See my post in this thread for Overdrive experts. I did think the best way around the problem might be to fit the MGB GTV8 or MGC pinion but I can't find anyone who lists that part. From here on you are best using a specialist because the overdrive needs dismantling to fit a Blue speedo gear that will mesh correctly with a White speedo pinion with 21 teeth. Unfortunately to put this right, the engine and gearbox need to be removed from the car (you can't just remove the gearbox leaving the engine in situ) then the overdrive has to be removed from the rear of the gearbox. I think what has happened in your case is someone has had an overdrive problem and found another LH type overdrive which has come from and MGC or a GTV8, and bolted this to the rear of an MGB gearbox. This gearbox casing will not bolt to the rear of the 1800 Engine. However both these cars used a larger diameter clutch which means a different gearbox casing. Also why might the pinion no longer be meshing with the worm gear?Īfter a bit of research I've discovered the MGB GTV8 and the MGC used a green speedo drive gear on the output shaft of the Overdrive and both used a higher rear axle ratio than the MGB so the speedo drive is geared differently.
I have checked that the worm gear is firmly located on the output shaft.Ĭan anyone shed any light on why I might have a green worm gear and what pinion it should use. (Later cars had red worm gears with red pinions.) According to the online parts catalogues of parts suppliers I would expect it to be blue if the pinion was white. Looking into the hole in the overdrive, I can see that the worm gear is green. Having extracted the pinion and its housing, I can say the pinion turns freely in the housing and the teeth do not seem to be worn. Investigation indicated the pinion was not turning again. That solved the problem until this summer when the needle started waving about and under reading until it stopped registering again. The pinion that came out was a white one so I replaced it with a white one. I bought them second hand in the mid 1990s.
The gearbox and overdrive are not original to the car. It was not turning as the prop shaft turned and the teeth looked worn. Investigation indicated that it was the pinion that was at fault. Last summer the speedo in my chrome bumper 1974 MGB failed.
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