I have copied below an article that I produced for Dolly mixtures way back.
Fitting Overdrive to a 1500cc Dolomite
A question that comes up from time to time is what is involved in upgrading a 1500 car to have O/D. In this article I do not intend to repeat all the details that are really available in both the Official dolomite workshop repair manual and the Haynes manual but I will attempt to identify possible pit falls and list the items required to undertake the upgrade as far as I can remember as it is now a number of years since I undertook this task.
Back in editions 107 to 110 of Dolly Mixture, David Anderson, who was formally the Chief Design Engineer at Laycock Engineering kindly wrote a series of articles on the J type overdrive and I have taken some of my details from those articles. For instant he points out that all O/D units have a serial number plate, which is secured to the front of the hydraulic casing. On the plate is stamped the identification of the O/D unit. As an example it could be 22/61985/0013296. The “22” relates to the Overdrive Ratio. The “61985” indicates it is built for a TR6 and finally the “0013296” indicates that it is the 13296 unit to be built to that specification. For our 1500cc Dolomites the Overdrive ratio is 25 so the first two digits are 25. Then because later 1500cc cars have a different rear axle ratio to the earlier cars there has to be two specific build standards of O/D units. The later models are fitted with units identified by 25/115868/**** and the earlier units 25/115867/****. If you fit the wrong one your speedo will read incorrectly. I have not been able to identify at what point in time Triumph changed the rear axle ratio or the reason behind it. It is not a widely known fact and the only place that I have seen it documented is in the official Triumph Dolomite 1500HL handbook Edition 6. That states on page 69 that the early models had a 3.89:1 and that the later models have a 3.63:1 ratio. See extract below.
In the late 1960 speedo instrument manufacturers standardised on a standard 1,000 speedo, designed and calibrated to operate at 1,000 speedo cable revs per mile. This meant that reduction gears in the tail of the gearbox or O/D had to be selected to produce 1,000 cable revs per mile taking into account the size of the tyres and the axle ratios. For J type O/D to be fitted to a car with the 3.89:1 axle ratio I believe the speedo drive gear is a part number NKC46 and this drives a drive pinion part number NKC49. If your car has a 3.63:1 axle the driver gear is a part number NKC45, which works with a drive pinion part numberAEU1578.
Unfortunely I do not know at this time the numbers that indicate that the O/D has been built for a Sprint or 1850.
In 1977 I purchased a new Brown 1500HL, which came with O/D and I have driven it daily ever since. We purchased a non O/D blue 1500HL for my wife in January 1995 and we soon decided that we would upgrade it to have O/D. Luckily, some time in the early 90, when we had started to look for a second 1500 Dolomite and we met a man who was also looking for a good 1500 dolomite as he had a rust bucket with O/D that would not pass its next MoT. The outcome was that I offer to give him a few pounds more than he would have got from a scrap yard if he delivered the car to my home when it came up for its next MoT. One of the first tasks that I undertook was to raise the rear wheels and to run the transmission normally. Engage overdrive, and disengage with the clutch pedal depressed. (This releases the spline loading between the planet carrier and the on way roller clutch). Failure to do this makes it difficult to separate the O/D from the gearbox. I then spent a number of months stripping the car down. This was the source of the bits for the blue cars upgrade. The list of bits required in addition to the gearbox, gear lever extension assembly, adaptor plate and J type O/D are as follows: (I hope that I have not missed any out and it has been difficult to confirm that the Part numbers are correct so I recommend you do not just quote part numbers list to any supplier but discuss your requirements before you purchase):-
Rear engine rubber mount (This may be part number 219227 (Added Sept2016 or TKC1044)
Restrictor plate (Fit to bottom of rubber mount) part number 160268
Mounting bracket to connect O/D to rubber mount. I can’t identify the part number for this item. (Added July 2012 It is part UKC2797)
Bracing plates (These secure the next item the cross member to the car body and can easily be fabricated) 2 in number required. ( part number 155304)
Bracket (Cross member). ( Correction Sept 2016 Part number 218519)
Bracket (To clamp exhaust pipe to rear of Overdrive - part number Not known) (Added Sept part number 2016 UKC4098)
O/D speedo cable part number GSD 300 (It is longer than the non O/D)
O/D gearlever UKC3283
Gear knob part number UKC7614 - (Note I believe there is a difference between a sprint and the 1500 gear knob caps in respect to the engraving as reverse gear is in a different location)
O/D switch part number 520999
Harness part number UKC4612 (connects to the inhibit switch & reversing lamp switch) plus Extension harness overdrive part number 153726 (goes up the gear lever). Alternatively part number part number 159613 may be the complete harness requirements.
Angle drive for speedo cable 120694 (It is worth checking that this works before it is fitted in the car). A tip from clifthanger on the clubs internet discussion board recently was “If the angle drive is broken (usually the little bit of cable that goes into the gearbox) they can be repaired. Take apart by carefully prising the disc back off. needs the little pressed tangs either bending/removing. Take the internals out and clean up in white spirit/whatever. Now clean out the remains of the old bit of broken cable.
Now the tricky bit. I use old speedo cables, cut the end off to the correct length, I use a disc cutter, snips tend to cause fraying. And pop into the angle drive "bit". This needs to be fixed, a friend has used JB weld epoxy, I have applied a careful dab of mig, and more recently silver solder. Sounds a faff, but the cheap nasty repro items which can slip after a short time are £30+, and originals are getting harder and expensive to find, hence the repairs.”
Before I fitted the upgrade I took the time to strip and rebuild the gearbox using a Rimmers reconditioning kit, which included new bearings, synchro rings, layshaft, thrust washers, circlips and gaskets. It was well worth the effort as some of the bearings and the layshaft were coming to the end of their life. The layshaft and its roller bearings in particular. The layshaft and its associated 50 rollers are a known weak area for these gearboxes. It may not have been helped because there was a period when some lay shafts were not case hardened correctly and the bearing surface failed. There was also a period when at least one supplier was supplying the nearest metric rollers in place of imperial rollers and these would seize up when they got hot and expanded. In addition to the standard reconditioning items listed above I also made a point of replacing the selector shaft part number UKC4619 as this had been worn by the selector plunger. On all high mileage boxes that I have stripped and rebuilt I have found that the shaft has worn at this point hence my recommendation is check it and change it if it is showing signs of wear. When it comes to rebuilding a gearbox one of the critical tasks is ensuring that the rollers do not get dislodged from the lay gear cluster. The official dolomite workshop repair operations manual states using grease install the needle-rollers (25 each side) in the lay-gear and fit dummy lay-shaft tool 18G 1208. I have read that the trick is to embed them in vaseline as this will melt as soon as the gearbox oil starts to heat up. I made my dummy lay-shaft by reducing the length of the old lay-shaft that I had taken out of my first gearbox, so that it would just fit inside the gearbox casing. I was then able to fit this inside the lay gear cluster before it was lowered into the bottom of the gearbox casing. If you get the length just right it will also keep the thrust bearings in place. Lay-shaft as I have previously stated should be case hardened so it was not a simple hacksaw job.
When it came to the O/D I would have liked to have stripped and rebuilt it but all the available workshop manuals indicate that a lot of special tools are needed. The outcome was that I was somewhat limited in turns of what I could tackle. I ended up by obtaining a sump gasket part NKC76, so that I could take the sump off. This is very straight forward as it is just held on by six bolts. Under it is the rectangular sump filter, which just pulls off. I now have stripped three or four O/D units and I have never found much crud in this filter. However under the largest of the plugs, which are revealed when the sump filter is removed, is the pressure filter. It is relatively simple to make up a tool to remove this plug and based on my experiences it is well worth making an effort to clean this filter. I use either paraffin or white spirit as the cleaning agent but then dry the bits off before I reassemble them. There are a couple of seals that I would recommend changing, as they are relatively easy to do. These are the rear oil seal, part number NKC39A and the speedo pinion drive seal, part number NKC105A. I have included this second seal because for a long time on one of my cars I thought the rear oil seal was leaking and it turned out to be this seal.
Having refurbished the gearbox and undertaken as much work as possible on the O/D it is a matter of joining the two units together again. The workshop manuals make this sound a relatively simple task but I have never found it to be that easy. It is one of the few jobs that I have had to get my wife to assist me, as a second pair of hands is needed to hold the gearbox while the O/D is slid onto the drive shaft. Once they are back together they form a weighty lump.
One all important point to remember is that once you have fitted an O/D unit to the gearbox under no circumstances should anti-friction additives be added to the oil. Hypoy oil should not be used even though it’s recommended by Triumph (the gearbox maker) because Laycock (the overdrive maker) specifically excludes oils with Hypoy additives from its lubrication chart.
When it comes to removing the old gearbox and fitting the new gearbox complete with overdrive this can be done by leaving the engine in place if the front seats, parcel shelf and the gearbox tunnel section of the car’s floor are removed. Once these are out the gearbox complete with O/D can be lifted up and out through the body of the car. That’s the theory and I have done it this way more times than I like to remember because I do not own an engine hoist and it is only relatively recently that I have fitted a beam across my garage to which I can attach my chain block. A problem that I have found doing it this way is that unless the engine is moved forward until it is just about touching the radiator the rear coupling of the O/D will not clear the hole in the floor pan. You can get a non O/D gearbox out but you are unlikely to get an O/D gearbox back in based on my experiences. I remove the bolts from the front engine mounts to the sub-frame and this allows the engine to be slid forward sufficient to clear the hole in the floor. Now that I have a beam across my garage I prefer to lift engine complete with gearbox & O/D combination attached. To do this the lot has to be tilted at quite a large angle. Then once out the gearbox & O/D combination can be separated from the engine. It is a lot easier to refit the gearbox complete with O/D to the engine when it is out of the car. I have laid on the floor under the car and lifted the gearbox & O/D into place but it is very heavy.
A number of dolomite owners have stated that the prop shaft for a car with O/D is shorter than the one fitted to a non O/D car. I did not find this to be the case when I upgraded my wife’s car, however, when I upgraded my daughter’s first dolomite I did find that the front section of the prop shaft that was fitted to that car was longer than any of my other prop shafts and that this would not allow the O/D gearbox to be fitted. I obviously do not know if this is how the car left the factory or whether a replacement prop shaft had been fitted during the life of the car. The sliding joint in the front section of the prop shaft was sufficient to accommodate a non O/D gearbox but it could not cope when O/D was fitted. The front section had to be shortened to 32 cms..
When it comes to the electrics I have identified in the list of parts that a different gearbox wiring loom is required, this is because additional wiring is required by the O/D unit. Part of this loom or loom extension goes up through the gear stick to the O/D switch that is in the gear stick knob. I recommend that you replace this final section as the standard wires are prone to getting damaged where the wires come out of the top of the gear stick with piece of normal 240v 5amp twin sheathed cable. (note addded Feb 2017 the cable needs to be heat resisting such as used for lighting pendants) The sheath provides additional protection and since I carried out this modification I have not had any reoccurrences of the problem. If you fit the standard loom and the insulation breaks down under the high voltage that is produced when the O/D solenoid is de-energised by the operation of the O/D switch you get a very nasty voltage kick from the switch. It will make you sit up. (I think it only happens when you disengage O/D but it may happen when you also engage O/D.
Another electrical problem that I have encountered relates to the reversing light switch. This is somewhat difficult to get at once the gearbox and O/D is fitted back into the car. My advice is check, the connections and the switches operation before you refit the bits into the car. I have also taken to filing about 1/16 to an 1/8 inch off the casting where the switch mounts as I have found that the replacement after market switches fail to operate unless this modification is made. It is not easy to do once the unit is fitted in the car so it is well worth undertaking this modification before the unit is fitted.
The one switch that I have not mentioned so far is the inhibitor overdrive switch. This prevents O/D being engaged in 1st, 2nd and reverse. In general these are very reliable but I have had two fail. The most difficult to find was the one where the contacts would open and hence O/D would drop out when it got hot. In my case it turned out to be a very expensive fault as I took the car to an O/D specialist who told me the O/D needed to be replaced, as the hydraulic pressure was low when the oil was hot. I paid and the O/D was replaced only to find that on my way home the O/D was still dropping out. When I took it back the mechanics scratched their heads and then stated that as they had reused the original inhibitor switch. Once this was replaced the fault was cured.
Before replacing the engine in the car my recommendation is that you check the flatness of the exhaust manifold / downpipe flanges as one of the common problems with the 1500 engine is it blows the gasket at this joint. I have found a number of these flanges to be convex especially on new stainless steel exhausts. If the exhaust manifold flange requires attention it will require the studs to be removed to correct the problem but I think it is well worth spending the time and effort to correct the problem. The rear part of the exhaust downpipe also needs to be clamped to the O/D unit by a bracket. Unfortunately I have not been able to identify the part number for this bracket. If you can not obtain one from a supplier it is well worth making something up as the bracket helps to stop any movement at the exhaust manifold / downpipe junction. See photo below for details of the bracket plus exhaust clamp.
Note (added Sept 2016)
In the moss-europe spitfire parts catalogue in gives the following info for the O/D speedo drive. The spitfire has 13 inch wheels like the dolomite
NKC46 Gear speedo drive 3.89:1 axle ratio
NKC45 Gear speedo drive 3.63:1 axle ratio (I believe blue)
NKC49 Pinion, speedometer drive 3.89:1 axle ratio
AEU1578 Pinion, speedometer drive 3.63:1 axle ratio (I believe red)
I believe the axle ratio for a sprint is 3.45:1 which means it would have different again
These could be based on info in the triumph parts catalogue.
NKC45 Gear speedo drive 3.45:1 axle ratio
NKC52 or AEU1577 Pinion, speedometer drive 3.45:1 axle ratio
Last edited by Richard the old one on Sat Feb 11, 2017 9:03 pm, edited 1 time in total.