# How to calculate the compression height of a piston?

To better help understand about pistons, I have the following questions:

• What is the compression height of a piston?
• How is it calculated?
• Why is it important to know when building an engine?

The piston compression height is the distance from the centerline of the piston pin to the flat part of the top of the piston. The reason it’s an important number to know is, if you get it wrong, the piston can travel up past the top of the deck and hit the head, destroying the engine. To figure out the best compression height for your engine combination, you need to know three things: the deck height; the length of your connecting rods; crank stroke length.

The block deck height is the distance from the centerline of your crank main journals to the block deck surface. For instance, the deck height of a standard Chevrolet LS (Gen III & IV small block) block is 9.240”, while a Gen I small block Chevrolet engine is 9.325”.

Using the LS engine as an example, the deck height would be 9.240”, and if you wanted to use a 6.125” connecting rod, and a standard LS1 stroke of 3.622”, you’d compute the following …

First, divide the stroke by two and add that to the rod length:

• 3.622 / 2 = 1.811
• 1.811 + 6.125 = 7.936

Next, subtract this answer from the deck height:

• 9.240 – 7.936 = 1.304”

This will put the top of the piston at the top of the engine block when fully at top dead center. If you want to put the piston above or below the top of the deck, add or subtract that amount from the total. Leaving it to be directly at deck height will give you the best quench and performance, though you might want to bring it below the deck height to allow for a lower compression ratio for forced induction applications.

You'll need to know this figure when ordering pistons, if you change out the rod length or have the deck shaved. When ordering from reputable piston manufacturers, you can have them double check your figures to ensure you've done your math right.