The overall geometry of the actions of the valve train must be affected,
if the head is skimmed. Adjusting the rocker adjuster-screw up will NOT
compensate for it, as the push rod is still the same length. FOR
INSTANCE....MG must have been right as they shortend the push rods on
the XPAG at engine number TD2/17298 on the TD, and SC2/17432 on the YB,
by 1/16" when modifying them. Because of the problems with geometry, MG
insisted only rockers and push rods may be changed as a SET.
Neil
Rick Morrison wrote:
>
> On Mon, 31 Aug 1998 20:29:15 -0400 Bill Schooler <schooler@erols.com>
> writes:
> >Lawrie Alexander wrote:
> >>
> >> Hate to jump in a criticize one of Mr. Allen's learned contributions
> >but I
> >> just gotta........
> >>
> >> If your head were machined a lot (say 1/16" or more), shims under
> >all four
> >> pedestals would restore the original geometry.
> >
> >Okay, I'm gonna jump in here without any of the usual "IMHO" caveats
> >and
> >say that this business about "changing the geometry of the rocker gear
> >when you machine the head" is not correct. If someone proves me
> >wrong,
> >I'll be the first to admit it and eat crow.
> >
> >Machining the head will NOT change the rocker geometry. Visualize a
> >cutaway, end view of the engine. Now imagine removing 1/16 in from
> >the
> >face of the head. What has happened? Everything is "lowered" except
> >one piece - the pushrod. If we were to remove 1/16 of an inch from
> >the
> >pushrod, all would be the same as before - with regards to the
> >geometry
> >of the valve train. Lo and behold, we don't have to machine the
> >pushrods - merely back off the rocker adjustment screw to restore the
> >normal .15 thou clearance and bingo! Back in business just as before.
> >
> >
> >As a matter of fact, I believe that the geometry would be screwed up
> >if
> >you installed 1/16 in shims under the rocker pedestals. Then the
> >"geometry would certainly be different since you have raised the
> >rocker
> >arm in relation to the valve stem tip.
> >
> >I think John Twist has made this point before, but can't find the
> >reference...
>
> Well, I'm gonna jump in on Lawrie's side.
> In theory what you say is correct.
> The problem tho, is as you state. The push rod is still the same length.
> With out shaving the push rod or shimming the pedestals, the relationship
> between the upper end of the pushrod and the rocker center line is now
> incorrect.
> It's not just the rocker geometry that must be considered, it's the
> geometry of the entire train, from pushrod/lifter to the valve stem.
> Check out Paul Von Valkenbergs "Race car Engineering and Mechanics" as
> well as Vizard's "Tuning the A series Engine" for some good texts on
> valve train geometry.
> Rick Morrison
> 72 MGBGT
> 74 Midget
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