Crazy numbers on pistons?? BB 427/390 HP

[Michael G.]

Good Sunday afternoon,

As reported in an earlier post observed a ink stamp on top of the pistons of my '66 427/390 hp while tearing it down. Have since cleaned the rest to reveal other numbers: 9, 10 and 11. Letters A, B, C and E have been observed on bottoms of pistons as well. Pistons appear to be standard original pistons to engine with part numbers 3869967 and 3869968.

What's going here . What the heck do these ink stamped pistons mean/indicate ?

[John H.]

Re: Crazy numbers on pistons?? BB 427/390 HP

Mike -

Those are probably production piston "sizing" indicators; back in those days there were up to eight different tolerance ranges for a 4" bore size (at Flint V-8 for small-blocks), and pistons were cam-ground to fit within those eight sizing ranges to match the individually air-gaged bore sizes for each block. I think the same process was used at Tonawanda.

At Flint V-8, the gaged bore size was stamped in the pan rail adjacent to each cylinder, and the eight size codes for each block were electronically transmitted to the piston department, who prepared a tray with the rod/piston assemblies for that block by cylinder number that was conveyed to the piston-stuffing operation; it was theoretically possible for an engine to have eight different size pistons in it, although that wasn't the norm.

Precision machining these days permits one bore size and one piston size, since about ten years ago.