Measuring motorcycle cylinder piston clearance is one of the most important measurements you will take when doing some DIY engine work.
To get this job done right you will need to know the factory specification, an outside micrometer for the measuring the piston, and an inside micrometer for measuring the cylinder. Make sure you follow the instructions on your micrometers, such that they are set up right and “dialed-in” (pun intended) before getting started.
Step 1: Measuring the Piston
Pistons come in a variety of shapes and sizes, but generally they are “fattest” about 1/2-inch up from the bottom of the piston skirt (bottom of piston) – measure both sides from bottom, mark with a sharpie, and then measure the piston diameter with the outside micrometer.
Let me bottom line it for you – the purpose of those step is to find the largest piston dimension so measure in multiple spots until you find it.
Step 2: Measuring the Cylinder
Now, you want to take your inside micrometer and measure the cylinder in numerous spots. Keeping it simple measure near the bottom, in the middle, and near the top. Also, measure front to back and side to side. Essentially measure in a plus sign manner in three spots over the cylinder height. Grab a pen and paper and write down all results.
Step 3: Interpretting Your Results
First, let’s talk about clearance. Subtract the (largest) piston diameter from the largest bore diameter. This gives you your piston clearance and should be compared to the value given in your service manual.
Next, take a look at the results from Step 2.
Is the cylinder out-of-shape (oval) – bigger front to back than side to side?
Is the cylinder significantly bigger at the top versus the bottom (taper)?
If the out of roundness or taper is greater than 0.001″ you should consider getting a professional cylinder rebore job done. You can correct a misshapen cylinder with a high quality cylinder hone and ridiculous levels of patience, but isn’t usually time or cost effective – read more about that here.
If the cylinder shape and clearance are okay, they you can get away with a simple honing job and a new set of rings. I talk more about cylinder honing here.
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