I am building a 1950 model 7 from a "basket case". I know they were spec'ed at 6.7:1 compression ratio for the poor quality post war fuel available back then. My question is what compression ratio is suitable for modern American non-ethanol 91 octane fuel that is readily available here? I have a pair of +.020 pistons that are marked 9:1. Andover Norton lists oversize pistons, but they don't specify compression ratio on their website. I know I will have to "cc" the combustion chambers to really know, but what should I shoot for? Also, is it advisable to skim a little off the piston crown, or should I make a compression plate for the cylinder base?
Thanks in advance for any advice
-Jerry
Norton supplied standard and…
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Model 7 pistons
Thanks, David. I am going to measure the comp ratio by "cc"ing the combustion chambers, but maybe I am over thinking this. Maybe I ought to just assemble it and run it, and retard the timing as necessary. I just never had any experience with iron heads, my experience has been with Commandos...
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Dont think the barrel was…
Dont think the barrel was the cause of the compression change, the introduction of the later 1960 alloy head ( fitted in late 59} raised ratios by approx 1 atmosphere . Best to stay with low CR pistons with the iron head..If it were mine and i intended to ride it a lot i would fit the early alloy head from an 88 or 99 which although stayed low comp gave better cooling with hard valve seats and saved weight .Paint it matt black? ,who would notice?.
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If the 9 to 1 pistons are…
If the 9 to 1 pistons are spec for 60 and on engines its possible that they would only be 8 to 1 with the early heads of iron or alloy.
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Norton supplied standard and high compression pistons. In late 50's (I can't find the reference) the 500cc barrel height changed. The old "low compression" pistons now gave a single ratio to the high compression pistons in the earlier engines, and the high compression pistons now stepped up the ratio still more. If you have genuine Norton pistons, you won't be in trouble with a Model 7. USA octane ratings are of course quite different from European. I understand you have two grades generally available. A Model 7 should be happy on the lower grade fuel today, even with HC pistons. It's much better than British Post WW2 'pool' petrol. Model 7 seems to have been 6.7 or 7.8 according to Dominator Service Notes.