WSpohn4@aol.com wrote:
>
> <<Yes, when the head is milled, the pushrod hits the rocker screw at
> a higher point, but by adjusting the screw to compensate, the point of
> attack at the rocker arm itself is unchanged. If that is true, then the
> valve train geometry is unchanged, with the granted exception of the
> point at which the pushrod bears on the adjustment screw.>>
>
> When you shave the head .065", the point at which the lash is taken up and the
> pushrod begins to actuate the rocker is .065" higher than it is on an unshaved
> head.
> Believe it or not, this small difference can wear the already wear prone
> rocker gear and guides at fairly modest mileages. The only way to minimize the
> sideways moment on the valve stem tip is to make sure that the rocker is at
> the right height. Even then, there is obviously a sideways component the
> minute (or rather fraction of a second) that the rocker pushes on the valve
> stem in anything other than a straight up and down direction.
>
> You can minimize this sort of wear by using tubular pushrods with pressed in
> tips. By using a parting tool on a lathe of exactly the same size as the slice
> taken off the head, you can shorten the pushrod and then heat it and press the
> tip deeper into the rod tube.
>
> I grant you that many racers neglect this sort of nicety, but in a real sense
> it is more important for a street car than a race car. The racer will no doubt
> be torn down (probably many times) before wear becomes significant. A street
> car will see many miles and you don't want to be doing head jobs every 20,000
> or so just because you didn't bother to do the pushrods properly.
>
> The best setup for the MG is of course to have no pushrods, a la Twincam.
> There is zero sideways moment on the valve stem with the overhead cams,
> although there is a sideways moment imparted by the cam to the cam follower.
> Because of it's much larger size and bearing area, it wears much more slowly
> than the pushrod engne will, and always pushes straight down on the valve even
> as it slowly wears.
>
> Bill S.
> (a real MG has 2 cams!)
Give this man a cigar! You elagantly stated the conclusion that I have
come to while pondering this question - i.e., the proper way to restore
the original valve train geometry is to shorten the pushrod by the same
amount that you milled from the head. And, yes, this is an admission
that I was wrong.
The valve train geometry is changed when the head is milled. It's a
very confusing thing to try and explain without a diagram, because there
are about 3 different rotary motions involved in the process of moving a
valve up and down. But Bill Spohn has pointed out the factor that I
didn't consider which is that the "point" of rotation where the pushrod
meets the adjusting screw has changed. One of these days I'll try and
diagram it, for my own satisfaction, but thanks to the list for making
me think more about it; I believe I understand it now. And Chip Old's
article was where I saw the caution about not using shims under the
rocker pedestals to compensate for head shaving. Agree with that 100%.
--
*Bill Schooler *Check the MGCC Wash DC Centre Web Page
*Woodbridge, VA *http://members.aol.com/mgccwdcc/
*schooler@erols.com
*53 TD
*60 MGA
*69 MGB/GT
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