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piston to head clearance

arizona satellite

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I am putting my engine together and want your opinion, I have a 440 with kb 236 pistons with the step dome and stealth heads. my question is, with the pistons installed the dome sits .033" above the deck, I know they need to be machined down, but how far? do I remove them completely or reduce them to a zero deck? the head gaskets are 0.047" thick(not sure of the crush factor, assume .015-.020") so if I zeroed the dome it would yield a .025-.030" clearance between the piston and the head, is this too close? thanks
Patrick
 
What cam and gaskets are you using and what compression ratio are you shooting for? The gasket manufacturer can tell you how much the gaskets will crush.....but I don't recall any gaskets crushing more than a few thousandths and for sure not .015-.020
 
the head gaskets are standard felpro and the cam is the comp 305H. I will check with felpro and see what the crush is.
 
standard felpro is most likely .040" crushed.
How are you measuring the clearance your referencing? Just put the head on and a few bolts in. Use modeling clay on the piston dome and roll it over. The dome should be above the deck,,,, that's how they increase compression....... They fit into the chamber area of your head, and those stealths use the standard stock D shape big Ol chamber design....

One more thing as reference... My pistons in a 440 have .140" dome height above the flat of the piston. They stick out above the block deck height too
 
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The dome is what goes up into the chamber. How far up does the flat side of the piston go and what is the part number of the gasket. Do a mockup and use clay to check how much you have on the combustion side...but I think engines perform better with flat top pistons so long as you have good quench and a decent compression ratio than an engine with the same specs and domed pistons because the domes help get in the way of the flame front during combustion.
 
the Keith black site says that with closed chamber heads the dome must be machined down.
 
Follow 67's advice, I used a camera scope through the spark plug hole to double check, mine are dished though :)
 
First and foremost quit using book specs.Measure everything and correct them to actual spec.Yes KB says they may have to be machined down,but not always!.
Stop guessing,take and assemble the motor with the head and gasket on one side.Before bolting the head up,put a 1"ball of clay on the dome and the flat of the piston.Bolt everything together,just snugging up the head bolts.Then spin the motor over.One other tid bit spray the head and piston with spam so no sticky!!!!!Spin the motor over,carefully disassemble and measure the pinch points.No less than 0.039 piston to head clearance is safe.You will do the same to check for piston to valve clearance later on or at the same time.
Chances are if your deck height is 10.715 or taller you are going to be fine.
 
Well, you can just do it because someone said you should,,, or you can measure it for yourself and know what you'll need for your engine. Domes set above the deck,,,,, that's how they work.... nothing more to it and nothing less. Have you seen some of the dome heights on pistons being used? Like I said,,, mine are .140" high and I have a closed chamber more than ANY factory closed chamber design would ever have... Including the Stealths.... I would measure it and then make an educated decision on what you need to do next....
 
use .040 minimum piston to head clearance with steel rods and stock bearing clearances. i've done them slightly tighter but .040 is safe.
 
use .040 minimum piston to head clearance with steel rods and stock bearing clearances. i've done them slightly tighter but .040 is safe.

Funny you mentioned that .040" for the safe factor... I put my heads on without the gasket and the pistons didn't touch, so I knew I was in the clear with a .040" gasket on top of that......

One thing you'll want to check is the plug to piston clearance with domes. they usually cast or forge a groove in the tops for the plug clearance, but you should check it out....
 
use .040 minimum piston to head clearance with steel rods and stock bearing clearances. i've done them slightly tighter but .040 is safe.
I did one motor that was .019!! Yeah, it was a mistake but it hauled *** until the guy hit the NOS. It ran a 10.0 right off the trailer. Problem was that my buddy thought I was checking it and I thought he was checking it. The engine was a 498 stroker with steel H beam rods and turned 7500 and it was a rush job. That's a big problem when there's more than one person working and both are buzzing around like bees trying to hit a deadline. Anyways, after starting over from a blown out cylinder, it ran 9.30 on a 150 shot in a 3500 lb car. I usually shoot for no more than .030 quench with steel rods....
 
Funny you mentioned that .040" for the safe factor... I put my heads on without the gasket and the pistons didn't touch, so I knew I was in the clear with a .040" gasket on top of that......

One thing you'll want to check is the plug to piston clearance with domes. they usually cast or forge a groove in the tops for the plug clearance, but you should check it out....
claying in sparkplug to compression dome clearance is a must. the pistons the op is using are quench domes so piston to head is what needs to be checked.
 
Good Grief boys, this is a quench dome piston. It is designed to stick up inside the open chamber of the 88cc RPM or the factory large chambered head like the 906. The piston dome should be milled down to at least zero deck if he is using the .040 thick gasket with the 81cc Stealth head. Milling even further would be appropiate if required to obtain the desired compression ratio. Whatever that unannounced ratio may be.
 
Good Grief boys, this is a quench dome piston. It is designed to stick up inside the open chamber of the 88cc RPM or the factory large chambered head like the 906. The piston dome should be milled down to at least zero deck if he is using the .040 thick gasket with the 81cc Stealth head. Milling even further would be appropiate if required to obtain the desired compression ratio. Whatever that unannounced ratio may be.
Missed that all together! :iamwithstupid: I would do some math on that one and try to get my quench a bit tighter than .040 so long as compression wasn't too high for that cam....which I did not look up.
 
IQ52..I gotta say, thats playin it TO safe ... granted your right !!!!!!!!!! BUT wouldn't it be a little better to know EXACTLY how much to cut by claying...then Guessing ??? I'm sorry I'm from the OLD SKOOL of engine building the measure twice ...cut once...law does apply here.... JUST MY Opinion
 
Maybe this will help
Quench is set with the piston deck height and gasket thickness. Piston deck height is the measurement from the piston crown to the flat deck surface of the block with the piston at top dead center and has nothing to do with the chamber cc's. When building a motor, you would bore and hone the block for the oversize pistons, then mock up the crank in the block with main bearings and bolt up the rod/piston assemblies to the crank with rod bearings, but without piston rings. You would then turn the crank so that each piston would be at top dead center, then measure the distance from the piston crown to the block deck. This can normally be done with just four piston/rod assemblies, one at each corner of the block. On a small block Chevy, this would be cylinders 1,2,7 and 8. On a Ford, cylinders 1,4,5 and 8. Having these measurements, you would then cut the block decks to achieve the block deck height that you wanted to achieve. It would be very unusual to find a block that is square corner to corner and I have found some over the years that were off by 0.008". That is to say, for instance, that cylinder 2 might have the piston down in the bore at 0.025" and cylinder 8 (on a Chevy,passenger side of the block) might have the piston down in the bore 0.033". In order to square the block in this instance, that deck would be cut 0.008" more on the end of the block where cylinder 8 is. Let's say that in order to clean the entire deck on one side of the motor, you might take off 0.005" at cylinder 2 and 0.013" at cylinder 8. This would result in the measurement of each piston on the passenger side of the block having a piston deck height of 0.020" This process is repeated on the driver's side of the block. Actually, you wouldn't do any cutting until all 4 corners of the block were measured for deck height and all cut accordingly to the shortest piston deck height. Now, let's assume that all pistons measured out at 0.020" piston deck height after you have cut both block decks. In order to achieve the optimal quench (or squish, whatever you want to call it), we will have to use a gasket that will give us 0.035" to 0.040" clearance between the piston crown and the underside of the head with the gasket installed. With a piston deck height of 0.020", we could use a 0.016" shim head gasket to achieve a quench of 0.036". Again, this has absolutely nothing to do with the chamber size of the head. We would choose a chamber and piston crown configuration to achieve the desired static compression ratio. You NEVER use different gasket thicknesses to achieve your target static compression ratio. Different gasket thicknesses are used to achieve your target quench.

Some fellows will go ahead and cut both block decks so that the piston is exactly even with the deck at top dead center. This is called "zero decking" and you would then use a gasket with a compressed thickness of 0.035" to 0.040" to achieve your quench.
 
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