pistons hitting the heads


Nov 12, 2006
I just received a call from the machine shop and they tell me that my pistons are coming up .020 to tall vs. my deck height so they are going to cut down my pistons. They said this will not hurt anything. Is that true that by them knocking.020 off of my pistons wont affect anything?:bang:
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i couldnt imagine cutting the pistons like that. all it would do is make the top ring land smaller and more prone to breaking and it could also cause a thin spot in the piston that could cause problems. stock pistons are not worth cutting and i sure as hell wouldnt cut a brand new set of pistons. if the pistons are in fact coming up .020 thats not a big problem. the only problem you will have is if you are running small chamber heads where the piston will actually contact the head. but if you are running a factory head you shouldnt have a problem. it will raise the compression ratio though. if my math is right that .020 will lower your combustion chamber size by 4 cc's
well i have heard of fly cutting pistons be for but that just seems like alot to cut off a piston , what kind of machine work did u have done to the block

you find someone to buy my heads yet?

anyways, im assuming they milled the block. but it looks like they milled it a little too much. we need a little more info about the motor. stock pistons? what heads are you using?

O.K. just talked to the machine shop again and what they are actually doing is cutting down the deck height. They are 20 out of the hole He also mentioned something about a clean up cut on the mill at 5. He says what they are doing is minute to them they do this all the time to race cars and stuff.
if they are cutting the deck more than it will make the pistons stick out more. depending on how much they are cut you are gonna have to look into milling the intake to fit the heads and possibly getting new pushrods.
Well .020 is not much and it will only really clean up the top of the pistion its about the thickness of a hair on the back of your neck. And from what you explained they are going to cut the deck .005 to just make sure the the deck is straight and thats why they need to take the .020 off the pistion the .005 will be made up with head gasket thickness. I had my block parrell decked to "0" which means the tops of the pistons are the EXACT same hight as the deck surface the only gap between the head and the pistion is the head gasket. And if your block was milled that much before then you can bet that the lntake was cutt and the pushrods are all ready differenet but you will still need to check the clearences the .005 dont worry about you can make it up in head gasket and the .020 is nothing off the piston
If they cut the deck .005 and the pistons .020 then the pistons will protrude out of the deck .005. run a .040 gasket and the quench will be .035. It will run good and there will be less chance for detonation. The ideal quench is anywhere from .035-.045 with steel rods. I'm sure they will still check the piston to valve clearance. I set my own motors up at .000 deck to +.005 out of the hole.
His block has already been milled. His pistons are .020 out of the hole. They're going to take the .020 off the tops of the pistons to give him a 0 deck.

Atleast that's the claim.
Whats the exact deck height, and what is the CH of your piston??? Your using a stock crank and rod I assume?

Cutting the pistons is not a big deal however it is less common with a stock style build up. With a factory 3" stroke, 5.03" rod and a 1.60CH piston and a nominal 8.200" deck you are usually anywhere from .007-.010" in the hole STOCK.

If the pistons are sticking up .020" and they need to cut another .005" off to square it up I would worry about the deck thickness of the stock block personally since they are all over the place. I have never had to cut a stock 302 block that far...

Like mentioned above keeping the quench area in the .035"-.041" range is ideal.