The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Pistons & Rings

I'm working on (2) GMC 270 engines from WWII trucks and am having difficulty sourcing pistons & rings. These are the dome top pistons.

I found that EGGE has piston blanks that they can finish out to whatever oversize you need, from std. to +.100.
When I went to order them I found they no longer have rings to fit the pistons.....they previously had Grant or Hastings rings.

Called Hastings...discontinued.
Called Grant....no stock but are supposed to get back to me if they can make them. Their p/n P2791 or C2791 (chrome)

Hastings suggested I talk to Total Seal but wont' be able to call until next week.

Ideally I'm looking for a .020 oversize set for one engine and a .080 oversize set for the other.
Worse come to worse I could bore the first engine larger for whatever size I can find, and could sleeve the second engine back to a smaller size.

Any suggestions for other sources for pistons & rings?

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.

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

Can you snap and post pictures of the TM with FSN/NSN's??? There are on-line resources to acquire more details searching by those numbers that might help locate appropriate rings for your needs.

I did a little web snooping and found the following for sale "270" piston ring sets that might be worth further investigation.

GMC truck piston rings 256 &270 engine .010 1942 1946 1947 1950 1951 1952 1956 (GMC# 2104772)

GMC CCKW Piston ring set .020'' 270 Engine (FSN/NSN: 2805-00-176-4064 - aka GMC# 2104970)

I can try to help with more research if you can provide the TM pictures.

These TM's are old enough (1942) that they predate the use of FSN/NSN numbers. The part numbers in the manuals are just GM part numbers.
The links you sent are some of the parts I've run across doing searches. The trick is finding a matching set of pistons & rings that match the bore I have. I may have found a set in Europe that will work for the .020 o/s block. The other block is already at .060 o/s, which is the max o/s that were factory supplied. My only options there are find a set of .080 o/s rings that can be used with the EGGE pistons, or sleeve all 6 cylinders back to a smaller size that I can find a set of matching pistons & rings.

Understood and thanks for the feedback. Just a suggestion, but if you want to, maybe post an ad in Truck Parts Wanted listing the desired part, including the original GMC #s, so folks can perhaps help in the search.

Never know, there might be a Bolter somewhere sitting on a box of old rings that may not know what they go to and the part numbers could connect the dots and find you what you need?

You might call Memphis Equipment (901) 774-0600 I have bought a lot of military parts from them. Stuff like that appears on eBay quite often. I had parts for 52 and earlier 270, but I'm not sure if I can find them.

Several years ago, I was able to use a set of 270 piston rings on a 256 engine by widening the compression ring grooves in the pistons to fit the only rings that were available. It was a pretty simple procedure, using a 4 jaw chuck on my lathe and a parting tool. I also straightened a twisted connecting rod. To the best of my knowledge, that engine is still running.
Jerry

That's a possibility. Also, you used to be able to buy ring spacers. As I understand it, you machined worn ring grooves wider to clean them up, then used the spacers to take up the clearance. If I had narrow rings and wide groove pistons they could be put together that way....but searching for ring groove spacers didn't yield much of anything.

I did order a set of NOS .020 o/s pistons & rings out of France. We'll see what kind of condition they're in when they arrive. The company I got them from has a tendency to sometimes send out parts that are a little rough. They claim they're ok but we'll see. I'm guessing the rings will have a conventional 1-piece oil ring but I'm not sure. At least if I don't have any luck finding new production parts I'll have these.

A 270 has a 3.781" bore, a 4" stroke, and a 7" long connecting rod. That converts to a 96MM bore size. There are a lot of 96MM and 96.5MM Japanese Diesel pistons available on Ebay that have a pretty close compression height to a 270, and a piston crown about 1/2" thick that can be trimmed down to the same deck height as the GMC piston. For the dome piston head, just contour whatever part of the piston sticks out of the top to clear the combustion chamber shape. There's also an H beam big block Chevy connecting rod available that's 7.1" long, with a 2.100" rod journal size, which can be used with an altered stroke crankshaft- - - - -about 1/8" extra stroke is available that way. All these parts cost less than the scarce (and ESSSSSPENSIVE) GMC stuff, since they're 20-30 years newer production. There's also a 96MM slip-fit flanged sleeve available for that Nissan piston that can be installed in the GMC block so the engine can be rebuilt repeatedly without ever reboring it again. The cylinder head holds the sleeve down into a counterbore in the top of the block like a Cummins 855 engine or a Detroit Diesel. A "stock" engine is just a blank canvas to be used by a talented machinist to paint a masterpiece!

www.ebay.com/itm/195096233829?

www.ebay.com/itm/184354665596?

www.ebay.com/itm/403395951735?

Check your private messages- - - -the flashing envelope icon at the top of each forum page.
Jerry