'34 Tourer, 2 synchro, 4 speed box

Speedo (Magmo) has stopped working. It is a new cable, everything looks and feels good all the way to the drive gear at the gearbox end - the one in the brass mounting that you can unscrew and have a look at.

So - what has gone wrong in the gearbox? Presumably the speedo gear is fixed in some way to a shaft, and that device has broken?

I have never got that far into a gearbox, am asking for information and whether leaving it for the time being and fitting a bicycle speedo will do, or is more urgent attention required?

Cheers

Simon

Location: On a hill in Wiltshire

If I remember correctly the gearbox end of the speedo cable has a squared female end that matches up with a squared shaft on the speedo drive pinion.
You can remove that part of the mechanism from the gearbox without much problem just by undoing the brass nut. The speedo drive pinion and its little shaft will come out. You should then be able to see whether the square shaft is capable of driving the inner cable. Often there is wear which rounds off the corners and allows it to slip. You can also check on the state of the drive pinion which can also wear. Replacement is a reverse of removal. The speedo drive worm is a press fit onto the end of the 3rd motion shaft and accessible after removal of the prop-shaft flange and rear cover if you're going further down that road.

Location: Gard, France 30960. Used to be Languedoc-Roussillon but now it's Occitanie

Thanks.

I took the drive bit out of the gearbox, the squares (male and female) are fine, at both ends of the cable.

So the "press fit" could have loosened? Seems a bit unlikely, but at least there isn't anything to drop out and get chewed up.

Or I have missed something, which is quite likely.

Cheers

Simon

Location: On a hill in Wiltshire

Doubt it. If you jack a rear wheel off the ground and rotate it (gearbox in neutral engine off) you should be able to see the output end of the pinion shaft turning if the internals are ok (you might need a Baldrick unless you've got 10' long arms...)At least you can then determine if that's where the problem is.

Same applies at the speedo head where the female part can round off.

Location: Gard, France 30960. Used to be Languedoc-Roussillon but now it's Occitanie

Great. That gives me something to try. Take out the drive thingy into the box, shove a pencil or similar up the orifice, twiddle the propshaft, and feel if the gear is slipping.

Thanks.

It has to be in the box, speedo square is good fit to cable square, cable square is good fit to 'box square. With cable removed from the speedo, push middle hard down and drive along, there is no discernable movement, no drive at all.

Location: On a hill in Wiltshire

Looking at the parts list, it looks as if the drive gear might be sandwiched between the bearing and the output flange, pressure being exerted on it by the flange fixing nut.

If so .... one of the things I have changed is the propshaft, which meant changing the drive flange, which might be a bit shorter than the original, which means a shim might fix the problem.

Or the nut has come loose....

I will be crawling underneath tonight to take a look.

Simon

Location: On a hill in Wiltshire

Looking at the drive set I have on the bench the drive gear/worm is only showing wear on half of its width which suggests that if yours is past redemption you should be able to draw it off the 3rd motion shaft and reverse it so the pinion bears on the unworn part. Sending photo.

Location: Gard, France 30960. Used to be Languedoc-Roussillon but now it's Occitanie

Having got under and removed the drive thingy, I find that not only is the speedo gear free on the shaft, but it has considerable fore-and-aft movement.
At least 1/16th of an inch, if not more.
The cunning plan is to pinch it up between the output flange and the bearing/gear cluster, using a washer as required....
but that amount of movement needs more understanding of the set up.
Fortunately, I have a spare gearbox that I can play with.

To be continued...

Simon

Location: On a hill in Wiltshire

To conclude this: I took the speedo drive housing off. I didn't think this could be done with everything in the car, but it can, no problem.

The splines for the flange on the output shaft go all the way to the bearing, ie. the design is that the speedo drive worm gear is held in place by being sandwiched between the propshaft flange and the bearing.

It may or may not be an interference fit on the shaft, but I don't think it is engineering practice to rely on an interference fit on a set of splines.

I found that the very secondhand flange I had acquired was tight on the splines, but not up against the gear. Some cleaning with small files allowed it to slide, with some persuasion, all the way to pinch the worm gear.

So, is all good in the speedo department? No, I find that the cable has snapped.
It has snapped because the input shaft on the speedo head is tight, it turns, but not freely. It is tight on the spare head also.

So that is the next little job. But if the speedo hadn't stopped working, I wouldn't have known that the output flange wasn't back where it should be, which maybe could have had consequences for the gearbox in the long run.

And I have done a load of gardening today, so still have some Brownie points!

Cheers

Simon

Location: On a hill in Wiltshire