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1972 Roadster - Speedo gearbox setup

Submitted by tom_mortensen on Sun, 18/03/2012
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Commando

Having crushed the 2nd speedo gearbox in only a few miles, something must be wrong with the setup or order of spacers. The first gearbox came with a flanged spacer to go inside the hole in thegearbox. It clamped the gearbox securely, and seemingly it turned freely, when the wheel spindle was tightende up. But it gave up the ghost in about 100 miles. The gears inside the box were so worn that they rattled inside. I blamed the bloody Indians forproducinga poor imitation of the original item.

But now the second gearbox has given up too. So I am not so sure about the Indians after all! This gearbox did not have the flanged spacer it fitted snugly over the spindle and clamped up nicely.

Any tips or experiences out there, to prevent me from taking violent action against producers of speedo gearboxes?

One important question: Should the gearbox be clamped up tightly or should it be free to rotate on the spindle?

Submitted by david_henshaw on Sun, 18/03/2012

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The speedo gearbox you refer to , if not connected to the speedo cable should be free to rotate around the wheel spindle. The attachment of the speedo cable prevents this. Also I believe the spacer you are talking about, with the flange is an item that is needed ,at least it is an integral part on the parts breakdown in the parts manual. Sounds like there is something wrong in your rear wheel bearing assembly. I would check your rear wheel bearings and make sure you have the correct 15/12 drive assembly before you destroy any more. At the worst it sounds like you may be courting a high speed rear wheel lock up, something that will ruin your whole day. I hope this helps you. Craig

Submitted by tom_mortensen on Tue, 20/03/2012

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Thanks, yesit helps a lot.You have identified at least two problem areas, that I will have to look into. One is the free movement of thegearbox and the second is the flanged spacer, which must be the one around which the gearbox should be allowed to revolve. Thanks so far!

Submitted by david_evans on Tue, 20/03/2012

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Your speedo gearbox should be clamped between the spindle spacer and the bearing spacer. It should not be free to rotate about the spindle unless the spindle is loose. If you have crushed the gearbox, I would suggest that the bearing spacer is either not the correct length or maybe the bearings are not seated correctly. Craig's suggestion that the cable stops the gearbox from moving is incorrect. Sorry Craig.

The quality of the gearboxes is poor with most of them coming from overseas. Even the original ones were not a particularly robust design. The body of these is made from a mazac type of alloy similar to some carbs. When tightning the RH rear wheel spindle you will compress the thin section of this alloy slightly. If this area of the gearbox is being distorted, the spacing is incorrect.

Submitted by david_henshaw on Tue, 20/03/2012

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I am sorry Tom if I gave you some bad information, I was going by my experience with my 71 commando. I will leave future solutions to the experts in England. Craig

Submitted by david_evans on Wed, 21/03/2012

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Craig, I have a 70 Commando and the speedo gearbox should be clamped, not floating around. If your 71 bike has a loose speedo gearbox, something is not right. I wouldn't ride it with a loose speedo gearbox if I were you.

Submitted by richard_payne on Wed, 21/03/2012

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The most important thing is to have a look at the parts book and make sure that all spacers are present and correct. The spacer on the inside needs to be hardened or it will compress.

I've found out by experience that a couple of pumps on a high-pressure grease gun is also not a good idea. I managed to quite badly distort a pattern body (and on the 850 Mk3, this forces the sealing ring against the sprocket) - on an earlier model, I'd imagine that it could take out the hub cover.

Submitted by tom_mortensen on Thu, 22/03/2012

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Thanks for the updates on whether the gearbox should rotate or not. This coincides well with my experience from other British bikes, with a similar setup, where the gearbox shell is clamped up tight. I will proceed and make sure I have both the correct inside spacer and wheel spacer. On my Matchless I have had similar problems, accentuated by a very illogical order of assembly of spacers and seal cups! For this bike I was fortunate enough to find an N.O.S. Smith gearbox, which is the only one that has lasted, now for more than 10.000 miles. Lately I have seen advertised repro gearboxes, made in England! Can this really be true, anyone know about these?

 


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