Squish, also referred to as Quench Band refers to the distance in thousandths of and inch between the flat cylindrical area located on the outside diameter of the piston and the cylindrical area located outside of the combustion chamber (See Photos)

Squish is one of the most important elements in building horsepower and improving fuel mileage in virtually any engine build whether stock or for racing applications.

Surprisingly it is one of the most overlooked processes by less knowledgeable engine builders, but is one of many areas that must be recognized in the engine blueprinting process.

Squish is and important factor in any internal combustion engine, automobile, motorcycle, marine etc.

Typical Squish measurements range from .040 to .045 thousandths of and inch for street and racing engines running on gasoline. Engines running on such fuels as methanol or ethanol can be set up with a slightly tighter squish (perhaps as low as .025-.035) since these fuels produce little or no carbon buildup in the combustion chamber or top of the piston.

As the piston moves towards TDC the compressed mixture of air and fuel is squished into the remaining area of the combustion chamber, creating turbulence and a more efficient and complete combustion process.

Using a Harley Davidson Evolution based motorcycle engine as and example, that produces 65 hp with a squish of .080 thousands (not unusual as delivered) and correcting it to .040 would likely produce 4-5 additional hp with only this modification, fuel mileage would also improve due to a more efficient combustion process.

The Measurement Process:

Once again using a Harley based Evolution. engine (For Illustration purposes only)

First determine the actual deck height of the piston squish band (see photo) in relation to the top of the cylinder at TDC (TOP DEAD CENTER)

With the cylinder firmly forqued in place, rotate the the piston to ATDC.(see photo) record this measurement using a dial caliper a depth micrometer.

Next measure the thickness of the compressed thickness of the head gasket.

By adding the 2 numbers together you have your existing squish band.

NOTE

(If the piston protrudes above the cylinder deck or engine block, this measurement becomes a negative number and is subtracted from the gasket thickness).

How to correct excessive squish:

Using the Evo based engine shown, which as shown has a .060 squish, the simplest method would be to machine .020 from the base of the cylinder, assuming our target squish is .040. Further using for example, on a typical automobile engine given the same measurements the block deck would need to be machined .020 lower, a term called decking.