Squish Band & Deck Height Calculator

 

Definitions:

The Squish Band or "Quench" is defined as that area between the flat of the piston and the flat of the cylinder head at top dead center (TDC). On the compression stroke, as the piston approaches TDC, the compressed mixture of fuel and air is "squished" to the remaining space of the combustion chamber where the spark plug and valves reside. The "squeezing" of the mixture creates turbulence and is expected to promote a better and more complete combustion.

Typical figures for this measurement are in the range of .040" to.045" which allows for rod stretch, carbon build-up and other variables. RB Racing's Harley Big Twin ORCA motors are designed with .035" squish to accelerate the turbulence and to further concentrate the mixture in the remaining combustion chamber while leaving a small margin for carbon build-up. Pure race engines with short duration applications may reduce this figure to .025" as some builders aren't happy till the pistons "kiss" the cylinder head. At .025" when you factor in high rpm rod stretch and piston "rock" at TDC you effectively reduce squish to zero.

It's a simple fact: The closer the flat of the piston to the flat of the combustion chamber, the more power you will make. In a race engine with all the factors considered you practically want the two surfaces touching one another. In F1 they machine each piston individually even for the spark plug relief.

For those of you who have gone backward in time, seduced by bathtub "hemispherical" combustion chambers, this calculator won't do you much good as their "squish is defined by the peripheral intersect of the spherical piston dome and the dome of the combustion chamber...There is no "flat" area of the head and piston to measure. Whereas we all love "Hemis" and all this word stands for, from Richard Petty and the Superbirds to all the current supercharged Rodeck, Keith Black, Ed Pink, and Arias hemis, it's not the chamber design we see in modern pushrod engines like in NASCAR. Swirl, turbulence, squish, quench, smaller combustion chambers, and higher compression ratios with narrower valve angles are the norm these days...not whacking big relief pockets into domed pistons. This is the era where we want minimum surface area on our piston tops and where we wish to direct our "burn" to the exhaust valve, not to the far reaches of the combustion chamber. Hemi designs compromise the combustion process.

Deck Height: The distance between the flat "quench" surface of the piston and the top of the bare cylinder (no head gasket). Typically an engine is set to zero deck height with the head gasket (compressed value) defining the "squish band" value. If the value is negative this indicates the piston's "quench" surface extends above the top of the bare cylinder.

This calculator allows you to define the various "variables" under your control like cylinder length, gaskets, and piston pin to crown dimensions. It is imperative that you physically check all dimensions as there are always variances to deal with.

    • Stroke -- Distance defined by the total vertical movement of the piston in the cylinder from bottom dead center to top dead center. Evo Big Twins are 4.250". 113" ORCA motors are 4.500". 135" ORCA motors are 4.500". 120" EVO/TC ORCA motors are 4.000".
    • Connecting Rod Length -- This is the center to center distance between the big and small ends of the rod. Evolution Big Twins are 7.440". 113" ORCA motors are 7.440". 120/135" EVO/TC ORCA motors are 8.000".
    • Crank Centerline to Case Height -- Evo Big Twins are 5.375". 113" ORCAS are 5.875". 135" EVO ORCAS are 6.375". 135" TC ORCAS are 7.000".
    • Cylinder Height -- Evo Big Twins are 5.550". TC88 motors are 4.937". 113" EVO ORCAS are 4.995". 120" EVO ORCAS are 4.8645". 135 EVO ORCCAS are 5.1145". 120" TC ORCAS are 4.2395". 135" TC ORCAS are 4.895".
    • Base Gasket -- Typical base gaskets are .020" thick. 113/120/135 ORCAS do not use base gaskets but are machined for an o-ring seal.
    • Head Gasket -- Typical head gaskets are .040" thick. 113/120/135 ORCAS do not use head gaskets but are machined for metal seal rings.
    • Piston Pin Diameter -- Evo Big Twin pins are .791" in diameter. 113" ORCAS use .892" pins. 120/135 ORCAS use .927" pins.

Squish Band and Deck Height Calculator

Engine Stroke inches Enter your engine's stroke in inches. OEM Harley-Davidson EVO Big Twin 4.250". TC88 4.000". 113 ORCA 4.500"; 120 ORCA 4.000"; 135 ORCA 4.5000".
Connecting Rod Length inches Enter your connecting rod length measured center of wristpin to center of crankpin. OEM EVO Big Twin 7.4375". 113 ORCA 7.440"; 120/135 ORCA 8.000".
Crank Centerline to Case Height inches Enter the distance from the centerline of your crankshaft to the top of your engine case. Do not measure your base gasket. OEM EVO Big Twin 5.375". 113 EVO ORCA 5.875"; 135 ORCA 6.375". 120/135 TC ORCA 7.000".
Cylinder Height inches Enter cylinder height (no gaskets). OEM EVO Big Twin 5.550". 113" EVO ORCA 4.995"; 120" EVO ORCA 4.8645". 135" EVO ORCA 5.1145". 120" TC ORCA 4.2395". 135" TC ORCA 4.4895".
Base Gasket

inches Enter thickness of base gasket. OEM EVO Big Twin .020". All ORCAS 0.00"
Head Gasket inches Enter thickness of head gasket. OEM EVO Big Twin .040". All ORCAS 0.00".
Piston Pin Diameter inches Enter diameter of piston pin. OEM EVO Big Twin .791". 113" ORCA .892"; 120/135 ORCAS .927".
Top of Pin Hole to Piston Squish Area inches Enter distance from the top of the piston pin hole in your piston to the flat of you squish band at the top edge of the piston. Stock 80 c.i. Evo .984". 113" ORCA .701"; 120/135 ORCAS .736".
Calculated Squish In": Deck Height In": Boss to Crown @ .045": Boss to Crown @ .035": Boss to Crown @ .025":