Cam choice r60/5

[StephenB]

Exactly, thanks for your confirmation.

's time, Spring sets in so that I can start assembling that engine and to put theory into practice.

Stephen

[barryh]

I mentioned temporary thinking it might go some small way towards demonstrating that a 308 cam was a move in the right direction. Very close clearances on a permanent basis are obviously more risky than the wider than stock clearances that I use.

There is less risk of close clearances being a problem with the inlet valve and in fact it's the inlet opening time that has the biggest effect here, so you could safely close it up some.

I too think the hot clearances will not change much, so 2 and 4 thou should be plenty safe enough to demonstrate the effects.

Back in 81 when the 50HP R65 was introduced, the original 4 and 6 thou clearances of the earlier R65 were changed to 2 and 8 thou. Later in the March 82 service bulletin they were they changed again to the universal 4 and 8. I wonder if along with the other changes, the 2 thou inlet was needed to gain that last bit of power to get the engine up from the 45Hp of the earlier version. Why else would they have reduced the clearance on, valves sizes apart, an identical engine.

[daz]

Good morning. "I too think the hot clearances will not change much, so 2 and 4 thou should be plenty safe enough to demonstrate the effects. " says BarryH.

How about getting the engine totally hot then quickly adjust your valves to NO clearance? Tighten by hand and then lock in with finger tight ( none) clearance.

But I'd feel a lot safer with the Barry's 2 and four, betting that 1 and 2 would actually be safe. I'd be prone to check valve tappet clearance more often. Very interesting topic.

[StephenB]

I like that 2 and 8 very much ...

0.008 on the exhaust, just because the exhaust valve gets pretty hot so you might want to keep some margin for material expansion.

[barryh]

About exhaust valve temperature, there's a popular train of thought that it's best to to keep the exhaust valve clearances wide because the valve will run cooler due to spending more time conducting heat to the seat. The numbers don't stack up to this being a big deal as on an R60/5 the exhaust valve is shut for nominally 540 degrees of crank rotation and open for 180 degrees. So reducing the time spent shut by a couple of degrees is only going to make a small fraction of 1% difference.

[SteveD]

Interesting read. Anyone remember what GSPD recommended? He was in the tighter corner wasn't he?

[Major Softie]

How about getting the engine totally hot then quickly adjust your valves to NO clearance? Tighten by hand and then lock in with finger tight ( none) clearance.

Which will give you a safety factor of: zero. Zero is not very much.

About exhaust valve temperature, there's a popular train of thought that it's best to to keep the exhaust valve clearances wide because the valve will run cooler due to spending more time conducting heat to the seat. The numbers don't stack up to this being a big deal as on an R60/5 the exhaust valve is shut for nominally 540 degrees of crank rotation and open for 180 degrees. So reducing the time spent shut by a couple of degrees is only going to make a small fraction of 1% difference.

Not really. What matters is not so much how long it is closed, but how long it is open. It's not just that the valve can transfer heat while closed, but that it is while exhaust gasses are speeding past the valve edge that it is being superheated. So, while opening (for example) 2 degrees later and closing 2 degrees sooner might only increase the time closed by less than 1%, the valve edge is being heated 2.2% less time.

Of course, we all like power, and having that valve open for 2.2% less time can't help but have some negative impact on power. R60's are not known for having lots of power to spare.

[daz]

Major Softie good evening. What a crazy idea! I totally agree one needs a safety factor. But wait. Valve tappet clearance is set to allow for expansion so that the valves can fully close when hot. No? So why not set the valves when really HOT? And maybe give it a bit like .001 inch. Obviously I don't know much. So I must be wrong. But I think I'd give it a try and then measure the clearance cold. Hell, I guess it would turn out to be on spec. Totally speculation but it doesn't cost much and is worth the same.

[Major Softie]

Major Softie good evening. What a crazy idea! I totally agree one needs a safety factor. But wait. Valve tappet clearance is set to allow for expansion so that the valves can fully close when hot. No?

No. It is set to allow for expansion so that there is no chance of it leaving the valve too tight, as in: no clearance. So, the goal is not for to close it to zero, but that, no matter how hot the engine gets (within acceptable running temperatures) that the clearance never EVER closes completely.

OTOH, Some have said here (it may have been Duane) that Airhead clearance changes little, if at all, from cold to normal running temps. So the most important value of the "safety factor" is to protect in the case of overheating and (probably much more commonly) clearances changing between adjustments.

[daz]

Major:
I said "allow for expansion so that the valves CAN fully close when hot".
My mistake. I meant CAN'T fully close.
That changes things. See what I mean? I am agreeing with you about the safety factor. And I'm saying if you did do it that way the valve tappet clearance would prolly end up close to spec anyhow.
When I was young, I used to say "Noisy valves are happy valves" about my VWs.