The Stovebolt Site Forums The Shop Area Engine Shop please explain this piston

Would someone explain to me the 20 steel inserts in the top ring groove of this piston pictured below? It was the only one of the six removed from a 1957 261 spare engine that was like this. I can guess, but am interested in what the others have to say.

Personally never seen anything of the sort???

Perhaps one of our much more experienced than me "engine guru's" can shed some light???

Dan

I've never seen anything like that on a gasoline engine, but some Diesels use various alloys in the crown area of the piston as a heat dam to isolate the combustion above the rings. I have a feeling that the alloy isn't exactly steel- - - -possibly something with a lot of nickel in it, maybe? Some versions of the old 2 stroke Detroit engines used a 2 piece piston- - - -cast iron crown with an aluminum skirt- - - -held together with the wrist pin!
Jerry

I'm interested in this answer also. There had to be a reason and the fact these pieces are (a) only in one piston and (b) apparently were cast in place and symmetrically located gives me pause. At least I think they would have been cast in place. If they had been pressed in, the different thermal expansion properties might cause them to dislocate and shift with time, stress, use, etc and it wouldn't be good if even one of those came loose (unless they're part of a cast-in-place ring). Did you notice in which cylinder this piston was?

I'm going to venture a guess that it's an asterisk-shaped ring (*), cast into the piston as a heat dam, or maybe for expansion control. Some cast pistons incorporate Invar struts in the wrist pin bosses to allow for cam grinding the piston for minimum skirt clearance. Invar has virtually zero heat expansion, so the egg-shaped piston gets round as it heats up to normal operating temp.
Jerry

I'll buy the asterisk-shaped ring idea but why was only one out of 6 made that way? Parts store substituted one when the engine was rebuilt in the past and they had 5 of another design they wanted to remove from inventory? I don't think it would be a balance thing. That doesn't make sense to me, anyway. One checks/confirms/adjusts weight after the piston is made. Maybe if the original poster could show us an image of one of the other pistons we might be able to see if we think it was the same mfgr. If it was the same maker, then we might have a trail to follow. If the other 5 were made differently it might argue the maker was different and if so there's a large clue to the answer.

Is the engine overbore or standard?

The piston looks like standard 261, it measures 3.750 in diameter. It has a GM part number 3758427 cast on the inside. The only other unusual marks are the six notches on the inside of the skirt. (This piston was in the number six hole.) The inserts are magnetic. I can not take a picture of another of the pistons for a few days as they are in storage a the cottage garage.

Been following along on the sidelines of this thread, but don't have any more to add except that a GM parts wiki search for 3758427 comes up empty. Most GM part numbers are 6 digits, not 7, so that may have something to do with it.

Want the set?

https://www.ebay.com/itm/275804782421

Gone- - - - -I just bought 'em before somebody else snaps them up. No1300- - - -if you want the whole set of those pistons, drop me a PM- - - -we can do a swap.
Jerry

Thanks for the offer. I do not need any right now, just finishing assembly of a 261, new rings, bearings, oil pump, cam and lifters. Hope to have it running soon. I was just curious about the unusual piston.

Wow! That seller should send me a commission.

I just looked at the picture of the pistons in the ad. They have the same inserts! I guess it may not be so unusual after all

Sorry, they are sold. But you can definitely see the suspect crown area in the epay picture of all 6 pistons.

You missed the post where HRL said he bought them.

Just got a nice note from the seller on Ebay messenger. He's got an instagram post of the buildup of his engine. I told him about the 300+ cubic inch "Franken-261" I'm building with the H beam rods, stroker crankshaft, Buick V6 pistons, 9.5:1 compression, oversize valves, and a few other goodies!
Jerry