The C/GT has a new, more accurate, speedometer and I can't beleive I did
okay on the first try.
REVIEW
Early post about effecting the accuracy of a speedometer. Suggestion
from Barney that the easiest change would be in the air gap between the
spinning magnet and the flange of the 'pie tin' that is at the other end
of the shaft from the needle.
That is, the magnetic torque is what turns the needle. A clockspring
provides the opposite torque that, together, record a speed.
BACKGROUND
Bought a used speedometer from a swap meet last weekend. The little
number on the dial face after the model number is 1280. That is the
number of revolutions the cable must turn to register a mile. This is
different that the one in the C/GT which was 960(ish). As the C/GT has
the 3.9 gears from a 'B', then this used speedometer should be the right
fit. The downside is that I will no longer be able to tell how fast I'm
going beyond 120 MPH (the 'C' goes to 140).
PROCESS
FIRST: Installed speedo and went for a ride. Yipee!!! The odometer is
plenty close enough. The speedometer is registering 70+ at 60MPH.
THEORY: A speedometer can read too fast as it ages as the bushing
'rounds out' a little and the effective diameter of the spinning disk
and, thus, magnetic force, can increase on the flange of the pie tin.
SECOND: Took the speedo apart and dumped the 'first assembly' in Marvel
Mystery Oil for 3 days. This in an effort to swell the 'oil empreganted
bushing' inside the speedo. I don't know if this is bullshit, but this
is the way I've always done it with thngs that have bushings....
The 'first assembly' is where the speedometer cable terminates and
encompases a spiral gear for the odometer drive(s), the spining,
permanent, bar magnet, and the bushing assembly.
MORE THEORY: The idea was to remove a little bit from the end of the bar
magnet to lessen the torque applied to the flange of the pie tin. Barney
figured the effect should be about linear: To reduce the indicated speed
by a certain percent, increase the air gap between the ends of the bar
magnet and the flange by the same percent.
THE WORK
The air gap was .032 near as I could measure. Thus about a 15% indicated
speed reduction would be .002 off *each end* of the permanent bar
magnet.
Have you ever seen .002? Got a good feeler gAUge set? We are definately
in RCH territory. Actually YFBCH in my part of the country. But I
digress.
I wasn't at all sure that flimsy little feeler gAUge strip was going to
work. The gAUge is too thin to slide and besides, the damn magntism
fighting it makes it impossible to slide anything anyway. What you need
is a good 2 inch dial caliper of which I don't own (yet).
So I got out the Dremel, used an AL-oxide stone, and just sorta swpet it
over the end of the magnet on both ends. We are talking no more depth
than a thick coat of paint.
Then I bolted everything back together and *viola* the speed dropped
_alot_ for so little material removed. Maybe indicating a couple of MPH
*under* actual speed. So I am pretty happy. Having a speedo that matches
the rear-end gear is your first best bet as this gets the odometer close
then a slight shaving of the magnet will drop speed.
Biggest pain in the asses:
1) The dash. Talented 4 year olds needed with skinny arms and without
the propensity to pad center of mass through patient and consistent
application of intoxicating fluids.
2) The 'bolts' holding the speedometer guts together. They are vey small
and are easily the size for a jewler's screwdriver. But the gAUge face
is in the way from head-on so you unscrew from side with pocket knife.
3) New speedo uses two little cranks with matching little, tiny springs
to ratchet the odometer and trip odometer. Minor PITA to get that back
together.
4) Major headache resulting from eye strain (see 2 & 3, above). I should
have listened to my mother when I was a kid. (And become a doctor --
these computer terminals are blinding me!)
But, all things considered, I now have the most accurate speedometer of
any of the LBCs I've messed with in the last decade.
Bob Allen, Kansas City, '69CGT, '75TR6
(Barney: thanks for tips and ideas.)
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