Measuring Your Deck Height

Step-By-Step

 

       When you assemble an air cooled VW engine, it's important to check your deck height (DH) prior to assembling the top-end of your motor.  The main reason for this is so that you can get all the correct numbers to properly calculate and adjust the compression ratio (CR).   Also, you must ensure that the piston does not go past the end of the cylinder walls.  If it does, the pistons will hit the heads!!  This isn't as big of an issue with a stock stroked motor (69mm), but will definitely be a problem with a "stroker" motor (an engine built with a crankshaft which has a stroke longer than stock).

    So what is "Deck"??  Deck is the distance from the top of your piston to the top of your cylinder wall when the piston is at the top of it's stroke.  Note the two photos illustrating the bottom of the stroke and then the top of the stroke with the gap pointed out with the screwdriver.

bottom-of-stroke.jpg (36858 bytes)             deck.jpg (29103 bytes)

    In order to check this clearance you need to either need to buy a special DH measurement tool, make one of your own, or pay a shop to check this for you prior to finishing the assembly of your motor.  Since I feel the $40 - $50 this tool costs is just a bit much, I decided to make one of my own.  I found a piece of 1/4" plate steel and had a 4.5" X 4.5" square cut with a monster set of shears.  I then measured 1.25" up from two opposite corners and had a triangle cut off.  I then had three 3/8" holes drilled which match the stud pattern of the head studs.  Now that I had my plate finished, I cut some SS hydraulic tubing to  use as spacers for the studs.  Below is a photo of my finished "home-brewed" DH tool.

 dh-tool.jpg (35329 bytes)

    Now that you have the proper tool, first you needed to set the crankshaft at "top dead center" (TDC).  Then attach one piston to to either the #1 OR #3 piston rods.  Set the crankshaft to "bottom dead center" (BDC) for checking cylinders #2 & #4.

TDC.jpg (44276 bytes)             BDC.jpg (24781 bytes)

    For DH, piston rings are NOT required.  Then slide a piston into place and seat it into it's bore.  Then install the DH tool with the cut off corner at the top.  Now slide the two long spacers on the bottom studs and the appropriate top spacer into place and install one head washer and nut to each.  Torque the nuts to 24ft. lbs in a "X" pattern.  I leave the odd bottom bolt for last.

    I use a set of pre-bent feeler gauges for taking the actual measurement, but plain straight ones will work too.  Keep in mind that straight feeler gauges are less accurate when bent.

dh-feeler-gauge.jpg (20118 bytes)

    You will have to stack feeler gauges in order to get a correct reading so ensure they don't have any dirt or corrosion on them.  I always start with about .060" worth of feeler gauges.  If it's too tight, trade one of the gauges for one a little smaller; the opposite for too loose. 

dh-checking.jpg (42856 bytes)

   Once you have a nice snug fit, pull out the feeler gauges and count up the numbers.  The total is your deck height. 

dh-measurement.jpg (41450 bytes)

 


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Revised: August 29, 2003 .