Compression ratio

[Bill Cannon]

Does anyone have advice on best method to measure cylinder head volume without removing cylinder head please?

Bill

 

[clevtrev]

Put in place a spark plug with no central electrode, put a condom over the plug, turn over engine to fill condom, remove and knot without losing any air, (I know you must have had some practice) immerse in a jar of water, the displaced volume is what you require.

 

[Bill Cannon]

Gave up condoms many years ago (snip, snip) but sure I can find a party balloon somewhere. Good idea, thanks Trev.
Bill

 

[b'knighted]

Won't that give the swept volume rather than the cylinder head volume?

 

[bmetcalf]

Subtract 499cc?

 

[b'knighted]

Surely that is the original swept volume?

 

[Bill Cannon]

Yes, of course, b'knighted, you are right! Back to the drawing board.
I've had a suggestion on JTAN to use fork oil. I guess I could suck it out after with a fine tube on the syringe.
Bill

 

[timetraveller]

I have only done a careful measurement of one Vincent cylinder head and the result was 103.5 ccs. That was done with a calibrated pipette and a thin piece of Perspex.. Books on tuning will explain what to do if it is not clear. That, of course, needs the head to be off. What is it that you really want to know? If it is the compression ratio then compression testers are now only a few pounds. If you can gain access to a calibrated pipette, or similar, then you can use thin (5 grade oil) and put it down the plug hole ensuring that the valves are shut and the engine/bike is leant in the correct direction to ensure that the plug hole is exactly at the top of the combustion chamber. Fill to half way up the plug thread. Given the above figure of 103.5 ccs and the swept volume of 499 ccs you can see that a flat topped piston that did not protrude into the cylinder head would give about 5.8:1 compression ratio. That is why pistons have 'bumps'. The Terry Prince heads have a much smaller cylinder head volume.

 

[clevtrev]

Surely that is the original swept volume?

Only joking, oil could be used, it would drain through overnight.

 

[Bill Cannon]

The original question is motivated by my ongoing problem with starting. I can't kick the bike over compression nor will the electric starter get it over compression.
I recently remembered that when Terry Prince supplied my 1200 kit he was out of stock so he converted a 998 kit. So I want to check that the compression ratio is close to the nominal 9:1 that it should be, so great accuracy is not essential.