word from machine shop!

[HAM1974]

called machine shop today they told me they only had to take my block out to 40,which is a relief.so i ordered my pistons from jegs,part #844-l2256f30 speed pro forged pistons. Now for the question this part calls for the comp. to be 9.72 with 64cc heads this seems low,the piston are flat tops with 4valve reliefs.

[Rinkrat456]

Sounds like a solid build so far. Assemble your short block and then measure where the piston is at TDC in relation to the deck height. If memory serves, the piston should still be a few thousandths in the hole at TDC. Measure that distance, then pick your head gasket to achieve an ideal quench distance between .037-.041" for a good street motor. Say if your piston is sitting .005" in the hole and you're aiming for a .040" distance between the piston at TDC and the head chamber, you'll want a head gasket about .035" thick for a total distance of .040". Say if the piston was out of the hole (perhaps the machinist had to deck the block a few thousandths) and the piston at TDC sat .005" out of the hole, you'd want a .045" head gasket for the same target distance of .040". Got it?

After you've assembled the short block and have the proper head gasket set, THEN continue on to mill down the heads as needed for more compression.

Then again, I'm not entirely sure where you're getting the 9.72 info from, unless you're just looking at the Summit/Jegs description. I come up with 10.31:1 assuming a .040" head gasket and untouched 64cc heads. I calculated 6.1cc of "dish" for those flat top pistons with the deep valve reliefs in them which only takes the compression down to 10.3:1...and even if I calculate completely flat top pistons with 0cc of dish or valve relief, the compression only goes up to 10.8, not down to 9.72. Also, the pistons that come up for that part # are 4.030" bore...aren't you looking for 4.040" bore?

[HAM1974]

I was looking at what they had in the book,but i ordered the pistons in a kit with rings and pins. Thanks for the helpful info.

[rayL]

they bored it without having the pistons??

[grimpuppy]

they bored it without having the pistons??

Exactly what I was thinking.

[Rinkrat456]

they bored it without having the pistons??

Hopefully not, but then again the machine shop could be just that darn good and get it spot on. Haha

[stevo67]

Exactly what I was thinking.

Hopefully they still have the block, and will be "finish honing" to your piston's "actual" size, and have not just done a generic 0.040" oversize bore and hone.

[stroker1]

YEP, it's always smart to have the pistons in hand when boring a block. But it's really not all that uncommon to bore a block without the pistons in hand. Especially if they are going to be forged. The hyperuectic pistons usually fit a little snugger in the bores and it's always a good idea to mic the pistons or at least read the inspection report from the manufacturer before boring.

I prefer to have all my cylinders bored to match the assigned piston also, but most high end piston manufacturers don't have much variation from piston to piston. I have HRC pistons in my motor which are made by ROSS for Howards Racing Components. I measured each piston and they where within .0003" of an inch. And yes I can measure that close and even closer when need be. I make my living as a tool & die maker.

I wouldn't sweat it too much, they can always double check before assembling. That's the point in blueprinting, so you know what everything measures and how much clearance or interference you have.

[HAM1974]

they are waiting to finish hone to piston size

[spacecadet]

I would have rather had mine bored 1st then, ordered the pistons. I had to send the 1st set back. THe last hole had a low spot that needed more machining than the 020 pistons fill.
Space

[HAM1974]

in my sitiuation my walls were scatched so the were not sure how far they were going to have to bore.40 or 60! they are a very reputable shop in my area.