DragBikeMike
Serious Thumper
Offline
SuzukiSavage.com Rocks!
Posts: 4194
Honolulu
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I don’t want to step on any toes here, and I mean no disrespect. I just want to clear up some obvious confusion.
First, checking piston to cylinder clearance with a feeler gage is a perfectly fine alternative to a dial bore gage and micrometer. It’s not as accurate, and I wouldn’t use the feeler gage to resize a cylinder, but its just fine for verifying running clearance and also a great cross check of the more precise measuring instruments. I always cross check with feeler gages after measuring with precision instruments. Actually, there are spring scales available to verify correct drag on feelers when used for fitting pistons.
Second, Dennisqb, if you can fit an .008” feeler between the skirt of the piston and the cylinder (assuming the drag is proper and the piston is positioned correctly) your piston to cylinder running clearance is .008”. That works out to .004” radial clearance but you don’t use radial clearance to evaluate piston to cylinder.
As Armen said, you always do this check in the fore/aft direction, perpendicular to the wrist pin. That’s because pistons are not round, they are egg shaped. Pistons are also tapered top to bottom, the top being way smaller than the bottom. So, the tightest clearance is found close to the bottom of the piston in the fore/aft direction.
This picture shows a cylinder/piston combination that has right about .003” running clearance. Note that the drag on the feeler is just enough to hold the piston suspended in the cylinder, just enough to prevent gravity from pulling the piston down. The piston will continue down with just a light finger push.
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