1901-1953 Indian Motorcycle – Parts – Accessories

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  5. 109 – Instructions For Refinishing “T” Slot, Cam Ground, Semi-Finished Pistons


At the factory, pistons are ground to a “Van Norman ‘E’ cam contour”.

In refinishing semi-finished pistons in your own shop, use this same contour or equal.

Piston clearance in the cylinder should be five thousandths (.005) smaller than the bore of the cylinder. Clearance
measurement should be made at the skirt on the thrust face sides of the piston. The thrust face sides are the sides
opposite the wrist pin sides, or what might be called front and back.

Clearance measurement on the wrist pin sides should be at least thirteen thousandths (.013).

For Illustration

I If the cylinder bore is 3.290

A. The piston dimension across the thrust faces at the skirt should be 3.285.
B. This will give a thrust face clearance of .005.
C. The piston dimension across the wrist pin sides at the skirt should be 3.277.
D. This will give the piston a cylinder clearance of .013 on the wrist pin sides.


When pistons are ground, be sure to have the lower edge of the skirt beveled. If. the piston is not beveled and
is left with a sharp edge, it will have a tendency to scrape the oil from the cylinder wall.

On unfinished pistons supplied from the factory, the “T” slot is not cut all the way through. Be sure to cut this

Erle Armstrong
Service Engineer

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