Piston Marks? 73 400 CR

Recently received my 73 400 CR (see other post). Pretty sure this is the original
piston, can anyone tell me what the numbers mean. Original bore is 81.50, piston is
marked 8142. Surely someone on this board knows what the numbers mean.....

8142 is 81.42mm diameter piston. The other marking closer to the center indicates it is the original Mahle piston.

Marty

That's strange, the standard bore is 81.50 I haven't seen anything about an 81.42 anywhere. That's what threw me.

Yup... stock bore on many machines is usually the nominal bore size, like "81.50mm"... Obviously there needs to be clearance between the piston and cylinder so the piston need to be smaller, like "81.42mm"...

If cylinder was exactly 81.50mm, you'd have 0.08mm or just over 0.003" piston clearance. Standard bore sizes sometimes vary a little either way during mass production so often a factory has a few "standard" pistons of slightly different sizes marked maybe 81.41, 81.42, 81.43 so on the assembly line the correct piston can quickly be selected to match the bore and get the clearance right.

On lower production... like a re-bore... the machinist measures the piston and then adjusts the final bore size to obtain the correct clearance... so you don't hear about the way "standard" pistons were sized all that often.