What is Deck Height?

Deck height is the amount of space between the top of the piston and the top of the cylinder.  This area combined with the volume of the combustion chamber in the cylinder head, is what determines the compression ratio of your engine.  With stroker engines, you almost always need to use cylinder spacers as opposed to cylinder shims to create the required deck height needed for your compression ratio.  The difference between cylinder shims and spacers, is that cylinder spacers need to be machined by someone like RIMCO, whereas you can buy cylinder shims at almost any VW Parts house.  Also, cylinder spacers cost about $40 more than cylinder shims.  Cylinder spacers provide a more stable foundation for the cylinders to seat against the case, and are the preferred method over cylinder shims.

 

Tools Needs

The following tools are needed to measure the deck height:

1. Dial Caliper with depth gauge
2. Dial Indicator
3. Calculator

Procedure

We will use my situation as an example to show the procedure used to find out the deck height required for my engine.  First, I need to decide what compression ratio I want to run in my engine.  I decide to run 10:1 since I have a sandrail engine that won't have any problems staying cool and since I know I will be running Aviation fuel.  Once I make this decision, I need to measure what my current deck height is without any shims or spacers.  To do this, I install the piston and cylinder on my short block, removing the piston rings first, then I attach the dial indicator to one of the head studs and rotate the engine to find Top Dead Center (TDC) of the piston (see the image).  TDC is located once the dial indicator shows the top of the stroke.
 

 

Once I found TDC, I use my Dial Calipers and determine how much negative deck height I currently have.  I have negative deck height, becuase of the 82mm stroke, and the lengthened 5.6 inch connecting rods.  Look at the following image that shows how I checked for the negative deck height.
 

 

This measurement showed that I had .200 inch negative deck height.  After doing some calculations, I determined that I need an overall positive deck height of .060 inches to get my desired 10:1 compression ratio, so that means that my cylinder spacer needs to be .260 inches thick (.200 + .060 = .260).  With .060 deck height, my compression ratio will be 9.8:1, pretty close to 10:1. If you have Excel 97, you can use my Engine Calculator to figure out your compression ratio, engine displacement, etc.

 

Now that I have received the .260 inch spacers, I put modelling putty on the piston to check for valve-to-piston clearance.  I installed one piston and cylinder, with the rings removed, and the other cylinder, without the piston, so that I could bolt the head onto the engine (see image).
 

 

After bolting everything together, I rotated the engine at least through 2 full rotations of the crankshaft, this makes sure that you have opened both the Intake and exhaust valves completely.  I then removed the head and inspected the modelling clay to make sure there was adequate clearance (see the image).