Where to measure piston diameter?
 

Do you measure the pistons in just the one dimension under the pin bore for 76' and later pistons? This seems to be the only area showing the slightest rub/burnished finish. I couldn't find the information in any of the books in "detail".

In Wayne's excellent book it shows in "A" to be measured here for 76 on engines. I don't see reference for any other plane or area to measure the CIS Mahle piston, these being from a low mileage 79.

I don't have rings yet for these and if I replace them I'll likely go with JE's. Could someone give me an idea of spec on top ring land so to justify ordering a set of rings or not? I found specs upto a 2.7 and then it simply says NA (these were listed at 1.540-1.590). I don't want to buy rings obviously for a set of pistons that I may not be able to use.

TIA

I know its Saturday and there isn't much traffic, but let me further explain my question and mystery.

First, are Porsche pistons "cam ground" to compensate for expansion rates? Are we just to measure across the major thrust faces perpendicular to the pin bores? I know the pistons are tapered above the oil ring to allow for thermal expansion, so where else is the dimension to be taken? Some pistons will measure .040" smaller across the top so I'm just trying to determine what particular measurement is to to determine the "limit".

As has been discussed with JE pistons and other racing pistons that these skirts are not always round to compensate for thrust pressures. I know that in V8 racing that measuring off center 30 degrees will sometimes yield a smaller number.

I may be clear as mud, but I'm trying to clarify my question. Thanks a million for reading.

really, no replies?? HENRY or other experts, please.

No answers yet? No expert, but as you surmise, measure perpedicular to the pin axis, about midway up the skirt. That's your piston OD. No different from any other piston. The pistons will grow in size slightly as they come up to operating temperature. Mahles will remain more dimensional stable compared with other brands, thus less piston slap when cold.

Measure cylinder OD at two locations: top and bottom in every direction. The bottom receives less wear. Shouldn't be too much of a difference if the cylinder is within spec.

Hope this helps,
Sherwood

Thanks, Sherwood!