What is a Cam Ground Piston?

Aluminum expands much more than Cast Iron, so when the piston to cylinder wall clearance is measured at room temperature there must an allowance to allow for expansion when the piston heats up. The amount of clearance depends upon the design of the engine and the anticipated operating temperature. Air-cooled engines run very hot so they require more clearance than water-cooled engines. On a Cushman piston the reference point for clearance measurements is at the top of the skirt perpendicular to the piston pin.

All areas of the piston do not expand the same; the top of the piston runs the hottest so it expands the most. The skirt runs the coolest so it expands the least. For this reason the diameter at top of the piston will normally be smaller, and the diameter at the bottom of the skirt will normally be a little larger, than the diameter at the top of the skirt. Also, the piston does not expand the same all the way around its circumference. The diameter measured across the piston pin will expand more than the diameter-measured perpendicular to the piston pin. Therefore, the piston is “cam ground”, or made slightly oval, with the smaller diameter measured across the piston pin. When the piston heats up to operating temperature it will then be round. If the piston were not cam ground, the piston to cylinder wall clearance would have to be extremely high when the engine was cold to allow for expansion, and it would not be round when at operating temperature. This could cause piston slap and other problems.

Cushman engines are usually set to a clearance of about .006 inch measured at the reference point described in paragraph one. The clearance at operating temperature will be much less, and if sufficient allowance is not made for expansion, the piston will expand to the diameter the cylinder wall and freeze up. During break-in everything is running extra hot due to the increased friction, and the piston temperatures get very high. If the engine is run at high speeds during this period it will almost always freeze up and may damage the engine.

The following measurements are from a Cushman NOS +.030 cam ground piston:

Top of skirt perpendicular to the piston pin 3.024 (.006 clearance)

Top of skirt measured across piston pin 3.011

Bottom of skirt perpendicular to pin 3.025

Bottom of skirt parallel to piston pin 3.012

Top of Piston and ring lands 3.005

In this case the clearance to a nominal 3.000 inch cylinder bore is .006 inch, the elliptical amount is .013, the skirt is .001 larger at the bottom than at the top, and the top of the piston is .019 less than the widest point on the skirt. These measurements are for illustration only and individual pistons may vary considerably.