Tuesday, June 1, 1999

Probable Cause of Most Rod Failures

To help prevent engine failure, here's a few tips:
For the most part, a quality billet connecting rod doesn't just break. There are several things that can really shorten the life of a rod of which bearing clearance is probably the most important and too much is worse than not enough.
Visual inspection (or eye balling) is not good enough. The only way to check a crankshaft, rod or bearing is with micrometers and dial bore gauges. Dial calipers or digital calipers are just not accurate enough. Even with the ones that have .0005" graduations, the accuracy is generally + or - .001" which means outside to inside measurements could have a total error of .004".
The Crankshaft:Just because a crankshaft looks good, doesn't mean it is to size and round.Using a 1" micrometer, measure the rod journal of the crankshaft in 4 places. A crankshaft with more than .0005" wear or out-of-round will probably not last very long in a 9500 rpm race environment.Keep in mind, what you might get by with in a stock engine, will not be as forgiving in a high rpm, high horsepower engine.
Surface finish of the rod journal is also very important especially when a babbit bearing is used. Most of the new Raptor III cranks I've seen and checked are way too rough.
Bearings: One of the most common things we hear is "The bearing looked ok, so I reused it"Again, looks can be very deceiving. Take a 1" ball micrometer or use a ball anvil attachment on your mic to measure the bearing thickness in several places. The ball anvil is necessary because of the curvature of the bearing.The measurement should be .075" and, on a crankshaft that measures .998" on the rod journal, this should give you .0025" clearance.
On a bearing that has been run or has been honed or sanded for clearance, carefully mic each half on the outside edges and in the middle.Remember, a new bearing should measure .075" which should give you .0025 clearance. If the top bearing measures .073" and the lower bearing measures .074", you now have .0055" clearance.
Some engine builders use a ball type hone to clearance bearings and this can cause a major problem. The Babbitt material is very soft and easily removed. What happens is that as the flex ball hone enters and exits the bearing bore, it removes far more material from the edges than from the center producing an hour glass shape. Using a plastigage or measuring the bearing in the center may give you a false and possibly fatal reading.You may think you had .003" clearance (and you did in the center) but, it will be a very narrow contact area and wear rapidly and as this clearance increases, it compounds the problems.This additional clearance pounds the bearings until the rod & piston assembly becomes a 9000 rpm slide hammer. This is when you have a failure.
Special note: Never file or grind the ends of a bearing. If the installed i.d. of the bearing is too large (measured inside diameter of the bearings installed in the rod and torqued too 150 inch/pounds), gently and slowly remove material from the parting edges of both halves with fine emery cloth on a flat surface until you get an installed i.d. of .9995" to 1.0015" for stock .998" cranks and .8765" to .8785" for stroker .875" cranks. It is better to sneak up on this slowly.
These bearings are designed to crush in the bore of the rod which holds them in place and prevents them from spinning. The tangs are primarily for location. If your crank measures less than .997" or .874", it is probably a good idea to replace it. In any case, it is not a good idea to try to compensate for a worn crank journal by reducing the i.d. of the bearing.
Inspect and/or replace the bearings regularly. By examining the bearings, you can set up a schedule of how often to replace them - plus - by measuring and looking for excessive smearing or wear-through of the babbit material, you'll be able to tell if your oil is doing its job.
Rod Bolts: Proper rod bolt torque is VERY important. In order to keep from backing out, it is necessary for a bolt to stretch a specific amount so the threads lock into place. This is a little known or understood requirement for a bolt to do the job it is designed to do. The proper stretch for a bolt is usually achieved by torquing the bolt a calculated amount based on the bolt's design and the characteristics of the application. ARC's rod bolts are custom designed to achieve thread-lock at 170 inch/lbs. This MUST be measured with an accurate inch/lb torque wrench. Failure to properly torque the bolts is a all to common cause of rod failure, second only to the oil clearance issues explained above.
Treat your engine as the piece of precision equipment that it is. The environment in which it operates is extremely harsh and attention to detail along with precise measurements is absolutely necessary.

Thursday, January 7, 1999

The Crankcase Vacuum System (CVS)

By: The ARC R&D Team Date: January 7, 1999
Re: Crankcase pressure management

For as many years as I can remember, crankcase ventilation went like this:
If you were blowing oil and having gasket and seal problems, you just added another line from your motor to the catch can. I have seen as many as 6 lines running from a motor, but the problem was still there.
In case you didn’t know it, we were just adding insult to injury.

Viewed from the crankcase, the single cylinder motor is an excellent air compressor. The more air you take in, the more you have to push out somewhere. Consequently the more air that is being passed through the crankcase, the more oil that will follow the air out.

All small engine manufactures have done a fair job in their crankcase ventilation system, but it only works up to about 3,000 rpm.

NASCAR and the Drag Racing industry have long used a dry sump oiling system that also creates a negative pressure (or partial vacuum) in the crankcase.

A multi-cylinder engine is a little simpler to deal with because one piston going up is canceling the pressure of the other one coming down. The basic thing you are dealing with here is called blow-by.

Without getting into a long technical and complex discussion on the subject, let me just make this statement. The rings on the piston are designed to work at their maximum with pressure from the top and vacuum from the bottom.

Common sense will also tell us that the piston will function much better and develop more horsepower if it is being pushed down in a negative pressure environment. In fact, with vacuum in the crankcase, the piston is actually being sucked down the cylinder wall.

Any positive pressure in the crankcase will allow a certain amount of oil to get passed the rings and contaminate the fuel charge. This can and will reduce horsepower.

Our Crankcase Vacuum System is very complex in design and every hole, groove, passage and vent are critical to its successful operation. Even the length and size of the tubing used in the catch can model for Kart racing are critical.

While the design is very complex, the operation is very simple to explain.

Without the CVS, the tappet room (valve spring area) is continually being flooded with oil and, contrary to popular opinion, this volume of oil is NOT being caused by the length or design of the dipper on the connecting rod - the cam gear is the culprit.
A little side note right here on horsepower. If the tappet room is flooded with oil and the valve guides are a little on the sloppy side, oil can easily be sucked by the valve stem and contaminate the fuel charge.

Our CVS works like this:
As oil and air are being pushed up to the tappet room, the first baffle is atomizing the oil and lubricating the valve stem with a mist. The second baffle is now starting to separate the air from the oil.
The 2 check valve discs are sensing the blow-by of each down stroke of the piston, regardless of how minute the amount, and are opening and closing on each stroke.

When the piston is at the top of the stroke, we will have our maximum vacuum. When it nears the bottom, vacuum gives way to the amount of blow-by pressure from the rings. The best calculations we can come up with is that we have vacuum 95% of the time and little or no pressure 5% of the time.

As the mist of oil and air move through the CVS, we are continuing to separate the two in our maze of holes, grooves and passages.

In the final step of this unique process, we are now using the vacuum in the crankcase to pull the oil, which is heavier, back into the crankcase, and the oil free air is vented.

While the operation is simple, the R&D on this project has been the most intense of any project we have ever undertaken at ARC Racing. The Dyno testing has been extensive, not only by us, but other engine builders as well along with track testing that has been on going for months.


Dual Plane Balancing

We receive lots of calls about balancing and one of the first questions is always, "What is DUAL PLANE BALANCING".
If you read the book on it, and you had a background in engineering and physics, you could get a good understanding of it. The two terms that are used in the explanation are Force and Couple balancing. What we are going to attempt to do is reduce all of this down to a non-technical explanation.
Lets go back many years to tire balancing. Your rim and tire assembly was mounted on a shaft and then placed on a frame with the shaft resting in a ball bearing V fixture. The tire assembly would then rotate around until the heavy part came to rest at 6 o’clock. A wheel weight was then placed at 12 o’clock on one side of the tire. You kept adjusting this weight until the tire would not move regardless of how you repositioned it in the V fixture.This process is called, Static or Force balancing.
Then an improvement was made in this process. Instead of putting all the weight on one side of the rim, it would be split with ½ of the weight going to the inside. This was the first form of Couple balancing in the tire industry. Not perfect, but an improvement.As years went by, and the advent of much wider tires came into being, the need for Couple balancing increased.
We know have electronic tire balancers that spin the tire assembly and calculates the amount of weight needed to Force balance. Then it calculates what amount goes on the inside of the rim and what goes on the outside. This is Dual Plane balancing of a tire.
How does Dual Plane Balancing apply to the Briggs Crankshaft ?
A bob weight is attached to the rod journal of the crankshaft that represents 100 % of the rotating weight and a percentage of the reciprocating weight. This assembly is then placed on the ball bearing V’s of the balancer. It is then spun up to the operating RPM. Now the balancer can read both sides of the crankshaft and precisely tell the operator the amount of weight that needs to be added or removed from the left or right counter weight.This completes the Force and Couple (Dual Plane) balancing.
Special Note:If you change the connecting rod and/or piston, wrist pin and rings, the crankshaft may need re-balancing.