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RE: MGA tach on crutches (both legs cut off) help!!!!

To: "Clayton Kirkwood" <>
Subject: RE: MGA tach on crutches (both legs cut off) help!!!!
From: Barney Gaylord <>
Date: Fri, 07 May 1999 23:10:23
At 07:53 PM 5/7/99 -0700, Clayton Kirkwood wrote:
>OK, here is where I need some further guidance. The inner shaft connected
to the bar magnet spins fine driven by the drill and end piece of shaft.
The outer disc, which is connected to the needle moves freely also,
however, without much effort, the needle can be held still and the disc can
move free of it. It would seem that at this point, the disc and needle
should be in firm lockstep. In addition, the clock spring is distorted and
is not in a single plane: it's as if the spring has been sprung.

Sproingggg, bong!!!  Yup, spring is sprung.  It should be a thin flat strip
wound in a multi-turn coil all in one plane.  The (outer) metal disk and
the needle hub should both be a tight fit on the output shaft and all that
should turn together.  Is seems like the wheel with the magnet may have
come into contact with the magnetically driven disk, possible a result of a
lot of vibration when it was making all the noise.  With some physical
contact the input shaft could easily spin the output shaft beyond the
intended 3/4 turn limit, probably knocking off the indicator needle in the
process as well as twisting up the clock spring and possibly loosening up
one hub or the other.

See if you can determine which hub is loose on the shaft.  If the needle
hub in front is loose, probably all it needs is a small axial push to make
it tight again.  I think that hub has a slightly tapered hole in it such
that you can pull it off (carefully), rotate it to get the desired speed
reading, and press it back on again with just finger pressure.  If the
driven disk at the back of the output shaft is loose from the shaft, all
bets are off, 'cause I don't think that one is supposed to come loose.

>.... 1) is the needle and shaft supposed to be loose with respect to the disc

No.  Both hubs should be tight on the output shaft and all should turn

>2) do I have cause for concern for the spring being out of plane and
apparently "sprung". If so, do I send it in to a rebuilder?

Yup, and yup.  I personally have never disassembled the output shaft other
than to remove the indicator needle, and I do not know how (or do not have
the fine skills necessary) to repair or replace the clock spring, let alone
have a source for the part.  I haven't priced it lately but I suspect the
reconditioning cost will be over $100.  Before you spend that much on the
repair you might ask if anyone on this list would like to supply you with a
working tach for a reasonable price.  Just a thought.



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