M.G. TC & TD Lucas PLC 6 Alternate Reassembly Method

Note: It’s advisable to read these instructions completely before you begin disassembly. All the parts are identified in the photograph & legend below. Check to ensure that the two tabs on the Mazak lock body cover 5(b) aren’t broken. Be very careful, as these two tabs are unbelievably fragile! If the tabs are at all bent over at the back of the switch, it may be possible to very gently straighten them just enough to remove the cover. Great care must be used here! With one tab broken it can still be used but if both tabs are broken, unless the lock body cover is modified, the switch is toast! A soldering iron & a 6 BA spanner are the only tools required.

Prior to assembly ensure you have noted the OFF position on the body of the switch. The first F in OFF coincides perfectly with the projection in the body beside the single grub screw near the top of the switch. If using a decal it is necessary to complete this step before fitting the decal to ensure it is located correctly. Temporarily refitting just the face plate into its indents will ensure the decal is in the correct position. The decal supplied by “From The Frame Up” is a good reproduction of the original lettering however the artwork is fragile & the white lettering rubs off all too easily. I found it prudent to protect the lettering by spraying a coat of clear lacquer over the decal before applying it. Carefully examine the 3 tabs on the bezel face plate (12) as these are often damaged by PO’s who had no idea that the bezel simply untwists!

The phenolic insulator disc (10) with the four ears (not illustrated) may or may not be fitted. Later models saw this disc relocated to the small end of the large spring. Do not omit this insulator or smoke will be released! To assist in reassembly of an earlier switch such as the PLC2 it is suggested that the phenolic disc (10) with the 4 ears be replaced with the later disc (10a) & relocated. These insulating discs can be easily made from plastic ice cream containers.

The photograph and legend follow……


1Switch Contact Bridge or Ignition Contact Ring
1(a)Contact Ring Insulator (Note this is glued onto the contact ring)
1(b)Small Insulator
2Metal Contact Cover
3Small spring
4Cylinder Extension or bottom half of the Lock Barrel
4(a)Brass spring loaded Pin & Cap
5Cylinder or Barrel (MRN series TC & early TD only. FA series later TD)
5(b)Hollow Lock Barrel or Mazak Lock Body Cover
8Brass Contact Plate
10(a)Large Insulator
12Bezel or Face Plate (Note the bezel with the window is incorrect)
13Knob or lever (Note horizontal black lever only. The 45 degree lever is incorrect)
14U shaped Retainer
17Washer & 6 BA Nut (Note this has a 4 BA thread!)

(A) Examination of the (4) & (5) assembly will reveal a small brass button (4a) protruding from the lower part of the cylinder lock barrel (5) into a corresponding hole in the bottom half of the lock barrel (4). Depress the brass button (4a) to release the cylinder lock barrel (5) from the bottom half (4). Be careful not to lose the spring & brass pin (4a) as this is very easily done. Apply a little Vaseline internally to (4), (4a) & (5) as an aid to later reassembly.

(B) To reassemble the switch, first start with the key switch contact bridge, also known as the ignition contact ring (1). This is a small circular brass bridge with its arc shaped projections facing the bottom of the switch & with its axis in the one o’clock / seven o’clock position, when viewed from the front. Two insulated washers (1a) & (1b) then sit on top of it. Carefully place this assembly into the recess in the body, followed by the metal cover (2) with its projections also facing the bottom of the switch, then fit the small spring (3) large end first. Maintaining alignment, place the bottom half of the lock barrel (4), also known as the cylinder extension, into the body ensuring that the brass button recess is at 7 o’clock. Fit the fibroid fish paper & then secure it with a washer & nut (17) ensuring that it is not overtightened. This nut requires a 6BA spanner but has a 4BA thread! I made a tiny spanner from a small piece of 1/8” flat mild steel & an angle grinder. Next position the hollow lock barrel (5b), also known as the Mazak lock body cover, over the cylinder extension (4). It is essential that the deep cut out in the hollow lock barrel (5b) fits over the central molded stop in the body (7) & ensure that it doesn’t rock. Don’t bend the tabs on the lock body cover as this is unnecessary & will render the switch inoperable if they were both to break!

(C) Assemble the bezel face plate (12), the knob (13) and then the U shaped knob retainer (14). The cupped shaped keeper (15) slips over & secures the retainer (14). Note that the retainer (14) should be fitted first. The keeper (15) will have its convex side facing the back of the switch. On some switches I’ve dismantled, the positions of the keeper & retainer are reversed, which seems to have no effect on the operation. Next fit the phenolic insulator disc (10a) or a suitable replacement (See note). If a decal is used it must also be applied before (12), (13), (14) & (15) are assembled. The large spring should now be compressed & held with two thin wire twist ties opposite each other. I used multi strand copper household wiring however MIG wire can also be used. Place it in position, with the small end closest to the bezel (12). Next place the brass contact plate (8) over the 4 projections on the back of the knob (13) ensuring that the bumps on the plate (8) are facing you. Line them up with the corresponding 4 cutouts in the knob (13). It will be noted these projections & cutouts are different sizes & they will only fit together in one position. The assembled face plate is now complete. This method of assembly with both springs compressed is much simpler than the 2 other methods I’m aware of & ensures that the mating of the two halves is straightforward.

(D) When offering the assembled face plate (12) to the switch body make sure the contact plate cutout (9) engages with & is centered on the molded stop in the body. This is what provides the stop for the 3 position lighting switch and prevents it from going too far clockwise or counterclockwise. Ensuring that the wire tails protrude from the slits in the body, gently press the two halves together. There is only one way the 3 tabs on the face plate bezel (12) will line up with the 3 notches in the rim of the switch body. Check to ensure the lettering on the bezel face plate (12) is properly aligned with the body of the switch. As noted above the OFF position should be observed on the switch body prior to disassembly. Take care not to rub off any of the decal lettering during reassembly. See the note on using clear lacquer.

(E) With the two halves of the whole assembly together so the 3 tabs have engaged with their notches, ensure that the face plate (12) is fully home. While maintaining slight, even pressure give the face a ½” counterclockwise twist so that the three bezel tabs now come to rest in their respective indents. It may be necessary to apply a little Vaseline to the lip as an aid to rotation. Place the key into the cylinder lock barrel (5) & insert the barrel into the switch ensuring that the letters are on the right & the numbers are on the left. Once engaged remove the key & the cylinder should remain. Gently untwist & pull on one leg of each of the two ties to remove them. This will release the tension on the large spring. Check the switch operation mechanically then secure the nut (17) with two 4 BA brass hex nuts, the second acting as a lock nut. 4 BA nuts with 6 BA bodies aren’t available & the use of solder just makes subsequent disassembly much more difficult than it needs be. Ensure that the nuts are not too tight.

It is essential that some insulating material such as fibroid fish paper electrical insulation (199-9620 from Element 14) be fitted under the nut to ensure that the nut & ignition terminal post are insulated from each other. Finally test the switch with an ohmmeter before installation.

Revised alternate method (Jan 2016) by Peter Hehir © based on text originally supplied by Lew Palmer.