QuestionHi, i have a 1978 MGB and found that the driven gear does not rotate with the drive shaft. Hence why my speedo is not working. I thought it would be simple to pull out the sealing washer and bearing which holds the driven gear in place but seems to truly stuck and cannot pull it out by hand. I even broke part of the flange that sits on the outside of the OD unit when trying to prise it off with a screw driver. Is there a technique in removing this driven gear to examine it?
Secondly, is it quite likely to repair the driven gear i will almost certainly have to repair the drive gear - which i believe means taking the engine, gearbox and OD out of the car ??
Answer
Overdrive speedo gear
Hi Emilio
This is a very common problem with the MGB overdrive unit. Unfortunately, if the speedometer drive gear is stripped, then you will have to remove the engine, gearbox and overdrive unit in order to replace the drive gear.
Here is an extract from my book. (I refer to the driven gear as the pinion gear):
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The MGB overdrive unit has a small nylon pinion gear (which is easily removable without stripping the gearbox) and an internal nylon speedometer drive gear (which is not). I attach a photo of a stripped speedometer drive gear.
Why on earth they didn’t make the internal gear of metal I have never fathomed. If the speedometer cable gets kinked and difficult to turn, these gears invariably strip their teeth, and fitting a new pinion gear by itself will not cure the problem.
The speedometer gears are colour coded, so make sure you use the right ones. Chrome bumper cars have a speedometer gearing of 1,280 turns per mile, and the gears are either blue or clear nylon. Rubber bumper cars are geared at 1,000 turns per mile, and the gears are red.
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You should be able to pull the driven gear and bearing assembly out using pliers. They are shown as parts number 33 and 30 in this diagram from Moss Motors:
http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29017
The bearing assembly is a tight fit, held in by O rings, so pull hard. Before you can remove it you must first remove the retaining bolt and lock plate (shown as part number 34).
