scott12180

Valve clearance on Lhead straight eights

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I've always been under the impression that as the engine gets hot, valve clearances would decrease. I've a lot  of experience with Franklins where you set valves cold, but admittedly never measured clearances both hot and cold. 

 

On my 32 Packard standard eight, I've been setting the valves to factory spec 0.004 with the engine hot and running. No easy job. 

 

With the engine stone stone cold this morning I see that all intake valves have essentially zero clearance and exhaust valves are 0.001 or 0.002. In other words clearances get bigger when the  engine is hot. 

 

Is is this the way it's supposed to be on L head straight eights? If so then shouldn't valves be set when the engine is cold?

 

--Scott 

32 Packard 902 5-p Coupe

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Interesting. Cast iron gray has coefficient of linear thermal expansion = 10.8 x 10-6 m/m.K. Pure Iron is 12 x 10-6 m/m.K. "Steel" is 11 to 12.5 x 10-6 m/m.K. Stainless steel ranges from 9.9 for Ferritic to 17.3 x 10-6 m/m.K for Austenitic (304) stainless, just in case one has installed modern stainless steel valves. Thus a steel valve say 175 mm long would expand 0.175 x 11 x 10-6 m/K temp. rise, = 0.00005"/K temp. rise, or say 0.0027" over cold to hot range.

 

So one might expect the valves to lengthen more than the block expands over the same length. Maybe the block gets hotter than the valves?

 

For interest, the Duralumin coefficient is 23 x 10-6 m/m.K.

Edited by Spinneyhill
typo (see edit history)

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I'd still appreciate any other ideas for what's going on, but this afternoon I set all valves to 0.004 COLD. I'll give it a good run and then check them hot. See where they are. If this is true then they ought to be more than 0.004. 

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2 hours ago, scott12180 said:

I'd still appreciate any other ideas for what's going on,

 

Me too!

 

This is what Dyke's said about it in their 18th Ed. in 1937.

IMG_0594.thumb.JPG.ae7871e0fc628d2feebf2eefea7c83ff.JPG

Edited by Spinneyhill
Added more information. (see edit history)

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I never "got" the hot setting either.

You mean the engineers didn't know how much expansion was going on?....... :blink:

Depending on the length of the stems and heights of heads I've always set them .001-.002" loose and never had a problem.

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I would set them cold with the engine off too. Much safer for one thing, and there should not be any noise from those small clearances. Much better than having any valves running without a clearance.

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I gave the car a good run to get it hot then checked the clearances with the engine running. 

The exhaust valves which I set to 0.004 cold were now generally about 0.004 or 0.003. 

Intake valves which were also set to 0.004 cold were 0.007, 0.008 and 0.009 hot. 

So the exhaust valves got a tiny bit longer hot --- clearances smaller--- but the intake valves got alot shorter when hot --- clearances bigger. 

Any ideas?

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I think the intake valves got looser because of the head.

Intakes are being continuously cooled by incoming air so are less prone to stem expansion.

 

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