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post #1 of 3 (permalink) Old 09-30-2017, 10:59 PM Thread Starter
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Are these stock pistons?

So after tearing down the motor, I noticed that the pistons have numbers stamped in them. They all have "1956PX .030" stamped into them.

When I tried looking for replacement pistons, I can't seem to find any that have the valve reliefs that look like mine.

Are these OEM?
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1966 Lemans w/ 1965 GTO 389 Motor
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post #2 of 3 (permalink) Old 09-30-2017, 11:57 PM
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The pistons are old 389 .030 Sealed Power forged pistons, forged replacements from the '70's-80's.
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post #3 of 3 (permalink) Old 10-01-2017, 09:01 AM
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They are the old TRW forged pistons. 1959-1965 389CI. 10.25 compression. .030" over. The one in the picture is .040" over.

Now owned by Federal Mogul. You want to have the bores checked before ordering a new set of pistons if that is your goal. It may clean up and you can go with .030" over as you have. If not, don't go any more than needed and see if the .040" will work. I went .040" over on one of my engines as it was already .030". This leaves you more material and another rebuild should you need to go .060" over. Butler offers the Federal Mogul brand for the 389CI as well as other piston options.

Build the engine/compression based on the available gas octane in your area. If you plan on running high octane racing gas, 10.25 compression will work. If you plan on running 91-93 pump octane, keep the compression between 9.0-9.5 with 9.0 being the better number. Match the cam to the lower compression and you will still have plenty of power.

You can email Butler for a recommendation on piston and cam selection so as they match.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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