Compression Ratio revisited

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Dec 22, 2006
A couple of weeks ago this topic came up and it was decided that the MK 111 had an actual compression ratio of 7.9 to one, give or take, not the 8.5 to one stated in factory specs.
While I had the head off, I accurately measured the cubic volume of the RH4 head with it inverted and set level on my work bench. After filling the head with varsol three times to be sure, I determined that it's volume is 49cc. A full 50 CC fill causes the surface tension to break and the varsol to run out onto the milled gasket flat, but at 49CC the fluid all stays in the combustion chamber, so 49 it is.
The piston comes up to within .020 of the cylinder top at TDC, plus the firering head gasket is .030 thick , so these two factors add 5.9CC to the combustion chamber volume.
Total Combustion Chamber volume then is 49 + 5.9 CC = 54.9
Cylinder capacity plus Combustion chamber volume = 828CC/2= 414CC+54.9=468.9 CC
468.9/54.9 = 8.54 to one CR, pretty much bang on the stated number.
During its first rebuild of my mk III in 1980 I calculated the cr and mine was 7.8:1. My brain has muddled some since then and I cannot find any notes, but as I recall the primary difference in your figures is that the pistons on my bike were about .060" from the top of the barrel at TDC (vs your .020" reading). This difference amounts to right at 5cc's which makes up the variation in cr readings (7.8 vs 8.5).

Since I was intent on following Brian Slark's "performance mods" in the Clymer manual at the time, I had both the barrel and head machined .060" to bring the piston even with the top of the barrel and the cr up to 10:1.

I suspect that mine was typical of a production flaw that can be traced to a series of bikes delivered from production. Given that this difference would seriously deteriorate the swirl effect of the squish band, I suspect that performance would also be negatively impacted rather badly on these bikes in addition to the effect of the lower cr.

Still have the bike which was again recently rebuilt frame-up ala cafe.

Dave Winship
Austin, TX
In Steve Wilson's 'Norton Motor Cycles' book referring to the new 850 models it says:

"These [850] engines were claimed to have a modest 8.5:1 compression ratio, but as Norton troubleshooter Bob Manns later told Which Bike? [UK motorcycle Mag.] 'none that I ever checked did! (this was usually borne out by others who have checked, usually to find the standard ratio was 7.7:1.)'..."

I believe the slightly thicker (0.040" = NOC notes) copper head gasket would likely have been used then, during the check?
The copper gasket would drop the ratio to about 8.4 to one in my case. My bike may have had the head milled or perhaps the cylinder milled, that may be the reason it measures at 8.5 to one.

A 60 thou piston top to cylinder top distance instead of the 20 thou distance which mine measures would also account for the difference.
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