Author Topic: M20/M21/M33 Speedo Drive Gear Queries  (Read 314 times)

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Mr Jones

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M20/M21/M33 Speedo Drive Gear Queries
« on: 19 April, 2022, 22:54:03 »
Ive just finished restoring a 1948 M33 which was built from boxes of bits. The speedo I had (which I presumed and still think was original), was sent off for full restoration and came back almost as new (lovely job). I also replaced the front wheel speedo drive gear unit and cable (bought on line). When I took the bike out for itís first test ride yesterday, I noticed the speedo wasnít working. Long story short, the cable is rotating in the wrong direction i.e. when looking at the back of the speedo, itís rotating clockwise and the speedo doesnít work. When I use a drill to drive the speedo cable in an anti clockwise direction, the speedo works perfectly. I am assuming that the new front wheel drive gear unit I bought is rotating in the wrong direction.

My question is can someone please remove their speedo cable from the speedo and looking in the direction of the back of the speedo, let me know if itís turning clockwise or anti clockwise. This way Iíll know if my drive gear is wrong or if in fact the speedo I have is not original to my M33.

Any help will be very much appreciated.

Pete C

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #1 on: 20 April, 2022, 06:46:21 »
I'm not familiar with the front wheel drive but I have heard of this problem before on the WM20 website. I suggest you post your problem there and I'm sure you will get an informed answer. They have helped me no end with my bikes  ;D

Go to the forum section here http://wdbsa.nl/

spaceman

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #2 on: 20 April, 2022, 08:16:08 »
I believe this M33 should be fitted with an S433/3/L speedo with the L signifying that the input should rotate CCW when viewed from underneath; that is, the cable should rotate in the same direction as increasing speed on the dial. Therefore, the output from the speedo gearbox should rotate CW when viewed from above.

Assuming you have a S433/3/L speedo fitted then, from what you have observed, the speedo gearbox has the wrong rotation direction.

idie

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #3 on: 20 April, 2022, 10:02:16 »
Although you haven't said, I take it you have girder forks, if so you need a drive unit that rotates in the opposite direction. These are quite hard to find. These units were fitted to the WDM20s during the war.

spaceman

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #4 on: 20 April, 2022, 10:18:27 »
An alternative would be to reverse the drive on the speedometer gearbox you have by popping the end caps, removing the worm gear and putting it back together the other way around. The only downside is that the speedo cable would emerge from the gearbox behind the fork rather than in front of it.
« Last Edit: 20 April, 2022, 10:20:38 by spaceman »

Mr Jones

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #5 on: 20 April, 2022, 11:13:50 »
I believe this M33 should be fitted with an S433/3/L speedo with the L signifying that the input should rotate CCW when viewed from underneath; that is, the cable should rotate in the same direction as increasing speed on the dial. Therefore, the output from the speedo gearbox should rotate CW when viewed from above.

Assuming you have a S433/3/L speedo fitted then, from what you have observed, the speedo gearbox has the wrong rotation direction.

I have a S433M as per the picture below, taken before it was renovated. It does not have an "L" but it operates exactly as you say i.e. the cable rotate in the same direction as increasing speed on the dial. So my problem is definitely that the new geardrive is turning the wrong direction.

Thanks for helping me define the exact problem.
« Last Edit: 20 April, 2022, 11:27:09 by Mr Jones »

idie

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #6 on: 20 April, 2022, 11:17:02 »
Unfortunately not all the drive units can be dismantled as many have no caps.

Mr Jones

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #7 on: 20 April, 2022, 11:35:20 »
An alternative would be to reverse the drive on the speedometer gearbox you have by popping the end caps, removing the worm gear and putting it back together the other way around. The only downside is that the speedo cable would emerge from the gearbox behind the fork rather than in front of it.

Thanks, I'll investigate if this is possible.

Mr Jones

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #8 on: 20 April, 2022, 18:47:08 »
An alternative would be to reverse the drive on the speedometer gearbox you have by popping the end caps, removing the worm gear and putting it back together the other way around. The only downside is that the speedo cable would emerge from the gearbox behind the fork rather than in front of it.

I can confirm that it has end caps, are they just press fitted?

spaceman

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #9 on: 20 April, 2022, 19:36:35 »
They are just pressed in but may have shallow longitudinal ribs to aid retention. I would recommend taking out the other innards including the felt washer/seal before trying to remove the end caps - a bit of heat to expand the casting might make extraction of the end caps a little easier.

Mr Jones

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #10 on: 20 April, 2022, 23:14:45 »
Noted, cheers!

Mr Jones

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #11 on: 29 April, 2022, 18:47:50 »
They are just pressed in but may have shallow longitudinal ribs to aid retention. I would recommend taking out the other innards including the felt washer/seal before trying to remove the end caps - a bit of heat to expand the casting might make extraction of the end caps a little easier.

Just to close this off. I couldn't get the end cap out without destroying the soft alloy body so in the end I machined it out and turned up a new end cap. Perfect end result, all working perfectly. Thanks for a great suggestion!


spaceman

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Re: M20/M21/M33 Speedo Drive Gear Queries
« Reply #12 on: 29 April, 2022, 18:57:34 »
Glad you managed to sort it - a nice bit of machining :)