Acceptable Piston to Valve Clearance?

Nourish said:

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!

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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.

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.

Nourish said:

But how deep to machine the piston pockets? I'd need to know the piston to valve clearance, I might not need to machine the head to increase the compression if I could minimise the gaskets!
I'm thinking somewhere in the region of 9 to 9.5 CR would be ideal for road use. In the UK 5 star fuel was available when the Combats were sold - sadly not so available now.

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Yes, trial assembly required. And lots of potential frustration. It is the one measurement I cannot find on the drawing, but I can see why not. Too many variables not under the control of those providing the guidance.

When I do things which change valve timings or lift, I usually put plasticine when the valve cutaways are on the crown of the piston, then put the cylinder head on and rotate the crank, so the piston passes TDC when the valves are open. Then I remove the head and section the plasticine with a sharp blade, to see what clearance I have when the valves are at maximum lift. Half a millimetre is usually sufficient, if you do not use a dramatic cam. As the cam lifts the valve, at max lift the valve probably becomes airborne, the closing side of the cam catches the valve assembly, so the cam closing rate is important. A closing rate which is too rapid can break valves. Some cams are valve-droppers.