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400 small block piston choice

7328 Views 6 Replies 6 Participants Last post by  rich1978
I have a 3951511 stock bore 2 bolt 400 that has stock dish pistons,crank and rods. I want to build this for a nice street manners motor . I would like to change the pistons to Speed Pro hypereutectics H615cp which have a comp. hgt. of 1.425 and change the rods to 5.7 . The block has not been decked; so I'm assuming I have 9.025 deck height.I want to see if I'm calculating this right so bear with me.
Stock stroke:3.75 /2 = 1.875
Comp.hgt of piston: 1.425
Rod length: 5.7
Total: 9

Should leave me .025 in the hole right?
Piston's have a .135 dish w/4valve reliefs. Will run cast iron Darts w/64 cc chambers.
Any experts out there let me know if I'm correct . Thank's in advance for your help. Don
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The SpeedPro Hypers ALMOST NEVER (I would say "never", but that's a very dangerous word to throw around...) have the same compression height as stock. That is, they are "designed to fit" an engine with a nominal compression height of 1.425", which is NOT the same as saying their CH is 1.425".

Nearly all of them add the usual "rebuilder" .020" of deck clearance, since it's alot less of a problem for a rebuilder if the motor comes out with slightly lower than stock compression and works, than it is if they have to deck the block to repair a blown head gasket scour or something, and then the pistons hit the heads.

Note that their compression spec is lower than you get when you put .025" deck clearance into whatever "calculator" you're using. Instead, it matches what you get when you put .045" deck clearance into the calculator. That should be a red flag to you that the CH is NOT 1.425".

Measure with a mic between the top of the pin bore and the top of the piston. From that number, subtract half of the pin diameter of .927", which would be .4635". If the pistons are REALLY 1.425 CH, the measurement will therefore be 1.425" - .4635", or .9615". More likely, the measurement will actually be .9415", indicating the extra .020" of deck clearance, or about .045" "down in the hole" at TDC.
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