Don Howard indicated that the confusion about rebushing may lie in the
difference between SUs and Zenith Strombergs.
SUs are common to older British cars, I am told they have bronze bushings
that wear over time. The bronze bushings are reamed and a new bushing is
installed to return the bore to the original diameter or a oversized
throttle shaft is installed. Moss provides such a reamer and specifies that
it is "for SU carburetors only".
Zenith Strombergs on the other hand have a steel insert that also provides a
housing for the throttle shaft seals.
To expand the frontier of TR6 "science" I expended a vast amount of time and
resources to take some measurements on my '73 Zenith Strombergs (yeah sure).
Please note these are "kitchen table" measurements:
0.312" - Throttle shaft diameter (caliper)
0.303" - my old worn out shaft minimum diameter (caliper)
0.009" - maximum shaft wear
0.315" - shaft bushing inner diameter (indirect - via dial gauge)
0.003" - difference between the shaft and bushing diameters
The shaft bushing was measured by inserting a shaft into the bushing and
noting the play at the other end of the shaft with a dial gauge. The end
play, shaft and bushing length are then used to compute the inner diameter.
This was checked at a 90 degree angle to ensure the bore was uniform (it was
to within 10%).
We know no more than we did before but we have some useless numbers to prove
it!
To summarize: rebush SUs, just put a new shaft in Strombergs!
Second observation is that I've always been wrong in the past, why change
now?
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