Some of you have been following my 454/250 B&M build. Well, here's a real humdinger of an issue:

I needed to replace some pistons from my original set. I wasn't going to bore the block because it wasn't needed, and to keep the rotating assembly's balance correct, I wanted to change as little as possible. So I picked up a set of pistons from another OSO'r. Same GM part number and everything. Pistons are pressed onto the rods etc. Get everything home. Set ring gaps, all good. Go to install the rings. Rutroh.


The rings (Hastings Moly for a standard bore 454) fit on my pistons, but not the replacements. The rings feel free and loose, and rotate as they should. Side clearance is on the money (.0015). Oil rings have plenty of clearance. These are on mine. The replacements area another story. The top and second rings are tighter than I'd like (.001 at best). The oil rings are a no-go. Very tight.

I brought them back to the machine shop just to get an opinion. We were all scratching our heads. One thing for sure... though the pistons share the same part numbers, they are vastly different. Mine have no number inside, and holes around the circumference of the oil ring land. The replacements have a nnn-nn inside the forging, and have long slots on the oil ring land.

I've read that GM did some monkeying around with oil ring depths, but part numbers for Merc rings (used as a reference point) were the same across all 454 (and 482) big blocks.

What the heck have I got here? Help?

i ran into a similar problem about 5 years ago with some trw pistons,same part number but the pistons were different.also seen it on some j&e pistons.sometimes you are the windshield,sometimes you are the bug.

I have seen some pistons with the ring grooves cut like a dish to see if that's the problem put a ring in each side and see if they aim up or down. Probably not your problem.

Just curious if you cked the groove depth from new to old. I hear what your saying and ya you wouldn't expect that with number matching Pistons however nowadays who's to say where they are made and to exactly what specs etc. good luck and keep us posted.

I have the old rings from the pistons I got from f_inscreenname. They work. The question... what the heck are these rings? Factory pistons both sets.

At this point I'll probably buy another set of pistons and then rebalance the whole thing.

I'll tell you... I have a number of good builds under my belt. This has been a f'ing goat rope thus far. Very frustrating.

I think I might have it figured out. There ARE two different rings sets depending on standard vs shallow depth oil rings. Both are 5/64 5/64 3/16 but the depths are different. Strange that they all share the same part numbers.

If i may ask , was the trw book broken out ,treasured cuddled ,and loved ? Them damn things sure do take up some room but when the sheet comes down the ass can be saved .
Rings indmar small blocks . And 460 king cobras
i did google search why google sucks now and actually got some relevant results lol
glad you got it figured , i still havent pulled the snout on my 250 .

Wow. I'm lost. sorry.

Tom, I dont know what year your engines are/were, but the old pistons from the mark iv 370's, 400's, 420s, 365, etc, were the same. The later Gen 6 454 pistons used a different part #.

Later style piston

737-850477T

Early piston

737-5536T

not sure about these pistons, but I know wiseco early pistons used std size rings, later they switched to metric so ??