The factory oversizes are usually in quarter millimeter increments. .25,.50,.75,1.0 and the like.
.25 mm is about .010". That's enough to clean up the bore, but it is not enough to increase the size of the engine by much. If you are unsure, measure the diameter of the bore and the diameter of the piston-
- with inside and outside micrometers.
Measure the piston about 1/2" up from the bottom of the piston skirt. Now measure the cylinder bore in several places at right angles to each other. Subtract piston diameter from the biggest bore diameter. This gives you the piston clearance. Piston clearance varies due to a number of factors.
1. Diameter of the piston.
2. Type of metal the piston is made of.
3. Is it a cast or forged piston.
4. Is the engine air or water cooled.
5. Is the engine two or four stroke.
A small (50mm bore) cast piston, 4/stroke may have half a thou (.0005" or .0127mm ) piston clearance, while a big, forged piston, 2/stroke (73mm, 2.874") can need .003" (.075mm) or more. You will need a shop manual to find the clearance for your bike.
Now this is the minimum.
What is the maximum ?
If it is an after market piston, use the piston manufacturers clearance recommendation.
Tuck\o/
