How to Set Squish Area Harley EVO Twin Cam M8 head to piston clearance

This is very important part of building a motor that most people overlook. You must set up the squish area to have maximum combustion chamber efficiency. With combustion chamber efficiency you get two benefits. 1 - Maximum horsepower per given engine combination. 2 - Better gas mileage. These both happen because you get better atomization of the fuel. In other words you burn all of the fuel getting your engine to do more work from the same given amount of fuel.

The squish area must be set between .030 - .040 inch. It's best to shoot for .035 inch. This way if your measurements are a little off you will still fall in-between the .030 - .040 inch.

The formula for figuring out the gaskets you need are as follows:

If you piston protrudes from the cylinder -  Base Gasket crush thickness (BG) + Head Gasket crush thickness (HG) -  Piston Protrusion (PP) = Squish Area (SA) or   (BG+HG) - PP = SA
In other words if my Head gasket is .020 inch. My Base gasket is .020 inch and my piston sticks out of the cylinder .005 inch my squish area would be .035 inch.  (.020 + .020) - .005 = .035 inch

If you have all the measurements but you need to figure out the thickness of the Head Gasket that you will need, the formula is: (BG - PP) - SA = HG  
So if you want a squish area of .040 inch and your Base gasket is .020 inch and your piston protrudes .005 inch you will need a Head gasket .025 inch.  (.020 - .005) - .040 = .025 inch

If you have all the measurements but you need to figure out the thickness of the Base Gasket that you will need, the formula is: (HG - PP) - SA = BG  
So if you want a squish area of .033 inch and your Head gasket is .020 inch and your piston protrudes .005 inch you will need a base gasket .018 inch.  (.020 - .005) - .033 = .0.018 inch

If your piston is below the cylinder deck surface (Piston Recess) of (PR) then on the above formulas you must change the Piston Protrusion to Piston Recess and make the number a positive number and add it instead of subtract it.
Formula for Squish Area    -     (BG+HG) + PR = SA
Formula for Head Gasket  -      SA -(BG + PR) = HG
Formula for Base Gasket   -      SA -(HG + PR) = BG

If you need further help email me your phone number to [email protected]. It's easier to help over the phone.

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