Most factories suggest changing the initial conrod after break-in, and after wards that connecting rods be subjected to a measured operating life, and replaced maximum after 5 liters of fuel under normal operating circumstances.

It is also very important to check the condition of the connecting rod periodically during their operating life. This check should be performed after 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), carefully disassemble the engine and measure the crankshaft pin and connecting rod, 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.

Also consider replacing the rod if the engine has seen any trauma such as a sustained over revving of the engine, or a significant over heating.

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 people 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.


AFM