Valve to piston clearance??

I am wondering what the more experienced engine builders an tuners among us would recommend as the minimum clearance between valve and piston?
I am just doing the putty on the piston then assemble an turn over strip an measure (as best i can) technique. I am thinking of trying a thin head gasket and maybe not fitting a base gasket also. The engine is an 850cc with big valve head an PW3 cam and i think it needs more compression to make the cam work as it should. The reason for going the thin gaskets route is that the pistons an bores are new (shame not to get some use out of new pistons) and as nothing is machined off head or barrels if it doesn't improve the performance or makes things worse it's just matter of replacing normal gaskets.

Minimums - .040 exhaust and .020 intake

I should add -this is with full length pistons. Jim

Jim is cuttin' it close but I agree. Be sure to check your valve clearances frequently on breakin, you don't want a receding valve (or whatever else might go wrong in a fresh valve train due to, say, improperly heat-treated pushrods) to cause a reduction in those clearances.

If you are running big valves double check the clearances between the valves and the edges of the valve pockets on your pistons. You might need to enlarge the valve pockets a bit.

Skip

Thanks. Yes they are full size pistons with standard compression (i was worried about going to high and having starting issues. Now wishing I'd gone higher :roll: ) with pockets. When checking the clearance i closed the valve clearance to nothing to hopefully give me the worst case scenario as it were in terms of lift at TDC. There is 1mm at inlet and more at exhaust but have not removed base gasket yet only head gasket. Inlet valve to edge of its pocket is the issue i have there is plenty of depth but the if the pockets were just a little closer to edge of piston I'd have much more space and nothing to worry about.

Am I missing something here? If Toppy is using standard 850 pistons, there's no valve pockets. I'm wandering along a similar path with my engine, since the Wiseco high compression pistons are currently not in stock. My plan is to try no base gasket & JS's 0.030" head gasket to see if this will work with the PW3 cam.

My cases are currently with Steve Maney to get the cam bushes sorted - I gave up on getting them done locally.

SteveBorland said:

Am I missing something here? If Toppy is using standard 850 pistons, there's no valve pockets. I'm wandering along a similar path with my engine, since the Wiseco high compression pistons are currently not in stock. My plan is to try no base gasket & JS's 0.030" head gasket to see if this will work with the PW3 cam.

My cases are currently with Steve Maney to get the cam bushes sorted - I gave up on getting them done locally.

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They are NOS +20 pistons with standard CR but have been pocketed. I got them from Mick Hemmings when he did the head an rebore work he advised Wiseco higher CR but i was worried about starting issues like i said. I should have took his advice i know but to late now.

I was also thinking of trying Jim's 0.003" head gasket (a used standard type measured 0.030" Steve) and with my admittedly crude measuring of pocket volume (1cc per piston) added to my equally rust maths of rebore volume an such i think the 0.003" gasket would give maybe 9.5 CR and if no base gasket was fitted also 9.9CR. Does that sound about right to others and do they think nearly 10 to 1 CR would be to high for road bike on normal fuel (good stuff but available from ordinary petrol station) not race or aviation fuel.

toppy said:

SteveBorland said:

Am I missing something here? If Toppy is using standard 850 pistons, there's no valve pockets. I'm wandering along a similar path with my engine, since the Wiseco high compression pistons are currently not in stock. My plan is to try no base gasket & JS's 0.030" head gasket to see if this will work with the PW3 cam.

My cases are currently with Steve Maney to get the cam bushes sorted - I gave up on getting them done locally.

Click to expand...

They are NOS +20 pistons with standard CR but have been pocketed. I got them from Mick Hemmings when he did the head an rebore work he advised Wiseco higher CR but i was worried about starting issues like i said. I should have took his advice i know but to late now.

I was also thinking of trying Jim's 0.003" head gasket (a used standard type measured 0.030" Steve) and with my admittedly crude measuring of pocket volume (1cc per piston) added to my equally rust maths of rebore volume an such i think the 0.003" gasket would give maybe 9.5 CR and if no base gasket was fitted also 9.9CR. Does that sound about right to others and do they think nearly 10 to 1 CR would be to high for road bike on normal fuel (good stuff but available from ordinary petrol station) not race or aviation fuel.

Click to expand...

Toppy, I run 10.5:1 in my 850 and it runs fine on 'super unleaded' etc. Yes it takes a hefty swing, but generally starts first or second kick, so in summary, starting and running are no problem. And it goes like stink!

I believe that the Norton head design means that at that CR the combusiton chamber shape is nice and compact and the squish works well, thus allowing a higher CR than would be possible on something like a Triumph for example.

Ref your own plans, I don't think something adds up correctly. I run JS pistons which have crowns .110" taller than stock, and that's what gives the CR (and nice tight squish).

If I recall correct, a fitted head gasket is around .030". A base gasket is less than .020" (maybe much less depending on type of paper used). So, you are probably only going to remove max .050" and if so, you'll still be a good .060" more gap than I have, so I would guess you will be somewhere a little over 9.1.

For ref, if you're on a + .020" bore, I believe every 1mm (or .040") removed takes approx 4.7cc out of the equation.

My own 850 had a base gasket fitted when I bought it, I measured the CR and it was 7.6:1 with stock pistons and no machining of head or barrel, which is quite pathetic really I feel. The reason I add this info is that it is wrong to assume that you're starting from a base of 8.5:1 as was claimed by the factory!

Personally, I can recommend a nice high CR if that's the kind of ride you're looking for. Which leads me to also say it is NOT too late, you could easily sell your unused + .020" pistons and put the dosh towards some nice Wiseco's ...

Just sayin' !!

Nice to know thanks Eddie. I do release that the CR figures as quoted by Norton are shall we say optimistic.
I am just trying to work out the highest CR i may get as a worst case scenario for running on pump fuel.
I have the Roadster tank so range would be an issue if special fuel or additives are needed to stop the engine literally going like a bomb

toppy said:

They are NOS +20 pistons with standard CR but have been pocketed. I got them from Mick Hemmings when he did the head an rebore work he advised Wiseco higher CR but i was worried about starting issues like i said. I should have took his advice i know but to late now.

I was also thinking of trying Jim's 0.003" head gasket (a used standard type measured 0.030" Steve) and with my admittedly crude measuring of pocket volume (1cc per piston) added to my equally rust maths of rebore volume an such i think the 0.003" gasket would give maybe 9.5 CR and if no base gasket was fitted also 9.9CR. Does that sound about right to others and do they think nearly 10 to 1 CR would be to high for road bike on normal fuel (good stuff but available from ordinary petrol station) not race or aviation fuel.

Click to expand...

Interesting - I did some similar calculations and with a standard, non skimmed head and 0.020" oversize pistons, I calculate a C/R of around 8.4 with no base gasket and a standard head gasket, just over 9 with Jims super thin 0.003" gasket. This is using a combustion chamber volume of 51.8cc (both of mine are very close to this figure).
When did you get these pistons from Mick? When I spoke to him about a month ago, I rather had the impression that he has stopped doing machining work, being semi retired now?

I got my work done a couple of years ago but many of life's no Norton related problems got in the way and progress on the bike was none existent for over a year. Mick and his ever charming wife Angela were most helpful and understanding they even stored my engine parts for over a year rather than risk damage in the post. I know Angela reads things on this site from time to time and so if she reads this i wish them both well and hoe they are enjoying their new life.

I do expect my CR to be more like 9 but as i said the calculations were rough i had to make a few guesstimates. The idea be more to get the highest possible ratio just to check it was not to bad an the real ratio would be lower but better than what i have and still ok for road use on pump fuel.