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)
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