Maybe I can offer something to help determine the correct lash for mechanical cam. In my experience with some "unknown" cams, I have found certain characteristics.
A hydraulic profile will have a very uniform, steady acceleration on the ramp of the lobe... right from the base circle. This can be observed using a degree wheel and a dial indicator on a lifter ( cam installed of course
:
). Just for clarity, I'll use a hypothetical checking example / procedure...
From the cam's base circle, JUST as the lifter begins movement, rotate the crank 5 degrees, and note the amount of lifter rise... lets say it was .015". Rotate another 5 degrees, and the lifter would rise another .015". On a hydraulic cam, this basic pattern will continue to full lift.
A mechanical cam is different. Using the same procedure as described above, you will find that those first few degrees of crankshaft rotation ( at the biginning of lifter rise ), will result in a VERY soft, gradual movement of the lifter. That changes... QUICKLY. The contour / ramp of the lobe makes an abrupt increase in acceleration... on a good profile, providing a much more aggressive rate of lift than what the hydraulic lifter can tolerate ( this is the fundamental advantage of a mechanical cam ).
By comparison to the hydraulic profile, a mechanical cam may show figures like:
5 deg rotation... .010" lifter rise. Another 5 deg rotation... another .010" lifter rise. then the next 5 deg rotation... additional lifter rise of .020". In this hypothetical example, it's between .020" and .040", where you will find that your lash is to be set.
This can be determined pretty accurately. Rotate the crank back to the zero(0) position ( just before the lifter moves ). Now rotate the crank clockwise ( correct direction ), and watch your dial indicator VERY VERY carefully. You will be able to see this sudden increase in rate of lift on the lobe. It's the point, maybe .002" before that sudden increase in lobe acceleration, where your valve lash should be set. All mechanical cams are like this. In the case of the "hypothetical" cam that I just made up, we would have found that this sudden "acceleration" occurred at around .022" - .025" lobe lift... indicating that our lash should be set at approx .022".
Lastly, this explains why it is "safer" to run your valve lash a bit tight, rather than somewhat loose. If the above "hypothetical" cam was to be lashed at say... .030", it would allow the lifter to be off the cam lobe until that lobe enters it's "maximum acceleration" mode... thereby "hammering" the base of the lifter and that flank side of the lobe. THIS WIPES OUT MECHANICAL CAMS:eek: .
Running the lash a little tight actually makes the valve train parts more comfortable, but will often have a dramatic effect on manifold vacuum at idle, and can tax the valve springs in their effort to maintain control of the the valve train.
Hope I made sense
If you try this, watching carefully, you'll see that it becomes quite obvious
Oh... hot / cold valve lash...
I've never been able to get the engine warm enough when it's on the engine stand, so I can do a hot lash
... so, obviously, the first lashing is a cold one. I set to spec.
However, I NEVER EVER EVER leave it like that
no1: . I break in my cam, good hot engine, then do a hot adjustment... to spec. Not until I have done that, will I run an engine hard.
In my experience, with iron heads on these engines, valve lash loosens up about .002" - .004" when the engine is at operating temperature.
Haven't you ever noticed how your hot rod will have this radical vicious idle when it's cold, then kind of smoothen out when it's warmed up ?
That's the valve lash loosening, and reducing the effective duration of the cam.
Gee, I didn't mean to get so long winded