Lucas 'Magmo' Speedometer overhaul The Lucas
Magmo speedometer used on Austin Sevens from 1932~1935
The rotating speed
reading dial was stuck at the near zero position and only a sharp tap of the
hand whilst driving would make the dial free to move. It would then work fine
until the next prolonged stop.
As I
have not been able to source much information [if any] on this type of
speedometer, this article is my experience and the parts names are probably not
correct to those given by Lucas.
Tools
required: 1 x set of
Jeweller’s screw drivers: 1 x tweezers; 1 x small vice or clamp; 1 x clean
cloth; 1 x small brush; 1 x small pot of cleaning liquid [paraffin].
Start
by removing the speedometer from the car and return to a room with plenty of
light.
1.Remove the outer
chrome bezel, by holding firmly the steel case and rotating the bezel until it
aligns with the cut out in the steel case.
2.Carefully using a
small screw driver prize the inner black facia from the housing.
3.Remove the two
screws at the back of the steel casing and gently pull out the inner speedometer
mechanism.
4.Holding the
mechanism in a vice, remove three of the 4 screws leaving just screw B in place.
Unhook the small arm spring and very carefully lift away the Odometer assembly.
Care is needed at this point to ensure that the non-return spring, (which is
held by screw A), is free and the odometer assembly can now be removed.
5.Now the very
tricky bit. You will need the smallest screw driver to remove the hair spring
screw, which is the one near the red pointer, and very gently lift out the hair
spring which is located in a slot under the screw.
6.The base plate and
the rotating speedo plate can now be removed.
7.Under the speedo
plate is a magnet which, when driven by the speedo cable, causes the speedo
plate to turn. The hair spring returns the plate back to the zero position. This
magnet assembly can be removed by unscrewing the single screw at the back of the
cast housing and removing the steel keeper plate.
8.Now this exposes
us to the Achilles heal of this drive [In my opinion] and is the most common
causes of the failure of the speedo odometer to stop working.
9.There is a small
brass gear wheel, which is the drive of a ‘worm’ drive behind the revolving
magnet, and if the small brass gear seizes in the housing [due to poor
lubrication or corrosion] this then results in one or more of the teeth being
stripped from the gear and the odometer is now ‘dead’. Spares are just not
available.
10.So it’s worth
removing the gear from the casing and cleaning it, then applying some clean low
melting point grease to the shaft. It’s worth noting that there is a small
thrust washer between the brass gear and the casing.
Inspection of all the parts and very careful cleaning is required. I would leave
the rotating speedo face alone as the painted-on numbers are very easily
removed.
Re-assembly
is basically the reverse of the above but great care and time is needed to
ensure that the rotating speedo plate is carefully assembled and the hair spring
is back under the screw. There is a very delicate spring finger which prevents
the speedo plate from
lifting and this can be easily distorted. If the job is not rushed, all goes
back as it was.
The
reason for the speedo plate sticking was the result of the plate with the
numbers on it becoming detached from the small spindle.
This article, written by Mike Davies, originally appeared in CA7C Seven Focus
in Feb 2009 pp 16-19.
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