When to change the connecting rod by R.Poage
Originally Posted by Grasshopper1
How much play in the big end of a con rod is normal?
What does excess play cause in an engine in terms of performance and reliability?
The connecting rod is one of the most stressed parts in any engine, and therefore needs to be checked frequently to avoid expensive engine damage due to failure. Some factories suggests that connecting rods be subjected to a measured operating life, and replaced maximum after 5 liters of fuel under normal operating circumstances. Some factories suggest visual inspection to determine rod life.
We feel that a combination of the two methods is best to ensure catching a rod before it goes bad. It is also very important to check the condition of the connecting rod periodically during this operating life to ensure that premature wear has not developed.
We recommend that this check be performed before each race day, and the easiest way is to remove the rear cover from the engine (raise the piston up into a compressed position first! before removing the rear cover) then gently rotate the crankshaft in both directions to determine the amount of play in the connecting rod. If the amount of play seems excessive (you can detect movement / play with your eye), we suggest carefully disassembling the engine and measuring the crankshaft pin and connecting rod for excessively wear, a maximum differential of 0.04 and 0.05 mm in diameters is acceptable.
The fact that the crankshaft pin must be absolutely round (as must be the hole in the connecting rod) is one of the most overlooked aspects of engine wear, and can result in damage even with a brand new rod. The proper way to measure this is to measure with a micrometer on one axes (in line with the direction the rod would travel) and then again on the axes perpendicular to the first. The difference of these two measurements should be < 0.002 if any. The connecting rod should also be visually inspected for signs of wear in the bushing areas, and the oiling holes cleared of any debris.
Obtain the correct factory replacement conrod and "new wrist pin clips". Install the new rod exactly like the old one was before you removed it. Note the oil hole in the conrod should be towards the crank.
We also suggest that you inspect and consider replacing the rod if the engine has seen any trauma such as a failure of the two-speed one-way (over revving the engine) or a significant over heating of the engine.
As a general rule engines that are run hard should have a rod replacement at least every 1.5 gallons of fuel regardless of visual inspection.
It is very hard to standardize how hard customers run their engines, or what kind of gearing or temperatures these engines see, but is impossible to rebuild and engine with a hole in it, so error on the conservative side is in order.