Originally Posted by Gdads51
Jim Carter offers ME369O as a special order "Piston Rings Oversize Special Order Only All GMC And Chevrolet" that gets drop shipped direct from the manufacturer.

Added detail indicate "For GMC Piston Rings, we need the block casting number, engine size, pistonsize, and the ring groove dimensions.".

Not sure if this might be an option, but can't hurt to make a call and check. headscratch

I did do a search through GM Partswiki and found these two pages Parts Group 8.1950A and Parts Group 8.1950A - cont. from a 1963 GMC Master Parts Book that shows the then listed 270 piston ring sets and part numbers.

It shows two different sets of listings for 270 rings, with my guess that one applies to the flat top pistons and the other the domed piston, but don't quote me on that cause I can't tell which is which??? The model year "Series" codes show Y, S, T, X and N, I know as Y = 1955, S =1958/59, T = 1957, X = 1956 and N = 1960. Not sure if these part numbers apply to your much earlier 270 or not???

Perhaps some of our GMC guru's can chime in with more info or clarification?

Thanks for the link...sounds like they can make just about any Chevy/GMC pistons & rings.
From what I understand the 270 up until about 1955 had (3)1/8" thick compression rings and later 270's went to 3/32" thick compression rings. The military parts book lists two types of rings "conventional" and "Expander" types, with over sizes of .010,.020,.030,.040,.050, and .060