Considering that your piston is only in the hole, .005, I consider this a time to take advantage of bringing your piston to head ''quench" in line to keep detonation down. Depending on who you talk to , .039 felpro head gaskets will do just that and at the same time , increasing compression within favorable limits. I decked my block to 0 and used felpro 1010 head gasket, putting the desirable between .035-.045. Just a matter of preference.The machine shop told me they decked my 1968 327 block to 9.005" (both banks). I am wondering what thickness head gaskets I need to use.
TYPICALLY, stock GM pistons are in the hole .020-.035 on small blocks. Not sure if it was explained to you, but when a block deck is cut , its normally cut parallel to the crank centerlines, from there you determine your rod lengths, then piston compression heights.For stock engines, I've heard a gasket thickness of .040" is used. I also read that stock height is 9.025". Assuming this is actually true and mine is 9.005", would I need to use a .060" head gasket? Or would the recommendation be to stick with stock .040" and happily accept the small increase in CR?
I guess I am looking for confirmation from experts that a 0.040" gasket thickness is typical for stock, and that 9.025" is the stock "0" height for decks and if a change in gasket thickness is necessary when you deviate from this.
Good info.TYPICALLY, stock GM pistons are in the hole .020-.035 on small blocks. Not sure if it was explained to you, but when a block deck is cut , its normally cut parallel to the crank centerlines, from there you determine your rod lengths, then piston compression heights.
If you are looking to increase compression and what most race builders mean by " blue printing the block" this is what is considered.
If you add a thicker head gasket , like a .060, you are basically lowering your static compression. If you are doing all of this on your own math , you will be fine running a thinner gasket like a .039 or the GM type steel shim of .017-.019, but remember, you do your homework, trust your math . If you are already at .005 in the hole and you go for the thinner gasket say .025, I feel thats cutting it too close for a street car. Best to err on the high side of your piston to head clearances.
Another factor is to pay attention to what your cam lifts are as they play a part in this game and I don't think you are in it , yet.
Here is one compression calculator, but there are many out on the internet. https://www.yourmechanic.com/article/how-to-calculate-compression-ratio
If you use the .060 gasket and your known in the hole is .005, you can safely say you are running a .065 piston to head clearance.
This does not take in account of piston rocking in the bore so as I mentioned before, do your math , if you want to increase compression in a simple way.
Its simple if you can understand it. Here is another calculator tool. Compression Ratio Calculator | Tools | Diamond Racing Pistons
I might add that to use some of these calculators, you need to know about the piston and its specs as wells as rings, bore, etc. Enjoy.
Thanks for all the advice. By "Zero Deck the Block", what do you mean exactly? Does that mean you machine the deck so that the piston at TDC is flush with the deck? I am planning to use Powerforged pistons that are rated at 10.4:1 with a 64cc head (which I am planning on using). Here are the specs:Good info.
Until you measure you are only guessing where the piston is in the hole.
The difference between a .060 and .040 gasket only makes about .25 diff in CR on a 327. You should also consider ideal quench to minimize detonation
To determine the correct head gasket you need to know piston dome and head volume cc. You should also consider ideal quench to minimize detonation.
That said if you just want to wing it use a .040 and check PTV clearance and you'll be fine.
btw when I build a performance engine I zero deck the block.
If this deck proves to be correct, he can still build off it as if it was 0, select his proper quench and put it together providing block square with crank. Be fine.Echoing Dave stating that the block is 9.05 does not mean anything with regard to piston in the hole. It could be .20 down or .10 up.
And all the calculations posted above are meaningless without actually measuring with the rotating assembly installed.
Squish is the space between the flat of the head outside the combustion chamber and the flat of the piston.