Restoration of the Lucas SM5 Switchboard

Whilst restoring my 3 Speed Box Saloon I turned my attention to the Lucas SM5 Switchboard or as Austin called it in some handbooks a Switchbox. I had previously removed it from dashboard and wiped over, it was complete, but corroded with a very poor faceplate 70 years old.

I dismantled by removing the split pins in the switch knobs and the 2BA nuts on the two brass through bolts; the panel will then separate from the faceplate. The Ammeter can be removed by removing the brass 6BA countersunk screws under the grub screws in the A and +B terminals. This centre zero Ammeter checked OK and was accurate when compared against a reliable test instrument.

sm5 2The chrome bezel was soldered to the brass ammeter movement case at two places at rear. I melted the solder and carefully prised the bezel off using a small knife around the casing. The bezel was dispatched for re-chroming. There is a rubber ring around the ammeter faceplate circumference. This face and glass was cleaned carefully with a slightly damp cloth and the needle/pointer was carefully painted in white gloss. After removing the small plate with LUCAS stamped on held by two-6BA chrome raised head countersunk screws from the face panel, the red ignition light lens was removed and polished. The ignition light is 6 volt 3 watt MES screw threaded pilot bulb and was cleaned and checked.

All brass fittings/terminals were cleaned and polished. A Dremel or Wizard type small power tool with a wire brush is ideal for this. The rust covered slotted steel terminal grub screws were cleaned, you could make new from 1/4 BSF threaded rod or setscrews. I cleaned all the switch contacts so good electrical contact can be made. Very light lubrication to these contacts with WD40 will help operation. You could lightly solder the fixed (riveted) terminals to the ends of the brass connecting strips if there is doubt about corrosion.

All terminals and circuit switching can be checked with an analogue test meter or a battery and bulb.

The task was now to repaint the panel face. I removed all paint with Nitromors automotive paint remover. Wear gloves and a mask when using, your skin and eyes do not like this stuff. It was cleaned down to the bare brass surface. All tarnished spots were removed to stop further corrosion and a small brass wire brush and wire wool were used to restore this surface to a clean dull finish. A small scriber was used to clean out the small lettering punched on the panel, which was finished in white originally.

A degreaser was applied to ensure surface was thoroughly clean. The panel was spray painted from a grey primer aerosol, two light coats will suffice. I also painted the heads of the two brass through bolts that sit on the faceplate.

sm5 3

Special primer paints are available for brass and non-ferrous metals, but I had a grey primer aerosol from Halfords on the shelf. I then left to dry in a warm clean place for a few days. The lettering was lightly cleaned out and the surface was cleaned again. I now painted with gloss Black spray aerosol paint. It is advisable to use same make as primer to stop interaction. Two light coats should suffice. However make sure it is warm where left to dry, as in any car painting +70 F (+20C). You could hasten the drying process with careful use of a Heat Gun on a low setting.



sm5 4I now painted the white letters. The method I used was:

Obtain a small tin of White (No.22) Humbrol Super Enamel, 99 pence in most good DIY stores.

Using this paint after reading the instructions cover the lettering with a small paintbrush until you cannot see the lettering or the black paint between letters.

Do not try to paint in letters with the white enamel paint but make sure the letters are well filled with paint.



sm5 5Now the method is when this new paint is near to getting touch dry rub across the letters with a very clean dry lint free cloth, and clean off the surface paint. You should now find the letters picked out with the white enamel and surplus paint removed.

Polish off surface and leave to dry.

If you are not successful, try the white painting again after you have removed the white paint from the panel surface. Don’t worry about paint left in the letters.

The Humbrol enamel does not react with the spray paint, as most of these are now acrylic based. This method can also be used for the Lucas MT7 Wiper Motor case used on later Sevens and other items. Whilst this may not be the only method, it is the one I was successful with.

On assembly all wire terminals should be firm and tight. No odd strands of wire should be loose, as intermittent connections with high resistance will cause voltage drop or shorting and sparks. You could lightly cover the top of terminal screws and other nuts with a dab of paint to make sure the screws do not work loose through vibration when assembled and connected up. This old practice often forgotten also indicated that terminals had been tampered with when paint seal had been broken. Especially in awkward out of sight places like behind the dashboard in these cars. You do not want to be removing the switchboard again.

The diagram of wiring connections to the SM5 in some handbooks are not always the same as originally fitted or may have been changed in the cars life. I have seen several versions of the SM5 with different layout of the terminals and ignition light fitments.

If you intend to withdraw your switchboard from the dashboard to examine or restore, disconnect the battery first as safety precaution and make a diagram and mark the cable connections.

This SM5 Switchboard (Austin Part No. BG 115) which part of is a basic ignition switch was used from about 1928 in various versions of Austin Sevens until late 1932, when the Lucas SA1 ( Part No.BG160) ignition switch with a key was introduced on a raised panel that had a cluster of gauges including a separate ammeter. Although Part No’s changed in high and low frame cars available with or without a wiring harness.

SM5 6



There are earlier versions of the Switchboard (SM3) (Part No. BG82) recognised by two wander plug sockets, which were provided for an inspection lamp or a trickle charger? Previously a C.A.V. Switchboard (Part No. BG17) had been used.





I hope this Switchboard will now survive another 70 years.



This article, written by Peter Kemp, originally appeared in CA7C Seven Focus in May 2002 pp 10-13.