Fast Eddie
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- Oct 4, 2013
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Yes, conrod material matters cos alloy stretches more than steel. Piston weight matters cos extra weight equals extra momentum and thus stretch.I'm reading quite a variation on what people are recomending for the Piston to Valve clearance on the 850.
Jim Comstock if I read it right recommended 0.020". The Norton release notes on tuning the 850 to Combat spec 0.030 - 0.040 " but Mick Hemmings on the AN Technical Notes a massive 0.060" Things I imagine would alter the recomdations would be Conrod material an length, Piston weight and expected RPM.
I'm building a pretty standard 850 with a combat cam for the street!
Personally, I like to see .060” but I would suggest that .040” will be just fine, especially on a street motor that’s not gonna see stupid rpm.
I know that at .040” piston to head clearance in a Norton things never touch. So provided your valves are kept in control with good springs, I think you’ll be good.
You should check either side of TDC too though, it’s possible the inlet cam will be opening faster than the piston is descending.