<VV> adjusting valves - I want to believe!

[Craig Nicol]

Jim wrote:
A little math is in order.  The threads on the rocker studs are 
3/8-24, so each turn of the adjusting nut is 0.0417 inches.  Since 
the slack take up is on the pushrod and valve tip at the same time, 
the adjustment is 0.0833 inches each turn.  The the adjustment range 
in most lifters is 0.180 to 0.200.   The engine grows about 0.006 
inches for each 100 degrees Fahrenheit.  So now you can make an 
informed decision as GM did.
Jim Davis

Craig replies:
I like where you are headed with this, Jim!  I do have a little correction
to consider. 

The notion that a turn of the adjustment nut causes .0833" at the pushrod is
possibly incorrect; here's my thinking:  I don't think the adjustment nut
equally affects the valve stem and the pushrod as you stated.

The valve stem does not move when the adjustment nut is turned, the valve is
stationary and is effectively the fulcrum of the lever. 

The rocker supposedly has a 1.58:1 ratio but let's imagine for a second that
it was a 1:1 ratio, which would put the adjustment nut in the exact middle.
At THAT ratio, .0417" movement at the nut would cause .0834" movement at the
pushrod. To simplify the math for our actual ratio, consider for a moment
that the rocker arm ratio was 3:1. In that case the pushrod would move 4/3
the distance of the nut movement. (4=overall units of length, 3=units of
length at nut, relative to fulcrum, hence the 4/3 effect) 

At the actual ratio of 1.58:1, the overall units of length = 1+1.58 (or
2.58) so using the same concept as the 3:1 example, the pushrod would move
2.58/1.58 (1.63) times the nut movement. Using this factor, I think a .0417"
movement at the nut would cause.060" movement at the pushrod. (.0417" x 1.63
= .060") rather than the .0833 you calculated.

Here's how that shakes out:

1/4 turn = .015 at the lifter (ignoring the pushrod angle to the lifter) In
an environment where the engine grows .006" per 100 degrees and the lifter
has an available range of .180 to .200., this theoretically doesn't offer
enough lifter stretch for higher engine temperature and uses very little of
the lifter's range. (.15" out of .100" to center of lifter's range)

1/2 turn = .030" (which should accommodate a 500F degree engine temp change
but still uses only a fraction of the available .100" "center of lifter
range")

3/4 turn = .045", not even halfway to the center of the lifter's range, but
clearly able to accommodate well beyond any conceivable engine growth.

1-turn = .060", still well short of the lifter's center of range. 

I'll also add observations that the more the preload there is the less time
it will take a collapsed lifter to refill and the less time it will take for
the pushrod to fill and deliver oil to the upper valve train; both worth
considering.

So the questions are: Why does 1/4 turn seem to work so well? 

If the preload is set to 1/4 turn when cold, according to this analysis it
should clatter when hot. Perhaps 1/4 turn works only if adjusted at
operating temperature. I wonder if this unspoken need to have the engine at
temperature is why we hear so much cold/hot, 1/4 turn-1/2 turn-1-turn
controversy?  Maybe 1-turn is the right answer if valves are adjusted cold
and 1/2-1/4 turn is the answer if adjusted hot.
Craig Nicol

Note to math wizzes (which I am not): please check my math on the effect of
the rocker arm ratio when the fulcrum is changed to the end when the ratio
calculates to 1.58:1 with the fulcrum is at the middle. ;-)