I am in need of the specs for the adjustment of the valves on my 61' 270 h.p. soid lifter motor. I adjusted them cold on the engine stand. (I) 8 thousandths and (E) 18 thousandths. Is there a different setting when I adjust them while the engine is running ? Thx Phil
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Re: Solid lifter adjustment
Phil-----
The specification "hot and running" is .012" intake and .018" exhaust.In Appreciation of John Hinckley -
Re: Solid lifter adjustment
Chevrolet's specification is per Joe's post, however, having lived with a Duntov cam for over 100K miles I made the following observations. First there is no measureable difference between hot clearance and cold clearance, which jibes with the fact that the steel pushrod and cast iron case/head expand at approximately the same right - slightly different, but very close.
Second, the top of the clearance ramps on the Duntov cam are .008" above the base circle for the inlet lobe and .012" above the base circle for the exhaust lobe. Multiply these numbers by the specified 1.5:1 rocker ratio and you get the .012"/.018" clearance specified by Chevrolet for the Duntov cam up to 1962, HOWEVER, based on my detailed measurements of SB rocker arm geometry, the ratio is neither 1.5:1 nor is it even constant! For a typical production machined engine it starts out at 1.37:1 just as the valve begins to lift off the seat reaching a maximum of 1.44:1 at full lift in the range of .400" to .450" which is the range of maximum net valve lift for OEM cams.
During engine operation we want the valves to be closed AFTER the lobe has reached the constant velocity clearance ramp so the valve is moving at ramp velocity as it seats. This means that the maximum RUNNING clearance we want to see is .012 (1.37/1.5) = .01096" for the inlet and .018 (1.37/1.5) = .1644".
Because the valves will be better off with clearance a bit too tight than too loose I recommend rounding these numbers down to .010"/.016", and the vavles can be set to these clearances with the engine cold. Further, the clearances on the Duntov cam may be set with the cylinder at TDC of the firing stroke, but this does NOT apply to the 30-30 or LT-1 cams. These two cams require special a special (but simple) engine indexing scheme to assure that the lobes are on the base circle. (E-mail me if anyone needs the instructions.)
With the engine idling or "hot" just after shutdown, the exhaust valve is relatively cool, and since about 80 percent of heat trasfer from the valve is through the seat (Taylor's IC engine book), the stem does not expand that much. Hard running will heat of the valve and cause the stem to grow, and this is why the exhaust lobe is designed with a taller clearance ramp than on the inlet - to allow for exhaust valve stem growth, which will tighten the clearance.
Bottom line is that Chevrolet's clearances are too loose,which will allow the valves to seat at higher than ramp velocity, and this will increase the rate of seat recession.
1963 models with the Duntov cam are unique in that the inlet clearance is specified at .008". The lobe design did not change. Chevrolet merely specified a tighter clearance to gain a bit more effective inlet duration. This .008" specification was first recommended in a fifties vintage Corvette News article as an optional setting for Corvettes used in "weekend competition events".
As a result of the preceeding research and analysis, I recommend basic clearance settings for the Duntov cam (hot or cold - your option, but setting them cold is a lot easier and less messy) of .010"/.016", with optional 283 settings of .008"/.016", and I recommend these latter settings for all 327 CID engines with the Duntov cam.
DukeComment
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Re: Solid lifter adjustment
Phil - The valve lash adjustment on the '097 "Duntov" cam changed during its lifetime. It went from .012 & .018 (I&E) to .008 & .018 (I&E). It is probably not a big deal which way you go. The .008 lash will give you slightly more lift and duration on the intake valve & a slightly rougher idle. As far as hot or cold adjustment - or with the engine running. I set them cold at spec. Then warm up the engine, shut it off, and them set them to spec. I don't do it while the engine is running - that is way too much of a mess...Comment
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Re: Solid lifter adjustment
Betty Crocker not want to gets her hands oily? Mr Go-Jo, #15Comment
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Re: Solid lifter adjustment
Duke. as always, its a real pleasure to read your well thought out analysis. keep up the good work. mikeComment
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