Barney.....
You've come up with what certainly is the most logical answer to this
question, which has puzzled me for years.
Thinking about your suggestion, I have to agree - for the following reason:
I have taken apart many engines where the rocker shaft itself has had very
noticeable wear grooves worn in it at the four points where it fits in the
pedestals. I always wondered how this could happen, as the shaft is simply
supported by the pedestals; there is not supposed to be any movement that
could cause wear. I cannot say for a certainty that the engines with the
worst wear on the shafts at these points had no shims (because I never
thought to relate the two ideas). However, I can say that most engines I
take apart do not have the shims in place.
Once again, Barney, you appear to have succeeded where others have tried and
failed...............!!
Lawrie
-----Original Message-----
From: Barney Gaylord <barneymg@ntsource.com>
To: neil.cairns@virgin.net <neil.cairns@virgin.net>; Vemarootoo@aol.com
<Vemarootoo@aol.com>
Cc: mgs@autox.team.net <mgs@autox.team.net>
Date: Sunday, September 06, 1998 10:13 PM
Subject: Re: Rocker Post Shims.......
>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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