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Things EXPAND when they get hot. My e-mail frightens me. If you set your lash cold, ADD 2 & 3 thousandths to hot specs DO NOT subtract! After a cold setting you should bring the engine up to temperature and measure what you wind up with. You may want to compensate say to 0.003 and 0.004 or whatever. Each engine is a tad different and if you do this once you'll know what your cold specs are for future lash adjustments. The factory hot lash specs are what you want to achieve and this is a very good short cut. If you go the wrong way by too much, you'll burn valves! (or break one of your aluminum rocker arms in half).
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Re: Cold Valve Lash
Dale - I've measured the valve clearance both hot and cold, and it doesn't really change. The cast iron head and steel valve stems have nearly the same thermal expansion coefficient, and since most of the valve heat is transferred to the jacket via the valve seat, the stems don't get as hot as you think. Of course, "hot" means idling, not WOT and 6500 revs. Specified clearances are a nominal values and there is allowance for running changes due to different operating conditions. It's important for the running clearance to be taken up before the end of the clearance ramp. Otherwise everything will bang together at higher than clearance ramp velocity and shock load the valve train. Same thing at closing. If the valve is set down on the seat at higher than ramp velocity, it can bounce.
Another thing to consider is the actual rocker arm ratio. It's not 1.5:1, but more like 1.37 at the end of the ramp and 1.44 at max lift on a small block, based on my measurements. Chevrolet's recommended clearances are based on the valve clearance being taken up at the end of the ramp with a 1.5 ratio. Ain't true, so you should set the 30-30 and LT-l cams a couple of thou tight, cold.
The original Duntov's were speced at .012/.018, but in an article in Corvette News the inlet was recommended to be reset at .008" for "weekend competition events". Later this inlet clearance was specified for the 327 because their longer stoke made the Duntov relatively milder and a bit more effective duration helped top end power. So all the late Duntov's were already running "tight" on the inlet, but I'd set the exhaust at .016" cold to compensate for true rocker geometry.
Duke -
Re: Cold Valve Lash
the best way to do this is set one cly hot, then let it set over nite and in the morning and see what the clearance is. this will be your cold setting. alum heads with a iron block set .004 tighter and all alum motor set .010 tighter as the alum will grow and increase the clearance. these are for start up only.
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Re: Cold Valve Lash
This approach sounds best to me. Maybe measure two or three cylinders. As I wrote before, every engine is a tad different. I like bottom end torque so loose settings and the racket that goes with it is fine in my book. If I was gonna race and needed high RPM then I'd tighten things up. Forrest Shropshire, who has "the world's fastest 56 Corvette" runs Bonneville with 0.001 and 0.002 clearances. He rebuilds his engine frequently. On second thought, your approach is appealing but I'd measure all the cylinders cold the morning after. Don't forget #5 & #7 have "special" thermal properties.
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