Calvin Krug wrote:
> I happened to notice while disassembling the head on my
> MGB that there are no shims under the two middle rocker
> stands. Is this a big deal? I'm a little concerned that
> if it ran without them in place, it might throw things
> out of alignment if I put them in. What's the concensus
> on this?
Calvin,
See Barney's post (9/6/98) below ...
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Safety Fast! ... larry.g.unger@lmco.com
'61 MGA 1600 MkII
BG> The Rocker shaft is a slip fit in the bores of the
BG> pedestals, and not a particularly tight slip fit. There
BG> is a tolerance on the diameter of the shaft, and a
BG> tolerance on the diameter of the bores in the pedestals,
BG> and a tolerance on the height of the bores in the
BG> pedestals, and so the bores in the pedestals are
BG> intentionally made a bit oversize to accomodate all these
BG> minor sources of misalignment. And, the shaft is loosely
BG> pinned in place by a dog point set screw in the top of
BG> the #4 pedestal.
BG>
BG> Now when all this is assembled without those shims, the
BG> shaft is free to wiggle around a bit in the bores of the
BG> pedestals. Yes, it has some upward force on it from the
BG> springs of the valves which are being actuated at any
BG> given moment. But as the rockers rock they generate some
BG> side loads on the shaft by virtue of their frictional
BG> contact with the valve stems and their not-quite-vertical
BG> motion at the tip of the rocker.
BG>
BG> Well, if you shim the two center pedestals up 0.010", that
BG> takes up all the tolerances in the assembly and maybe bends
BG> the shaft slightly, thereby creating a preload on the
BG> mounting of the shaft so it won't wiggle around in
BG> operation, which could cause wear on the shaft and the
BG> bores of the pedestals, which in turn would cause slop in
BG> the valve train. The small amount of bending of the shaft
BG> would not induce much stress on the shaft or the cylinder head.
BG>
BG> Maybe it just dawned on someone to start doing this in 1972
BG> to solve a perceived problem. Like I said, just a reasonable
BG> guess.
BG>
BG> Barney Gaylord
BG> 1958 MGA with an attitude (but no shims)
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