAT OIL CLEARANCE IS:
"The distance between two very smooth moving surfaces that will allow oil to be present at all times, coating both parts with a film of oil so that metal on metal contact never happens. It must be small enough to retain enough oil and large enough to allow a fresh cool supply to move through every microsecond."
If you do not check your parts and build 4 motors and 3 of them seem to have a pretty good life span you are lucky. If one fails out of the gate or shortly thereafter, shame on you. It takes less than 15 minutes to check and measure all the parts in a motor and the benefits are fantastic. It is possible to build an engine that will run until the cows come home, or something like that. There are many engine builders out there that do a fantastic job in this area, but on the other hand there are more that don't.
ASSUME NOTHING, BELIEVE NOTHING AND LAST BUT NOT LEAST, CHECK EVERYTHING.
I hope we have given you an insight to one reason for engine failures. If you need us, give us a call, it's all free. 1-800-521-3560

The Jury is Back and the Verdict is In

By: Carl Amundsen Date : January 18, 2000

The Case: To produce the best crankshaft ever.
The Jury: Engine builders, car owners, chassis builders from our customer base.
The Evidence:

The Material to be used
Much to do has been made about what kind of material should, could and would be used to make a crankshaft. Manufactures tout theirs are made of 4140, 4150, 8690, Stressproof, or other high mucky muck number designations. We investigated all of these options, but when the smoke cleared we came back to a material we had been using for 20 years. The material does not have a number designation because it is proprietary to the manufacturer and is patented. The brand of the material shall remain anonymous for our own reasons. The biggest user of this material is the mining industries, where it is used in augers, blades, conveyors or anything that requires a hard wear resistant material. We have used this material for 20 years in impeller blades for steel shot machines. Through its manufacturing process and mineral content it requires no hardening before it is used. It comes in the door with a hardness of 44 to 46 on the Rockwell "C" scale and the more it is used the harder it gets. This is all accomplished without it becoming brittle. The only negative side of this is that it is a nightmare to machine. It took about 6 weeks of trial and error to come up with the right feed rates and spindle speeds. The only tooling material that is compatible is solid carbide. Tool life at best is very expensive.

Design and Engineering
Instead of taking a brand new crankshaft out of the box and starting the process of cutting and grinding. This is not necessary with ARC cranks for most applications of camshafts and lifters etc. You will also find that there is no note in the box instructing you to deburr sharp edges, this has already been done. This was one of the more important items that we addressed in the manufacturing process. At least 15% 0f the machining time is spent removing sharp edges. This is an absolute must , this is also the reason other manufacturers simply leave you a note in the box. A sharp edge is an invitation for a crack to start, and no matter how small it is, it will grow just like cancer. I am sure everyone has seen the old trick of tearing a telephone book in half, same principal. We borrowed technology from NASCAR in designing the (airplane wing ) airfoil style counter weights. NASCAR has proven that it improves windage in the crank case and increases horsepower. The crank pin even has a true 1/8" radius for added strength and durability. Other crankshafts advertise that they are already balanced, NOT. We went through a very pain staking design process to make sure we could and should advertise a balanced crank. You will without a doubt notice the difference. ARC provides two differently balanced models of the +.563 stroker, one is balanced for the smaller bore up to .190 over and the 3 x 3 for the large 3 inch bore. Appearance, cosmetics and detail have always played a major roll in parts produced by ARC. I think we raised the bar on this one. Besides our logo, all ARC crankshafts are engraved with the part number and the date the part was manufactured. Little more can be said until you hold one in your hand, this crank is truly a work of art. Its worth mentioning , if you haven't already noticed, ARC has a habit of not following what the industry thinks or does. If we can't do it better, we just don't do it.

The Testing:
The latter part of August 1999 we made our first live test. You might think this is a crazy way to start testing, but we needed to build a motor that would come apart fairly easy, I mean explode! This was done with a blockzilla block, our new +.563 crank, an old .190 over piston with new rings, a used 4.225 rod with old rod bolts and a used 436 lift camshaft. We knew from the start that the rod bolts and/or the rod should break, they just had to much time on them. If our crankshaft was going to live it would have to withstand this. The tension and anxiety is growing. Over the next four weeks we made over 250 passes down our local 1/8-mile drag strip, seeing RPM's in excess of 9600. Nothing happened. Not being able to break it was becoming very frustrating. The next test we felt sure "The old wore out rod and bolts" would fail, and we would have our desired results. We put the motor on a kart chassis and ran it on the local 1/4 mile asphalt oval. This is the hardest test on any motor because you are constantly in and out of the throttle. We now had shipped a number of crankshafts to select customers with their promise to have them in motors very quickly, and report to us with results.

The results from the field are as follows:
"That's a beautiful part, works great.""Smoothest running motor I have ever seen, what did you do to it ?""I can't believe this , it's great !" "How soon can I get five more?" And so on. Everybody loves it and no one has had any problems.
We were now getting well into October and getting a little curious , what will break it ? We have over 125 laps on the kart and the motor just purrs. The only maintenance we had done was keeping oil in it. It was even suggested that we run it without oil, dumb idea of course but we were desperate for some results. We didn't do that obviously. At this time we turned the motor over to a local drag racer to finish the job. Finally some results. One of our crank shafts had made it through one of our renowned engine builder customers well west of the Mississippi and back to San Antonio , Texas. Charlie Bass Sr. and his son Charlie Bass Jr. are kart racers that race an outlaw class against the highly touted 2-cycles. However they use a Briggs Blockzilla. Over the past year they were using a 563 stroker crank of an origin I will not mention. They had the power they needed to lead and win races but kept breaking crankshafts. NEED I SAY MORE ? YES, I WILL! THEY WHIPPED EVERYONE AND WENT ON TO BECOME POINTS CHAMPION! A footnote to this is he always used ARC rods.
CONGRATULATIONS TO THE BASS FAMILY, WE LOVE YOU !
Texans have always laid claim to being a little different and I wouldn't debate that. Charlie Sr. had a strikingly different approach for checking for burrs and rough edges on our crank. He wiped the crank down with a pair of panty hose, and did not get a run in them. I didn't ask who the panty hose belonged to.
More Comments
"We no longer have to hold the kart down on the rack when we run the motor, it just sits there and purrs." " I can read the temp. gauge and tack now, I can see the flags." Charlie Bass Jr. " Is this smooth or is this smooth "
At their request we have put the Bass' in the unprotected witness program.
Charlie Jr. Leddfutt24@worldnet.att.net

Charlie Sr. DrBriggs94@worldnet.att.net Home phone 210-923-5730
THANKS AGAIN TO THE BASS FAMILY

Now back to our test motor. We were closer than we thought, it only took two more weekends of racing to do it. When we tore it down the rod bolts had broken along with the rod (as we predicted) and chunked the whole mess out of the side of the block. FINALLY RESULTS WE WERE WAITING FOR. WHEN WE BUILT THIS MOTOR WE INTENTIONALLY USED AN OLD ROD WITH OLD BOLTS, THAT SHOULD HAVE BEEN JUNKED. WE KNEW THE ROD WOULD BREAK , WE JUST DIDN'T KNOW WHEN. The crank was still straight, the crank pin was still in perfect shape and the only damage was some scuff marks on the counter weights. We checked the hardness of the crank pin and it had gone up 5 pts. on the Rockwell scale. Magnaflux revealed no stress or cracks. Anyone in his right mind would think surely this is enough, NOT US. What everyone wanted to see now was just what would break it. GET THIS, the stage was set, the crank was placed in a hole in our heavy duty welding bench which is two inches thick. The crankshaft was vertical with one counter weight resting on the table. Our weapon of destruction was a 10 lb. sledge hammer with a 36" handle powered by Randy Amundsen. Smashing down on the exposed counter weight, we wanted to see if the counter weights would touch before breaking the crank pin. To me this seemed cruel, it reminded me of the turkey losing his head Thanks Giving morning. This crank surely didn't have a chance.
LET THE CONTEST BEGIN
After 5 or 6 hits the crank was holding up fairly well. After about 12 hits the crank was bending but still refused to crack. Someone muttered " that crank thinks Randy is a wimp". After more than 25 over the head death blows the crank finally gave up. Before Randy could catch his breath another brand crankshaft was set up ready for the same challenge. Two smashes latter we knew who had the toughest crankshaft on the market. This was ARC's real world extreme testing at it's best and our new crankshafts finest hour.
CASE RESTED.

THE VERDICT
1. Its design and engineering is well thought out. 2. The appearance and cosmetics leaves the competition in dust 3.The balance is superb. 4.The strength is probably beyond necessary. 5.The price is higher but well worth it. 6.Go to market.

SUMMARY
ARC has created a crankshaft with limited sales potential. It is going to last the customer too long and greatly reduce repeat sales. It may create for the first time in history, a USED CRANKSHAFT MARKET.