850 MK II Combustion Chamber Volume?

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Not sure if this helps but this guy shows how to actually measure volume of a cylinder chamber on a classic mini head, for determining precise CR's etc:

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That’s the best way to measure combustion chamber volume, needed for balancing combustion chamber volumes etc. But it doesn’t take into account pistons, head gaskets etc, so isn’t a compression ratio measurement.

The best way to get a baseline before skimming is to actually measure in situ.

I do this thus:

Get the cylinder to be measured at tdc on the compression stroke (valves closed).

Lean the bike (or engine) over and support it so the spark plug is pointing as vertical as possible.

Carefully measure the amount of oil required to fill the combustion chamber, fill the oil to a coupe of threads into the plug threads to allow for the holow part of the plug around the insulator.

Thats your combustion chamber volume (CCV)

Static compression ratio is calculated thus: (swept volume + CCV) / CCV = compression ratio.
 
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Fast Eddie said:
One of us has got this wrong then, my understanding is:

(414+43.58) / 43.58 = 10.5:1

whereas:

(414+55) / 55 = 8.5:1
Click to expand...
When I set my head up, I seem to recall measuring about 53-odd cc for the Combustion chamber volume.
The only way to do this correctly is to measure your own chambers - there's so many variables when dealing with these old dinosaurs. Who says that your head has never been skimmed for example?

I went on much the same path as Fast Eddie (but not as extreme) with the JS high compression piston & rods, and ended up at around 10.5:1 Am very happy with the results. JS also does a very thin head gasket which also looks good - bought it but never used it, I went with his high compression pistons instead.

Lots of good info in FE's build thread though, it cost me rather more than anticipated - he's a dangerous man, FE is :)
 
FWIW, here are actual measurements I made of combustion chamber volume in various Commando cylinder heads (in the head only, not including piston, head gasket, etc.) back in 2009. I've highlighted in yellow the ones that might be of interest to you.

Many of these heads have been modified a number of times, including milling the head surface, installing different size valves, and opening the counterbore for larger bore sizes.

Jeff Law’s 750 head converted to 920. Milled and counterbore machined – 44.6 cc.

Steve Shivers’s 750 head converted to 920. Milled and counterbore machined – 45.8 cc.

Head from Marc Field’s Dunstall 810 engine. 850 converted to 750 bolt pattern, milled. – 42.4 cc.

850 head from Jim Schmidt’s race engine. Milled. – 42.4 cc.

Axtell 30 mm port 750 head – 51.0 cc.

Axtell 32 mm port 750 head – 48.8 cc.

Stock Mk3 850 head – 51.6 cc.

Mk3 850 head milled .060” – 43.2 cc.

Stock Mk2 850 head – 51.8 cc.

Short Stroke 750 head – 55.2 cc.

Another Short Stroke 750 head – 52 cc.

Ex-factory Flat Track Short Stroke 750 head as removed from John Hately’s bike – 57.3 cc.

Ex-factory Flat Track Short Stroke 750 head after milling – 50.7 cc.

Nominal factory spec for standard RH4 850 head is 50 cc, and for RH7 short stroke head is 53 cc.

Ken
 
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