Piston Volume Measurement

Knowing the volume (dish/dome) of the piston is an important factor in the engine compression calculation. In general the manufacturer provides the volume spec but what if you have a piston of unknown spec? In that case you'll need the measure its volume with a fluid and do some math.

 

First thing I did was get out a dial gauge and a dial gauge bridge. I zeroed the dial gauge of the deck surface where the Plexiglas cover for the volume measurement will rest. I also rubbed a light layer of oil near the upper bore to help assist the piston ring seal (since we will later be filling this area with liquid).

 

 

Next I positioned the dial gauge over the piston edge and rotated the crankshaft until the desired piston was near the dial gauge probe. Since I wanted the piston to be set .1inch below the zeroed deck (2 rotations of the dial gauge). (Side note: I like to use a rubber strap wrench on the crank drive gear as this make rotating the engine super easy and causes no damage). 

 

 

Now that cylinder 1 is at a known depth I prepared the other necessary items. These included Jeg's economy cc kit, Vaseline, rubbing alcohol (Water would be fine also) and food coloring. Food color your liquid for added visibility.

 

 

 

Fill the syringe with 60cc of liquid, purging any air bubbles. Apply Vaseline to the deck surface where the Plexiglas plate will sit to create a seal. Then dispense into the plate hole until the entire area is filled. This may require you to reposition the fill hole over any trapped air bubbles to get an accurate measure. You can suck out the liquid with the syringe to reuse it and make cleanup a bit easier.

 

 

Record the volume remaining in the syringe for the cylinder and repeat for remaining pistons. Subtract the remainder from 60cc to get the volume.

 

My measurements came out in the 55 to 56cc range. I used 55cc for the next calculations.

 

To determine the volume of the unknown space we need to subtract away the volume of the cylinder above it. This was why we needed to know the depth earlier so we know the cylinder height. The bore of this motor is 104mm, which is the cylinder diameter.

 

h = .1in = 2.54mm

r = 104mm/2 = 52mm

 

Volume of a cylinder = PI * r^2 * h

V = PI * 52mm^2 * 2.54mm

V = 21,576mm^3 * (1cm^3/1000mm^3)

V = 21.58cm^3 (cc)

 

Now we can subtract out the extra volume from our measured volume.

 

55cc - 21.58cc = 33.42cc

 

33.4cc is the piston (dish) volume.

 

Additional Note: Make sure you do a good job of soaking up all the remaining liquid in the chamber, then recoat the cylinder bore with a light film of oil. Turn the engine over a few times to be sure you got it all.