”In The Know” Pages

Figuring Compression Height of a piston

The compression height is the distance from the center of the wristpin hole to the top deck of the piston. (see attached pix)

Custom pistons are available in any practical compression height to compensate for stroker or destroker cranks, long rods, or blocks which have been milled excessively, etc. Winston Cup & Busch motors usually have a very low or small compression height. (1.245 to 1.12 inches) The reason is that they have a shorter than stock stroke with a longer than stock rod length.

1^st thing you need to know is the block height. To find this you need to measure from the crankshaft centerline to the deck (cylinder head mounting surface) of the block. 

2^nd Next thing is rod length. To determine exact rod length, you should have a good pair of calipers to measure with. Measure the size of the rod bearing opening (big hole) and the size of the wristpin opening, and divide them in two (or one half) Finally, determine the distance between the two openings (center of the rod) and add the half you just calculated. That gives you the rod length. It comes out to be the distance between the center of the two holes.

Finally you need the stroke length.

Stroke length is;

 twice the distance from the centerline of the crankshaft main bearing journals to the centerline of the connecting rod journals or ;

 It is also the distance the piston moves up and down in the cylinder

Now that you have all the info, you can calculate the compression height of the piston;

To calculate the compression height, use the following formula:

Block Height minus 1/2 the crank stroke, minus the rod length, minus the deck clearance (amount piston is "in the hole").

For example, a 350 Chevy engine with a stock 3.480 stroke, stock length 5.700 rod, standard .017 deck clearance and standard 9.025 block height would be:

3.480 stroke divided by 2 = 1.740

9.025 - 1.740 - 5.700 - .017 = a compression height of 1.568.