Valve gap: what happens to it after engine warms up?

[racing97]

I have measured the gap cold at .004 and hot at .007 many years ago


regards

[Wil Ferch]

Two comments....

1.) It amazes me that someone posts "categorically" that it goes one way.....when in fact it goes the other way. Leads to dangerous lasting posts for the archives for others to depend on....glad to see others responding who have actual experience stating which way it truly goes ( opposite).

2.) Spec has always been 0.004" ( actually 0.1 mm if you give more credence to the metric homeland measurement, or 0.0039"). Look under many 911 engine lids on the emissions sticker.....there is a lot more acceptable "give" than we think. The spec stated there is 0.004", (+/-) 0.002". So it seems the acceptable range is 0.002" to 0.006"

Lastly, my feeling is this. Designers are often forced into a situation where a numerical measurement has to be given for spec purposes, but that it may not be "critically" important. This may be one of these cases. The core concept is that there should be a "scooch" of air-gap clearance measured cold...and that it cannot be "closed". Then the fun begins....."what number should we show as "spec" ... ???

Yeah, yeah.... I know, tolerancing closer to open vs cold affects true valve timing overlap/lift/duration...and that it directly affects that last 1 hp you may be chasing....but I don't think we're talking this point here.

[sc_rufctr]

And then someone invented hydraulic lifter that run at zero clearance.

[dshepp806]

the gap will widen a small bit after it heats up nice and toasty,..that's been the info I've rx'd from the P-wrenches I know.

Best,

Doyle

[scarceller]

I know that some 24hr race engines widen the valve lash on the exhaust valves. I think this question could have different answer for exhaust vs intake valves. In a race engine running under very hi-loads the exhaust valve surely heats up. I suspect that a super hot valve will measure longer than a cool one. But that's just a guess.

[racing97]

Porsche usually mandates lash settings of .006 inlet and .008 exhaust Cold settings

[Ronnie's.930]

Originally Posted by racing97

Porsche usually mandates lash settings of .006 inlet and .008 exhaust Cold settings

Say what?

[racing97]

Sorry Bourbon fingers forgot to say on racing engines from 906 to current Cup .006 on intake and .008 exhaust. cam ramp+ expansion


regards

[island911]

Originally Posted by Wil Ferch

Two comments....

1.) It amazes me that someone posts "categorically" that it goes one way.....when in fact it goes the other way. Leads to dangerous lasting posts for the archives for others to depend on....glad to see others responding who have actual experience stating which way it truly goes ( opposite). ....

Yeah, and the "hot check" info is really great. ...but what if the hot measurement doesn't tell the whole story?

We are talking engine shut down measuring of valve lash, of a heat-soaked engine, right? That, however, is different than measuring the relative thermal expansions (the gap) when that exhaust valve is punching thru exhaust gasses at full load and 6000rpm.

All I'm saying is there are transient temperature gradients which you won't see with a stopped engine and check of the valve gap. ...the thermal mass of the valve is small (compared to the cylinder head)

[island911]

I should add... That minimum .002" would not be specified if valve lash "always increased with temperature." Because IF "valve lash always (and only) increased with temperature" then you could simply adjust them to net zero (cold) and not immediately burn valves on first run. --I expect that valves would burn pretty quickly with zero gap. ..don't try this at home.