Author Topic: Piston dome  (Read 14438 times)

Offline MotorGeek - Jerry Hall

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Re: Piston dome
« on: June 27, 2013, 07:47:32 am »
What are you trying to do?  Are you trying to calculate the dome volume using procedures like most four stroke builders use.  If so there will be some volumes not accounted for and erroneous compression ratios will be calculated  The domes on most two stroke are not a constant radius.

You need a $30,000 CMM to accurately measure the top of the piston to determine its actual dome radius or how many different radi it has.

Save your self a lot of time and calculation and just measure the actual clearance volume a running engine "sees". 

1.  Place the piston about and one inch from TDC

2.  .Put two little daps of grease on the bore where the end gaps of the rings are located. 

3.  Rotate the crank so that the piston moves to TDC.  This should seal and keep the end gaps from leaking liquid.

4.  Wipe off the excess grease

5.  Install the head and head gasket .

6.  Fill the volume on top of the piston with ATF or mineral spirits from a Burett (graduated cylinder) to the top of the spark plug hole.

7.  Subtract 2.1cc from the volume number you measured in step 6 if you are using a 3/4" reach spark plug.  I use 2.1cc because it is a good average number for the volume that a B8ES spark plug occupies in the spark plug thread.  Your should now have the actual volume needed to make a good compression ratio calculation.

8.  Now that we have an accurate compression ratio number what do we do with it??????.    What does it mean????

9.  This procedure should take less than 5 minutes.


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