Measuring combustion chamber volume

One very important aspect of a cylinder head is the compression ratio (CR) that it will result in. Of course, the compression ratio can be adjusted by adding or removing gaskets but this will also affect the squish clearance, which may not be desirable. When optimizing a cylinder head, gaskets should be selected to produce the desired squish clearance, the combustion chamber volume should be measured and the CR calculated, and then the head can be modified or replaced if the CR is too high or too low.

The basic procedure shown here to measure combustion chamber's volume is to bring the piston to TDC and fill the chamber with a measured quantity of oil until the chamber is full.

An alternative way to describe a combustion chamber is by measuring the pumping psi using a compression gauge.

What you need

  • Engine off the moped
  • Two-stroke oil
  • Small (~10 cc) syringe. Get it at a pharmacy. Remember 1 cc = 1 ml.
  • Tape

Step One

Set the squish clearance. Small changes in the squish clearance by adding/removing gaskets has a considerable effect on the CR.

Step Two

Bring the piston to TDC.


Step Three

Put the cylinder head on if it isn't already and remove the spark plug.

Head on no spark plug.jpg

Step Four

Fill the syringe with oil.


Step Five

Fill the combustion chamber with the oil until it reaches the top of the spark plug hole. The reason for filling to the top of the spark plug hole, rather than the bottom, is simply that it is difficult to tell when the oil level has exactly reached the bottom. This will be adjusted for later.


Step Six

Check the syringe to see how much oil was used and write the measurement down.


Take the head off, wipe out all the oil, and repeat steps two through six a few times to make sure your measurement is accurate. You might want to pour out as much oil as you can before taking the head off to minimize mess.

Step Seven

Now the volume of the spark plug hole will be subtracted from the volume measurement that was just taken. If you do not want to measure this volume, it can be approximated by a simple calculation. For reference, this volume on a common 14 mm thread by 12 mm spark plug will be about 1.8 cc.

To measure this volume, take the head off and put some tape over the spark plug hole (on the side that seals against the spark plug crush washer).


Step Eight

Fill the syringe with oil again and then fill up the spark plug hole and note this volume. Repeat steps seven and eight a few times to make sure your measurement is accurate.

SP hole oil.jpg

Subtract this second volume measurement from the first volume measurement to get the volume of your engine's combustion chamber. It should be noted that the volume between the spark plug's ceramic nose and metal threads has not been taken into account, and that this can be considerable in these small engines. The method outlined here should be reasonably accurate when using a surface-fire spark plug. If you are not using a surface-fire spark plug, you may try to measure this small volume of your spark plug by filling it with water or oil and adding that to the measurement found by following the steps outlined here.