Increase in valve clearance.

[John A]

The masters that cams are ground from get worn and as they age the discrepancy gets larger. P3 cams and Norris cams are noted for this. I learned by precisely timing performance cams in Guzzi’s when timing using the intake centerline method that the other lobes do not necessarily land in the same position according to the timing card. I talked to cam grinders who told me that the master cams become worn so I should either ignore the discrepancy or average it out and I would not be able to notice it. Some tuners use a fixture to map a cam before installing it in engines used for competition where a tenth of a second can result in a win or loss. I was on a funny car team and found a bad intake lobe on the cam when I was setting the valve clearance between runs. We didn’t have time to change it so we ran with it and set a track record and won the round because the track was slippery enough to make cars with slightly less power run faster.

[faffi]

I wonder if valve bounce from too high rpm could cause additional wear of the cam base circle in spots?

Anyway, the primary reason for valve clearance is to give the valve seats enough time to cool, especially the exhaust valves. Plus sufficient compression. The factory will give values that ensure that there is always enough valve/-seat contact to prevent burned valves as well as good compression. For normal use, clearance is not super-critical; a bit extra clearance will give a bit more noise and a little more low-rpm power, while a little less clearance will give less noise and a little more top-end power. If valve clearance vary a little during the closed phase, noting will happen as there is no load on the valve lifter or cam base circle.

[garbln]

Tappy valves are happy valves.

[Bulldog9]

Thanks for reply Acogoff, that's exactly what I'm going to do.
As clearance never seems to change I think it is what it is.

I'd be curious to know how much the clearance opened after TDC. I believe that was your initial concern.

[wirespokes]

I prefer the minimum clearance spec, not because of noise, but what the noise represents. The more clearance, the more the tappets are getting hammered. I'd like to keep that to a minimum. Pete Roper recommended .004 and .006 for square fins and a thousandth more for round fins and that's what I've been running.

[faffi]

The old Volvo OHV engines said 0.45mm of clearance, which is 1/64 or 18 thou. Presumably, they had "tamer" cam profiles, which would ease how the clearance was taken up before lifting of the valve began, but it is still a substantial clearance. Yet these engines would last "forever". Kawasaki, on the old Z1, listed clearance to be set between 0.05 and 0.20 mm IIRC, indicating that clearance is not critical in that engine. These engines also lasted a long time.

In my simple head, you would typically want more valve clearance with long duration cams in order to ensure that the valves stay close long enough. The cams on most Guzzis have quite short duration, though, so running the engine around the minimum recommended clearance should not hurt. Personally, I have stopped fretting about valve clearances; if the engine starts fine, idles fine, pulls well from low rpm AND does not clatter loudly, I do not bother to check the clearances. I do not run my engines hard, nor do we experience hot weathers, meaning the valve seats live a fairly sedate life. YMMV.