1966 427/425 valve lash spec is .024 for the exhaust valves when hot. If I were to set as specified hot and then recheck lash when cold (say .026 because cold would contract) would I then have a reliable cold lash setting that would allow for thermal expansion back to the correct tighter hot lash number ?
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Re: 427 Valve Lash: Cold vs. Hot
Measure lash cold and hot and idling and compare your results. My guess is that you will not see a signficant difference, which is the case for my SB. On iron block/head engines the thermal expansion rate of the steel pushrod is about the same as the cast iron, so there is not a signficant difference between cold and hot.
The L-78 lash specs are .020"/.024", and I recommend these for later SHP big blocks too (They have the same lobe.) Even though both inlet and exhaust lobes are IDENTICAL, including the ramps, the exhaust spec is looser to allow for exhaust valve stem expansion when the valve gets very hot when running hard. At idle the exhaust valve runs relatively cool, so valve stem expansion is mild. Expansion will increase at high load, but there's no way to measure valve lash at high load.
If you want the proper procedure to check cold, e-mail me and I will send you the instructions. Crank indexding is VERY IMPORTANT to ensure that you are on the base circle when checking and setting lash.
Duke -
Re: 427 Valve Lash: Cold vs. Hot
Peter,
The correct valve lash for your 425 HP 66 is .024 intake and .028 exhaust and are to be used with the engine "hot" and running. Also, it's important to understand the meaning of "hot" when these specs are used. If you just run the engine long enough to get some movement on the temp ga., that's not normal operating temp. An engine has to be at operating temp and that means when the oil temp has stabilized, not the coolant. This is about 20 minutes at idle or after a cruise around town.
If you want to know the difference between "cold" and "hot" lash, run the engine up to temp and set at least one pair of valves while running. Then allow the engine to cool to room temp and check the actual setting on the two valves that were reset. From that, you can determine the amount of expansion that is taking place during the warm up and you will have an accurate setting for cold engines. My 66 425 HP car setting changes to .025 int and .030 exh when the engine is cold and these are the numbers I use. Any setting less than these numbers will start to decrease power and anything larger will eventually start to break parts in the valve train.
MichaelComment
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Re: 427 Valve Lash .004 Difference ?
Michael
You mentioned "correct valve lash for your 425 HP 66 is .024 intake and .028 exhaust" hot. Both my original owners manual as well as the 1966 Chassis Service Manual state valve clearances at .020 intake and .024 exhaust hot. Where does the .004 difference come from ?
PeterComment
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Re: 427 Valve Lash .004 Difference ?
Peter,
The 66 manual does show .020 and .024 but these were actually supposed to be .024 and .028. The manuals for 67 and later show the .024/.028. There was a great deal of confusion about all of this that started in 65 with the specs for the 396, although it was the same cam and basically the same engine. If you look in a 66 Corvette News from the era, it incorrectly shows .022 and .024. I'll try to dig out some of the paperwork on this.
MichaelComment
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Re: 427 Valve Lash .004 Difference ?
GM changed the specs on this and other cams and i would guess to get them to idle smoother. i can tell you from my work with these engines at the drag strip the cars have more top end power with the tighter settingComment
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