by TMKits » Fri Nov 26, 2010 12:45 pm
Although not your engine, some info on rod clearance from Jett engines FAQ:
Question:
My rod is making a clicking sound. I fear it is too loose and about to break. What should I do?
Answer:
If your engine is new do not be concerned. The parts are inspected during manufacturing and during assembly. Your engine has also been test run just prior to shipping. Everything is OK.
Now, what is the reason for the clicking? Jett rods have about .003-. 004 (.07-. 1mm) rod-to-crankshaft clearance. This is approximately twice that of an O.S. This is why you see the extra motion. This is not a problem and here is why: During high-speed operation the engine loads the rod in one direction only—the piston pushing down on the crankshaft. This means that the rod is almost always touching at the top of the shaft, leaving a gap at the bottom. The gap is where the lubricant is stored and the more the better. During idle, you may hear the click of the rod moving back and forth on the shaft. The loads are very low at this speed and do not cause a problem.
We have measured rods after years of hard use and find the wear almost zero. In fact, we often say, “rods don’t wear”. Years of pounding are more likely to fatigue the rod body itself, and for this reason it is reasonable to replace them every few years. In racing, once a year would not be unreasonable. Always replace your rod after a shaft run (no propeller).
Finally, do not measure rod play at top dead center. This magnifies the clearance. Place the piston about one-half way between the top of the exhaust and top dead center. You will find a big difference.