Valve to piston clear

I am just dialing my PW3 cam in with my new big valve head.
At TDC I have piston to valve clearance of left pot 0.082" and right pot 0.075". I understand that 0.080" is desirable but that 0.60" is adequate?

What I have noticed as I measured the distance at various points is that at 3* to 5* ATDC the gap is marginally less than at TDC by about 0.010" per pot so say at & 5 ATDC the left pot clearance is 0.073" and the right pot 0.065".

Do I understand correctly that once past TDC the piston to valve clearence is less important as the piston is starting to move away from the valve therefore I shouldn't need to worry if I miss a gear change?

The obvious solution if I want to irradicate concern is to have some small pockets machined into the pistons for the inlet valves. Am I being illogical? Opinions please.

When I put my 4S cam in, many moons ago, I used an old valve & ground cutting teeth on it. Put the head on without the gasket, & cut the pockets deeper. Great fun. That was with high comp. pistons & standard valves.
Still got the same pistons with a PW3 now. I presume I checked it, remember checking the valve springs.
If you have got std. pistons, suppose you could just dremmel where the edge of the valve is nearest, high comp. pistons will take a bit more cutting. Yes .060" is the min.
I suppose that if the valve is going to carry on with it's momentum after it reaches the top of the cam lobe, then ATDC don't seem bad.
Have you thought about reducing the valve O/dia., say about .015", or a little bit off the face.

I think your clearances are OK. I remember when 1mm / 0.040" was the minimum, but that was on expendable Triumphs. I'd call Mick Hemmings, he's a full bottle on PW3 and race specs.

Mick

I've always run .040" with a PW3. And I do what I can not to miss shifts!

Re: Valve to piston clearance

Flo wrote;

I used an old valve & ground cutting teeth on it.

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A very good idea, unfortunately I don't have any spare old oversize valves or I would have maybe had a go at this.

Homeslice wrote;

I've always run .040" with a PW3. And I do what I can not to miss shifts!

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I'm probably being a bit paranoid. My clearances are just within the spec, I suppose it's just a mental thing. If the minimum clearance is 0.060", I'd like to see 0.100" just to feel that I am very safe.

ML wrote;

I'd call Mick Hemmings, he's a full bottle on PW3 and race specs.

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I have spoken to Norman White proir to doing the specific measurements, and it was he that said preferably 0.080" preferably and a minimum of 0.060".

Thanks for the replies, I'll sleep on it. Common sense says leave it as is, but my illogical head says put some very small cut outs in so that I can rest easy.

I've just spoken to Norman White and he has advised making 0.20" inlet valve cutouts . I think that I will do this so that I don't feel nervous every time I miss a gear or get up around the 7K rev limit.

Thanks again for the advice.

Valve to piston clearance does also relate to the type of crankshaft and conrods. For instance using a Nourish one piece crank and Carrillo rods my Domiracer will do with 0.040" but using a standard 3 piece Norton crank (more deflection) and aluminium rods (more stretch) I would do 0.060" minimum.

Reggie said:

I've just spoken to Norman White and he has advised making 0.20" inlet valve cutouts . I think that I will do this so that I don't feel nervous every time I miss a gear or get up around the 7K rev limit.

Thanks again for the advice.

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Just rev it to 8k and make instant pockets. :mrgreen: