On 12/12/2012, Jim from Marino's Machine Shop in Freedome, CA. USA wrote: US
"County Motor Pistons CP306 at .040" over. It is a 5 Ring slotted piston. The sticker on the box say measure at 90 degrees to pin at bottom of piston skirt. That measurement is 2.9145". There was a note in the box that says to measure it at 90 degrees to pin just below the top oil ring. That measurement is 2.9125. Also the note has the clearance of .002-.0025". Please help us with getting the correct measuring point. Called APT in South California, they said to measure at 90 degree to pin at center of pin. So we are confused where it should be".

The short answer is, always measure piston size a right angle to the wrist pin axis. From there is gets much trickier.

Most pistons are not round, not cylindrical. They will be larger perpendicular to the wrist pin, slightly smaller across in the direction of the wrist pin. They are "cam ground" to be slightly obround in this manner. The one land immediately above the top ring may be round, and few thousandths of an inch smaller than the rest of the piston. The piston is likely to be slightly smaller at top and larger at bottom, to allow for thermal expansion in the area that gets hottest in operation.

In short, the bottom half of the piston (the skirt) runs cooler with less expansion, least diametrical clearance, and is used to guide the piston in the bore. The top half of the piston is subject to more thermal expansion, runs larger clearance, and is used to guide the rings. The rings are not allowed to change diameter at all, must remain in contact with the cylinder wall at all times. Thermal expansion of the rings is accommodated by starting with a generous gap at the ends of the ring, specified as 0.008-0.014 inch for a fresh engine.

When fitting a piston to a new perfectly cylindrical bore, measure the largest diametrical dimension of the piston, which will be at right angles to the wrist pin and on the lower half of the piston. 0.0020-0.0025 clearance is minimal for a street engine.

The MGA Workshop Manual (from 1955) calls for 0.0017"-0.0023" at bottom of the skirt, and 0.0035"-0.0042" at top of skirt.

The MGA Twin Cam makes more power and likely runs a little hotter on the pistons. The Workshop Manual for this application calls for 0.0035"-.0.0066" at bottom of skirt, and 0.0058"-0.0083" at top of skirt. That is about double the clearance used in the pushrod engines. Chat among the Twin Cam owners turns up piston scuffing if minimal clearance is any less than 0.0050".