Measuring Cylinder Volume, Day 2

Today I got started by getting a more suitable hose for the size of the spark plug hole, as well as an O-ring and a larger capacity measuring device that goes up to 60ml instead of just 25 as before. These new items should allow for better measurement and a good seal to prevent oil leaking from the spark plug opening as I pump it in and out.

I firstly drained any remain oil from the engine and then removed the cover that housed the crankshaft end bolt as well as the hall effect crank position sensor and the stationary portion of the magneto. This would allow the slow controlled turning of the engine to pump oil in and out of the cylinder. Next I removed the timing gear cover to disconnect the cam shaft from the crank shaft and keep the valves closed while the piston moved up and down. I turned the engine until the valves were closed and the timing marks lined up. Then I went to remove the 3 bolts that hold the cam gear to the camshaft only to find they were stuck. After freeing up one I stopped because the bolt heads starting rounding. Tomorrow Amber will bring some different sockets to try again.

Initial supplied volume @ TDC: 60 ml
Initial measured volume @ TDC: 41 ml
Clearance + Hose volume @ TDC: 19ml
Measured volume @ BDC: 3 ml (valve opens)
Swept volume: 38 ml
Hose volume: 4 ml
Clearance volume: 15 ml
Compression ratio: 3.53:1


In the mean time I attempted to measure the cylinder volume just by moving the cylinder from BDC at the end of the intake stroke, through the compression stroke, TDC and power stroke to measure the volumes.
Initially, 60 ml of oil were fed in to the cylinder. With the hose volume and clearance volume, the syringe measured 41 ml, meaning the clearance volume and hose volume added up to 19 ml. After several attempts, I found that the intake or exhaust valve would open when the syringe measured 3 ml, allowing air in to the cylinder and oil out.

I measured the hose capacity by starting with 19 ml in the syringe and pumping through until oil was produced at the other end of the hose which occurred at 15 ml. This meant the hose capacity was 4 ml, and the clearance volume was 15 ml.

With a clearance volume of 15 ml and a swept volume of 38 ml, the total cylinder volume is 53ml with a compression ratio of 53/15 = 3.53:1.

I do not trust the accuracy of my measurements. I will conduct the process again tomorrow and ensure I have a very good seal on the spark plug hole. I will also attempt to stop the valves from actuating in order to get a more accurate measurement. My understanding is that the engine is supposed to have a displacement (swept volume) of 48-50cc, so 38 is quite low, as is a compression ratio of 3.53:1.

A 48cc engine with a clearance volume of 15 ml only has a compression ratio of 4.2:1 which is also quite low. Therefore I will also get a more accurate measurement of the clearance volume by ensuring the engine is positioned precisely at TDC when the first measurement is taken, and again restricting any oil leaking and air entering from the spark plug and valve openings, and from the head gasket too.