Tcheairs38655

Removed pistons and rods from my 66 450 car. The pistons are marked GM and TRW with the number 388830. they are standard bore. I thought the original 11:1 piston was a 22268. Also the rods have no identification and are not the "dimple" rods which I thought were stock. It's a counter exchange engine cast 3 months after the build date of my car, and I do not think it has been apart since the replacement. Any help with identification would be appreciated.

DansYellow66

Through google I found a couple references to GM 388830 pistons but both were a bit dubious. One was from someone in the UK taking apart a 454 and the other was an EBAY seller of what he proclaimed to be an original 66 L88 427. So, I would suspect they were used in some engines, maybe over-the-counter engines, and possibly as a service replacement. Hard to tell. But anyway this will get you back to the top.

SWCDuke

The service replacement piston/pin ASSEMBLY part number, standard size, for 66-69 SHP 427s is 3888342. (GM never sold pistons without the fitted pin.) The number in the forging you posted has only six digits. Did you miss a digit? Six digit part numbers were used during the early to mid seventies when GM was running out of seven digit numbers, and they gathered up unused six digit numbers that dated back in the forties.

The number in a piston casting/forging is either a "forging/casting number" or part number of the finished piston and would likely be close to the service number for the piston/pin assembly. 427 pistons would most likely have seven digit numbers, but a 454 piston could have a six digit number since that engine went into production about the time GM started assigning the unused six digit numbers from the past.

I'm not that familiar with OE big block rods, but they did go through several design/processing iterations in that era to improve durability, and OE rods typically have a different "look" than aftermarket, so a photo might help.

Joe L. over on the NCRS TDB could likely accurately ID both the piston and rod.

Duke

DansYellow66

If they number is correct in digits and they are an early 70s piston then they would have been used with open chamber heads I think. That would make for more mystery as I think using a high compression, open chamber piston with an earlier closed chamber head would be problematic to say the least. What heads do you have on the car?

Also - are the rods fitted with 3/8 inch bolts or 7/16 inch?

Posting a couple of pictures might get more response.

1snake

You're missing the last digit. It should be 3888304 and that's the casting number. The part number is 3888342 which was used on all 66-69 L-71 and L-72 engines. Your block should have 4 bolt mains and you should have "dimple" rods.

Tcheairs38655

]Sorry, the rods are installed and it's difficult to read the part number..missed a digit..It SHOULD BE 3888304. The rod bolts are 3/8 (takes a 9/16 socket). the pistons are definitely closed chamber and were used with my correct (date coded) 425hp heads.I've attached a few photos too.

Thanks for the help. [/B]

427425

Your pictures are typical of the pistons and rods in a 425. The lathe marks on the dome are different from the over the counter TRW piston.The dimple rod was the "fix" for all the failed rods in 396/425 and 427/425 engines. I have only seen one 66 425 that had dimple rods and the assembly date was early July 1966. I believe the dimple rods were provided to the factory teams in early January 1966. I was racing NHRA stock eliminator during that time frame, and I remember a racer being disqualified for having dimple rods in a 427/425 1966 Biscayne.

Tcheairs38655

Well, the engine is undergoing an overhaul with an .030 overbore. Dimple rods are also going in with a balanced crank. this set of pistons and rods is going on sale on Ebay and in the parts for sale section of this forum. Nothing wrong with them, however a broken ring scored one of the cylinder walls enough to justify the overbore.

Thanks to all for your help