The Triumph Dolomite Club - Discussion Forum

If 1850and Sprint share a drive gear thats fine, but the driven gear tooth count will (or SHOULD) vary depending on which diff is fitted to the car in question. Standard Sprint driven gear is AEU 1577 or NKC52. (both are listed for Sprint with O/D, Superseded number perhaps?) 1850 (single rail) driven gear is AEU 1578.

Steve

Do you know the number of teeth that the sprint drive gear has?

I think it will have 21 teeth.

I have noted that the drive gear is permanently engaged in the centre of the pinion gear so the number of its starts does not alter its gear ration. The gear ratio is only changed by the number of the teeth on the drive gear. I believe that to get drive gear with different numbers of teeth to engage correctly without damaging the pinion drive the angle of the pinion drive teeth has to be changed slightly and when this is done it results in the number of starts changing.

Again I believe the pinion damage you have found would be consistent with the wrong drive being fitted.

I agree, the only way of getting a different number of teeth on a constant circumference and of constant pitch is by changing the angle of skew…but there’s usually scope with skew gears to achieve this as skew gears don’t mesh as tightly as straight cut gears. This is why we can achieve a quick steering rack by changing the number of teeth on the pinion.

I can’t imagine different overdrives having differences in the case casting to accommodate slightly different speedo drive angles…but happy to be proven wrong…

The other accommodation could be if the drive gear had slight ability for lateral movement along its shaft it could then cope with different angles, and if this movement were to be restricted it could cause the wear shown?

Just wondering, but the adaptor on the single rail box to join the OD to it is about 20mm smaller than the 3 rail one,
Is it possible that the two output shafts shown here are labeled the wrong way around? Im no expert.

I've just dug out a Sprint overdrive unit which I've had for years. It had a red driven gear with 19 teeth. I counted the starts on the driving gear (as Richard described earlier) and it has 6 starts. I then simulated the motion of both and the green 18 toothed gear was exactly 3:1 whereas the red 19 toothed gear was slightly different, as expected. Both seemed to run ok in the driving gear probably because there isn't much of a difference between them? I did dig out a white/clear driven gear, which has 22 teeth, but that seems to be out of something different.



As to Alun's question, the driving gear is locked solidly in position between the two main bearings. It has no Woodruff key or anything to stop it rotating other than the pressure of the torqued nut on the end of the drive shaft.

Just wondering, but the adaptor on the single rail box to join the OD to it is about 20mm smaller than the 3 rail one,
Is it possible that the two output shafts shown here are labeled the wrong way around? Im no expert.

Three rail gearbox adaptor face to face 42mm
Single rail gearbox adaptor face to face 64mm
Difference 22mm

Difference between three rail and single rail mainshafts 21mm

Approximately, by my measurements, the Single rail adaptor is longer by 22mm and the single rail mainshaft longer by 21mm so the single rail main shaft is correctly labelled as the top shaft. As an aside this is confirmed by the nose of the mainshaft (on the right of the picture) where the bearing between it and the input shaft fits as it is much larger than a three rail.

Most people will know that I only play with 1500HL dolomites but I am very interested in the speedo drive as back in 1992 I purchased an exchange O/D unit for £125 but when I fitted it the speedo did not read the correct speed. The outcome was that I had to take it off the car and the Overdrive repairer overhauled my old unit to ensure that the speedo would read correctly. Hence when Martin stripped and overhauled three of my O/D I asked him to advise me how many starts the drive gears had and how many teeth the pinions had. He found that the two O/D that were built for the 1500cc cars based on the O/D serial numbers 25/115867/**** they were for cars with 3.89:1 rear axles they had brown coloured drive pinions which had 18 teeth and that the drive gear had 5 starts. The other O/D based on its serial number was built for a 1850cc dolomite so its rear axle would have been 3.63:1 which is the same as the very late 1500cc dolomites. This had a drive pinion with 20 teeth and a drive gear with 6 starts.

I have been looking at various documents this afternoon and the official Triumph dolomite range 1976 onwards part catalogue document RTC9822CB indicates that all the O/Ds have a drive gear part number NKC45, so if this is true it does mean that you can fit a different drive pinion without doing any damage and this does not need the O/D to be removed from the car. I wish I knew this in 1992 and why was I not told to change the drive pinion?

I have just spoken with ORS again and have been informed that it doesn't matter how many starts the driving gear has nor how many teeth the pinion gear has, they are all interchangeable.

Mad Mart wrote: ↑Mon Sep 12, 2022 12:38 pm I have just spoken with ORS again and have been informed that it doesn't matter how many starts the driving gear has nor how many teeth the pinion gear has, they are all interchangeable.

Yes I have always believed this to be true, you CAN use any driven gear with any drive gear. HOWEVER, only one combination will make the speedo read more or less true!

Steve

I did dig out a white/clear driven gear, which has 22 teeth, but that seems to be out of something different.

Well, I found out today what this drive was from, as I'm in the middle of rebuilding one, a non-overdrive Sprint gearbox.