24-06-2021, 08:51 PM
A30 piston.jpg (Size: 41.12 KB / Downloads: 176)
The number in a diamond. Pictured is a piston from a 1955 Austin A30. Standard size. In 1962 it was explained to me that pistons were made, then measured, and graded according to the exact measured size. The grade number was stamped on the piston in a diamond. Bores were similarly bored, measured, graded and stamped.
The pistons were then selected such that a number two piston goes in a number two bore etc.
Effectively this gave closer tolerances than they could manufacture.
So I'm guessing this was used in the 1930s too.
I've remembered all this, and kept the piston, since 1962. I was eight.