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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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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. http://butlerperformance.com/i-24453244-federal-mogul-piston-set-1959-1966-389cid.html

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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