Scott Tilton wrote:
> The former owner sent me the spec sheet on the "PAECO Superlift Camshaft" and
>pointed out the non
standard valve clearances . . . it says
>
> Intake Lash .018"
> Exhaust Lash .020"
>
> If memory serves me correctly stock is .010"
>
> Does this seem pretty excessive to anyone else?
Depending upon the lift and cam lobe profile, probably not.
> I'm wondering what the theory is behind valve lash.
> I thought it was there because there was potential for thermal expansion and
>the valve stem could actually
lengthen a tiny bit and the relative geometry between the rocker and the
valve stem would change.
All the parts grow with heat, not just the valve stem. Lash is there, in
part, to prevent part expansion from closing up the gap, but the larger
reason, especially in the case of more radical camshafts, is to minimize
stress from high acceleration of the parts due to the ramp on the lobes.
If the clearances were very tight, the valve train would be subject to
greater forces than they could tolerate and would fail more quickly.
> Wouldn't want the engine to heat up and the valves not fully close.
>
> Well . . . if that was the theory . . .what in the heck would make a valve
>stem get that much hotter and grow in
length that much more?
Yes, so much for theory. But, in truth, if you were running the engine
as intended, the growth would be slightly more because of the increased
heat associated with greater power production. But, as mentioned above,
that's not the principal reason for the increased lash.
> Maybe they think I'm going to run the thing at 6500 RPM. (I drive it hard,
>but not that hard)
>
> Currently I'm fudging. . . the valve clatter sounded very loud with .016" and
>I adjusted it down a bit.
> Am I being stupid?
Well, not exactly. You're trying to make an engine with an exotic cam
behave like a street engine. Extra valve clearance will make more noise
at low rpm. That doesn't mean the clearance is wrong--it just means that
it's not meant for normal street use. If you want the performance
associated with the cam with any kind of reasonable valve train life,
you've got to run the clearances specified, noisy or not.
> Anyone got some tech tips on the right way to adjust valve clearances?
If you have a manual, use the instructions there. If you don't have a
manual, I suggest you find one. Everyone has their own peculiar methods,
but the manual is the proper starting place.
> >We used one of the Clikadjust tools, which is truly a miraculous device.
> >Set all 8 valves in about 10 minutes.
>
> Yeah I know John and should ask him myself. . . . but . . .
>
> Does anyone know what a Clickadjust tool is?
It's pretty much as described--it's a tool for setting valve lash. One
sets it to a specific lash, and then that clearance is repeatable for
each valve with that lash setting.
Other instructions on its use are at this site:
http://autoexpertproducts.webjump.com/
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
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