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

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Subject: RE: MGA tach on crutches (both legs cut off) help!!!!
From: "Clayton Kirkwood" <>
Date: Fri, 7 May 1999 19:53:05 -0700
So down to the really esoteric stuff...
BTW, Thanks Barney for your usual complete and detailed explaination.

 > First thing is that you will need a new tach cable.

This has become apparent and a part will be on order, thx.


 >  Since your old cable is shot anyway, clip a couple inches
 >  off the bottom
 >  end of the cable core, chuck it in your reversible drill,
 >  and use that to
 >  drive the tach with the drill in reverse at top speed (running
 >  counter-clockwise to match the sweep direction of the
 >  tach needle).

 >   So if your drill
 >  runs at 900 rpm the tach should show about 900x12/5 =
 >  2160 rpm, give or
 >  take a little.

fresh charged battery turns at 1100 rpm so I should see ~2640 rpm
I reoiled everything >:)

 >  The mechanics of the
 >  tach are quite
 >  simple, just two basic movong parts.  The input spindle
 >  has a bar magnet
 >  that spins.  The needle shaft (with a small clock spring)
 >  carries a metal
 >  disc that faces the spinning magnet.  The spinning magnet
 >  has a drag effect
 >  on the metal disk and pulls it around in the direction of
 >  the spin.  The
 >  drag is proportional to the speed of the spin and pulls
 >  the indicator
 >  needle up accordingly against the increasing resistance
 >  of the clock spring.
 >  So you should check for free spinning operation of the
 >  input spindle and
 >  for free rotation of the needle spindle.  Then put a very
 >  small drop of
 >  light oil on the shafts in the area of the bushings and
 >  give it another try
 >  by spinning with the electric drill.

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.


I can't get a consistent reading on the tach and the freespooling of
the disc from the tach needle  concerns me. Questions are:
1) is the needle and shaft supposed to be loose with respect to the
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?

Thanks again for the details...
 >  Good luck and God speed,
 >  Barney Gaylord
 >  1958 MGA with an attitude


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