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.