mystery sbc pistons need trw? catalog

Guy claims to be selling these old sbc forged pistons. He told me what they are, but I need some proof. He is a distance away and can't measure them. He says the number on the piston is 3987172 B Can anyone tell me what they are-then I will tell you what he said they are! Thanks, Kurt

 

They look like pistons.

 

I don't think they are trw's. They would be stamped on the tops with a letter and 4 digit number like L2445

 

I'm going with SBC, 302, factory pistons and rods. I see they are set up for full floated pins (like the SBC, 302's were), but, the factory did not bush the pin end of the rod like most performance applications were done. That alone should tell you IF it's a factory setup. The number looks like a GM part number also. What did I win? Butch/56sedandelivery.

 

Most likely, they look like small journal rods. Maybe he has the number wrong 3927172?

 

In the current (?) 7th edition Chevy Power book std bore 1968-69 Z28 pistons are part number 3946876 and ID number 3927173. 30 over are ID number 3927176. No other oversizes are listed in my modern books, but I believe other oversizes were often available back in the 70s.

 

Rods are early small journal low performance with 11/32 bolts, that have been full floated. Notice the similarity to six cylinder rods around the big end.
The piston pins have aftermkt snap rings. Factory pistons used spirolocks or pressed pins.
Early production Z/28 rods (pink rods) were full floated pins. Small end was dipped in bearing material. Later stuff was press pin.

Looks very much like what we used to make a small journal 301 back in the sixties.

 

This could be the winner. Guy tells me they are 301 pistons. I wish I could confirm the part number somehow. I guess if they are 302 pistons that would be ok too. He says they are 12.5 compression if that is possible with a 283. If they measure just under 4 inches guys, would that confirm the 301 size? Thanks much! Kurt

 

gm part# yes, but 12.5:1? i dont think chevy ever put out a factory smallblock with 12.5:1 compression.

 

the 60 Vette 283 had domed pistons for 11:00 comp. was the 275 fuelie. The aftermarket pistons didn't have the long skirts like Chevy did.

 

I did a napkin calc with an estimated 12cc dome and got 11.3:1 compression. You might have to measure it all once you get it all together and decide if you want to mill anything, use shims, or a skinny head gasket, or what. And factor in cam choice, of course.

 

Same piston. 301-302 GM just rounded the numbers off to the higher figure. You will find the skirts fron .004-.008" smaller than 4 in.

These pistons have a large dome on them and the uninformed may think they were 12.5 to 1. But the short stroke requires more dome to make compression.

I built several small journal 301-2 engines back in the day. Most came out to 11-1, 11.5-1 with the available (camel hump) heads of the day'

I worked in the Van Slyke engine plant in Flint, Mi. back when these pistons used to be stacked in containers waiting to be rod/hung and installed. There were several part numbers. Depending on several factors. Including method of pin retention, batch no. production or repair over/undersize. + or - up to .003" usually.

Back in the day lots of these pistons ended up on the local streets from "lunchbox" transportation and were plentiful and cheap on the underground market. Now they are fairly rare, as are large journal 302 cranks.

 

Thanks Bob, super great info you shared. Agreed on the compression comment too. Kurt