Barney Gaylord wrote:
>
> At 06:25 PM 9/6/98 -0700, neil.cairns@virgin.net wrote:
> >Vemarootoo@aol.com wrote:
> >>
> >> Lawrie- I'm now confused. Are the center 2 rocker post .010 short,
> requiring
> >> the shims? Are all 4 the same dimension and the rocker shaft is thereby
> >> bent?!? Why required ( or not?) on the 18V and not the ,say, 70-71 ? Is
> it the
> >> head cast and not the rocker posts? Why are some listers saying throw them
> >> away? If these puppies have value, what is it? TIA Vic
> >
> >All the posts are the same, the head is flat, so, yes the rocker shaft
> >must be bent by the ten-thou. I am still trying to find out why. John
> >Lawson of the Marina Drivers Club has sent me a pile of data on disc for
> >the 18V engine. I am trawling through it all.
>
> Being a mechanical engineer, I shall venture a guess.
>
> The Rocker shaft is a slip fit in the bores of the pedestals, and not a
> particularly tight slip fit. There is a tolerance on the diameter of the
> shaft, and a tolerance on the diameter of the bores in the pedestals, and a
> tolerance on the height of the bores in the pedestals, and so the bores in
> the pedestals are intentionally made a bit oversize to accomodate all these
> minor sources of misalignment. And, the shaft is loosely pinned in place
> by a dog point set screw in the top of the #4 pedestal.
>
> Now when all this is assembled without those shims, the shaft is free to
> wiggle around a bit in the bores of the pedestals. Yes, it has some upward
> force on it from the springs of the valves which are being actuated at any
> given moment. But as the rockers rock they generate some side loads on the
> shaft by virtue of their frictional contact with the valve stems and their
> not-quite-vertical motion at the tip of the rocker.
>
> Well, if you shim the two center pedestals up 0.010", that takes up all the
> tolerances in the assembly and maybe bends the shaft slightly, thereby
> creating a preload on the mounting of the shaft so it won't wiggle around
> in operation, which could cause wear on the shaft and the bores of the
> pedestals, which in turn would cause slop in the valve train. The small
> amount of bending of the shaft would not induce much stress on the shaft or
> the cylinder head.
>
> Maybe it just dawned on someone to start doing this in 1972 to solve a
> perceived problem. Like I said, just a reasonable guess.
>
> Barney Gaylord
> 1958 MGA with an attitude (but no shims)
I had thought of something similar to that as well. On some engines, (
Triumph, Ford, ) the post is split, so it clamps onto the shaft when
the nuts are tightend. Seems a cheepo way of sorting it though.
Neil
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