Can anyone help me Identify this Gm Piston?

[Ramjett54]

I have 8 NOS domed pistons with two eyebrow valve reliefs, its 4 inch bore, 3 in stroke (302) has a GM # stamped on the top 39217624 and TRW cast on the inside below the wrist pin also has 3 cast ribs on both sides of the skirt.
Can anyone shed some light on what these are?

[JohnZ]

I have 8 NOS domed pistons with two eyebrow valve reliefs, its 4 inch bore, 3 in stroke (302) has a GM # stamped on the top 39217624 and TRW cast on the inside below the wrist pin also has 3 cast ribs on both sides of the skirt.
Can anyone shed some light on what these are?

Sounds like the '67-'68 Z/28 piston, which used pressed pins with no grooves in the pin bore for retainers.

[Ramjett54]

Thanks
Your correct, no grooves. Can you tell me where you got your information?
Thanks again
Merry Christmas

[JohnZ]

Thanks
Your correct, no grooves. Can you tell me where you got your information?
Thanks again
Merry Christmas

Many years of experience - GM only made two 4"-bore 3"-stroke pistons; the one you have for '67-'68 302's, and the one in the photo below with a straight-across relief for the valves, floating pins, and grooves for pin retainers, used in '69 302's; it has 3927172 forged into the underside of the piston crown.

[rodent]

This piston is out of my '68 05A Z,part #3927172

[william]

The change to the '69 style piston was made during the '68 model year as noted in Jerry's book.

[Ramjett54]

Thanks
Is the latter style better than the early style, and if so, how and why?

[JohnZ]

Thanks
Is the latter style better than the early style, and if so, how and why?

Chevrolet must have thought it was an improvement, or they wouldn't have spent the money to re-tool it; the straight-across valve relief formation probably improved flame-front propagation.