1500 Midget Overdrive Conversion

Guy Weller


I had toyed for ages with the idea of fitting an overdrive to my 1978 1500 Midget. Although this model is fitted with the higher ratio 3.7 back axle it is still a bit too low geared for modern traffic conditions. I have also seen references to this conversion on a number of occasions, but have never seen any real details of what work is involved. I know there is at least one "overdrive conversion kit" on the market, but I prefer to do these things the hard way, working it all out for myself. After all that is where the real satisfaction of owning one of these cars lies, for me at least.

The opportunity finally came last October when I saw an advert in the MG Owners' Club magazine for an overdrive gearbox. The price was a very reasonable £60 and well worth a 500 mile round trip after work one Friday to collect it. Typically the chap who was selling it had bought it to fit into his Midget, but never got round to doing the job. It turned out to be in top rate condition, with a repainted and reconditioned gearbox and, most importantly , was complete with the gear leaver and gear-knob mounted switch. I understand that these can be very hard to come by.

The overdrive unit was a Laycock J - type, fitted to the single rail triumph gearbox. Mine was attached to a larger bell housing than the normal Midget/ Spitfire one, I have no idea what it came out of. I had a spare gearbox already so the first job was to swap over the bell housing castings so that it would match to the standard rear engine plate. It also had the wrong input shaft, so I simply swapped this from my spare gearbox also. Had the gearbox been a normal spitfire or Triumph 1500 TC then it would have fitted without any alterations at all. The only other modification was to cut and re-weld the gear lever to match the original Midget one. The lever has the wires to the switch inside it so these had to be removed first. I then checked with a meter that that the wiring and switches were all operating as I certainly didn't want to have to remove it all at a later date just because of a poor connection somewhere!

The real work then began as the underside of the car has to be altered to take the bulkier overdrive unit. Engine and gearbox removal is as normal, and the new gearbox is a straight replacement for the original although I took the opportunity to replace the clutch components at this stage. Alterations to the car involve cutting out the chassis cross member and replacing with a fabricated box section which is below the floor pan, giving the necessary clearance. The underside of the transmission tunnel also has to be cut out as far back as the inspection hole which gives access to the front joint of the prop shaft, and the tunnel itself has to be widened out by about 1 1/2 inches. This was actually much easier than I had expected, using a scissor jack and easing it out a bit at a time. I had prepared a card template to give me the cross section of the overdrive unit, so that I could judge when I had opened out enough space.

I re-inforced the chassis cross member either side of the transmission tunnel by tapping two 5 inch lengths of square section tube, which conveniently was a very tight fit, into the open ends. These were then closed off with steel blanking plates, welded into position. Each had a small hole drilled to enable me to inject waxoyl into the cross member before closing this off with a plastic plug. All the sharp edges were then smoothed off with an angle grinder and cleaned down, primed and repainted.

The new cross member itself was made up out of a length of 2" X 3" tube, 24" long and with the two ends cut and welded so that the bottom surface tapers neatly upwards at either end at about 30 degrees. I debated whether this should be welded into position but was worried about having sufficient clearance under the rear of the overdrive for engine removal. I chose to bolt this through the re-inforced sections of the chassis cross member, using two 4 1/2 " by 5/8 " high tensile UNF bolts each side. I next made up an "H" shaped frame of 1" square tubing which was to carry the new rear gearbox mounting. This assembly bolts up through the seat runner re-inforcing sections either side of the transmission tunnel with four 1 3/4 " by 5/8 " UNF bolts and is also welded to the new cross member so that the whole unit makes up a removable sub-frame, similar in principle to the MGB item.

At this stage I refitted the engine and gearbox. That sounds easy, but it is surprising how much difference that overdrive unit makes and with the extra weight the business of adjusting the angle of the engine on the hoist as you lower it into the engine bay is that bit more critical. The new removable sub-frame really helps here, and I believe that had this been a welded in unit then the job would have been all but impossible! I expected to have to repeat the engine installation process several times before the job was finished but in the event in it went and in it stayed! Once in position I bolted up the prop shaft, and despite what I had read elsewhere about it having to be shortened, it fitted without alteration. Somebody was on my side!

The gearbox mounting was really tricky as there is very little space. I made up a dummy from thick card first, and after some adjustments produced a satisfactory design that I could weld out of 1" angle and steel plate and then weld to the new sub frame. I used a rectangular engine rubber to support the weight of the unit and two "bobbin" rubbers angled at 45 degrees to control the torque twisting of the gearbox. Each is adjustable with shims as it is critical to get the whole unit positioned correctly to avoid the gearbox making contact with the bodywork when driving over bumps and the twisty bits !

I had heard of speedometer ratio problems, but thinking it through there seemed to be no logic to this. The speedometer pinion is on the output shaft of the overdirve, and as I had made no alterations to the tyre size or rear axle there was no reason for the speedometer to be inaccurate. The only problem was that I needed to find a supplier of a slightly longer cable, fitted with the appropriate ends to go with my new gearbox. A phone call to MGOC spares resulted in an MGB cable on a "try it an see" basis which fitted just perfectly.

And that, as they say, was just about it. I took it for its first test drive moved the gear lever switch - and nothing happened ! I flicked the switch again and again, I tried it in third, in fourth, flicking the switch, but it just stayed in the normal gear ratios. I was truly stumped after all that work! It also happened to be late, cold, dark and raining - isn't it always when things go wrong? I Packed up and had a long hot bath. Next day I check over the wiring.

Nothing wrong there. I went for another test drive, and this time after driving a couple of miles it suddenly started to work ! (and the sun was shining!) The only explanation I could think of was simply that the overdrive oil was cold at first and that once it had warmed up all worked fine. It has given no trouble at all since that first evening. Midget was just trying to frighten me!

Normal driving is transformed! Revs drop by about 20% when you flick that little switch and the overdrive unit operates almost instantly. The engine is much less strained in overdrive top on main roads and when driving on the twisty Lake District roads it is much quicker to switch in and out of overdrive third than changing between third and fourth gear.