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| | |-+  Can anyone help me Identify this Gm Piston?
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Author Topic: Can anyone help me Identify this Gm Piston?  (Read 1199 times)
Ramjett54
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« on: December 16, 2011, 12:24:49 PM »

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?
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JohnZ
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« Reply #1 on: December 16, 2011, 12:43:59 PM »

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.
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'69 Z/28
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Ramjett54
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« Reply #2 on: December 16, 2011, 01:14:34 PM »

Thanks
Your correct, no grooves. Can you tell me where you got your information?
Thanks again
Merry Christmas
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JohnZ
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« Reply #3 on: December 17, 2011, 11:34:09 AM »

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.
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'69 Z/28
Fathom Green
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rodent
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« Reply #4 on: December 17, 2011, 03:28:30 PM »

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

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william
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« Reply #5 on: December 17, 2011, 03:49:13 PM »

The change to the '69 style piston was made during the '68 model year as noted in Jerry's book.
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Ramjett54
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« Reply #6 on: December 17, 2011, 07:02:52 PM »

Thanks
Is the latter style better than the early style, and if so, how and why?
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JohnZ
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« Reply #7 on: December 18, 2011, 01:53:48 PM »

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.
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'69 Z/28
Fathom Green
CRG
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