Original Pistons?

[David C.]

I have a '66 BB 390 HP car (IM code) with the original block not in car, but in my possession. I also have pistons that came in a box of parts and was hoping someone could help identify if the are original or not. I am trying to do some detective work. Inside the piston skirt there is a casting of B 555 that I could not photo well. The rod has GM (or maybe CM) cast on one side and 57 on the other. The dome has a arrow on it. The rods and caps also have matching numbers stamped on the machined flat on the end, I assume for matching cap to rod at assembly. I have attached the best pictures I could take.

Thanks,

DC

[Joe L.]

Re: Original Pistons?

I have a '66 BB 390 HP car (IM code) with the original block not in car, but in my possession. I also have pistons that came in a box of parts and was hoping someone could help identify if the are original or not. I am trying to do some detective work. Inside the piston skirt there is a casting of B 555 that I could not photo well. The rod has GM (or maybe CM) cast on one side and 57 on the other. The dome has a arrow on it. The rods and caps also have matching numbers stamped on the machined flat on the end, I assume for matching cap to rod at assembly. I have attached the best pictures I could take.

Thanks,

DC

David-----


The rods appear to be original. However, I'm a lot less sure about the pistons being original. Although they are generally of original configuration, they just don't look like the originals to me.

There are a few "hints" here as to the engine's history. First of all, the fact that the rods and caps are number-stamped indicates a previous rebuild. I've never seen original factory rods number-stamped. Rebuilders and custom engine builders often do this, though, to ensure that the caps are kept on the rods they were originally part of. This is critical, especially for a re-sized rod.

1966-69 L-36 engines were among the few Chevrolet engines which used side-specific pistons. In other words, the pistons for the left and right bank were different. These pistons were of internal GM manufacture and were cast at the GM aluminum foundry in Bedford, IN. All that I have seen have a 7 digit casting number seen inside the piston which begins with "388" and the complete number was different for the left and right sides. If yours do not have this, then I highly doubt the pistons are original.

No aftermarket replacement pistons that I'm aware of use different pistons for the left and right bank for L-36 applications. However, GM SERVICE pistons, once-upon-a-time available, did use different pistons for the left and right side just as used in PRODUCTION and were available in several oversizes.

Have you measured the current bore size?

[David C.]

Re: Original Pistons?

Joe,

I have not measured the bore size, but will do. I suspected that it may have been rebuilt because of the matching stamp on the rod and cap. I will check all of the 5 other remaining pistons for casting on the skirt and rod castings - 2 disintegrated and all I have are the rods. It looks like cyls. 1 and 3 dropped a valve onto the piston and created havoc with the block. Based on info I received today, I believe the car now has the 3rd block in it, not the second as previously believed - hence my detective work.

Is there anyway I can get you more helpful info on the pistons themselves?

Thanks for the input,

DC

[Joe L.]

Re: Original Pistons?

Joe,

I have not measured the bore size, but will do. I suspected that it may have been rebuilt because of the matching stamp on the rod and cap. I will check all of the 5 other remaining pistons for casting on the skirt and rod castings - 2 disintegrated and all I have are the rods. It looks like cyls. 1 and 3 dropped a valve onto the piston and created havoc with the block. Based on info I received today, I believe the car now has the 3rd block in it, not the second as previously believed - hence my detective work.

Is there anyway I can get you more helpful info on the pistons themselves?

Thanks for the input,

DC

Dave-----


The main thing would be any numbers cast inside the pistons other than the one you provided. Also, there may be numbers stamped on top of the pistons that are obscured by carbon deposits. Those could be useful, too.

As a practical matter, though, if 2 of the pistons are destroyed, the remaining 6 are probably not of a lot of use to you even if they are originals. To use what you have, you would need to find 2 more just like the other 6. I don't recommend "mixing" different pistons. If the pistons are aftermarket, anyway, then it's really pointless to try to save them. I would just go with a complete new set.