Factory bore tolerance stamps
 

Pulled down a few engines recently that looked to be pretty original.
On the decks were stamped letters near each bore, possibly designating tolerance post hone?.
S, T etc.
Does anyone have any information on these tolerance stamps, if they are indeed that.
Stamping looks pretty uniform, so may have been machine probed, then stamped, and assembly guy picked a corresponding piston to suit to try and close up tolerance?

Go to 5:29 on this vid.


https://www.youtube.com/watch?v=9LuXf6gJ-jg

Those stamps indicated which piston size would go in that hole, pistons were "select fit".

What AG said. Patt Lobb is a big 409 guy(also owns a big Toyota dealership above Dallas in Allen). He has a conference table size blueprints of the 409 blocks and it has table for them what letter is for what bore size piston. And hence the term "fitted block" for factory blocks.

In some years they changed the letters. (SD 455 is one I believe)
I have a Craftsman Service News on one of the changes. It also showed the specs for each letter.


:)

Pretty cool, would be interested in the specs if available?