Replacement speedo gearbox '62 31CSR

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Andy G
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Location: Aberdeenshire UK

Replacement speedo gearbox '62 31CSR

Post by Andy G »

I need a replacement speedo gearbox for my 1962 31CSR, but before I click “buy” I have been reading around the many posts and just wanted someone to check my work…

Realising that the wrong selection or installation can be a potentially life-changing mistake, I find:
1. I have speedo (120 mph) marked SC3312/00 which Smiths list as the US export fit for Apache, but matches same g/box as their listed speedo for 31CSR (1301/02)
2. The Smiths part number of the g/box for both is 5330/267, which I assume to be same as…
3. 1960 AMC Parts list 024448 as g/box all models and…
4. AMOC parts lists 900520 as the relevant replacement. So far so good...
5. This part will require reaming to fit
6. My speedo is also marked “1548” which I believe is related to the ratio?
7. I have 19” wheel with AMC hub
8. Current gearbox is underslung on right side

So, as far as I can tell, I need AMOC 900520, but reading around suggests that this must be fitted with a modified washer/spacer and using the well documented procedures in archives etc. to avoid rear wheel falling off.

Could some kind soul suggest if I am about to order the correct g/box,
Point me at the part number or measurements for modified spacer if available, which I have not been able to locate,
Or indeed tell me that I am barking up entirely the wrong tree!
Regarding the reaming – without access to other engineering tools, can this be attempted with a file, or is there too much material to make this practicable?
And finally has anyone any additional notes before I open the wallet?

Many thanks
Bombling contentedly around rural Aberdeenshire
Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Replacement speedo gearbox '62 31CSR

Post by Mick D »

Hi Andy

As far as I am aware all Smiths gearboxes of that era are dimensionally the same, just get the correct ratio.

Repro gearboxes seem to have some 'tolerance' issues which need to be addressed, easy enough if you are aware of them - I'd buy the box and then assess the route to installation and use.

What's wrong with the one you have? First choice would be to repair it.

Regards Mick
Andy G
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Posts: 358
Joined: Mon Jun 01, 2020 9:42 am
Location: Aberdeenshire UK

Re: Replacement speedo gearbox '62 31CSR

Post by Andy G »

Hi Mick and thanks.
The original appears to have worn worm gear, keeps skipping. It was missing a grease nipple when I got the bike (replaced with a screw) so had quite possibly been uncared for in former life!
I can drive the cable end with a drill and the speedo happily steps up and down to at least 100mph, but when connected and on the road it maxes out at around 30mph and trip registers around 70% miles traveled.
If I hold the speedo end of the cable and spin the rear wheel I can feel the mechanism catching and missing intermittently.
Disconnected both ends the new cable rotates freely in the fingers.
Would you interpret fault differently?
Is it possible to obtain replacement worm gear?
Bombling contentedly around rural Aberdeenshire
Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Replacement speedo gearbox '62 31CSR

Post by Mick D »

Hi Andy

I'd agree with your diagnosis and I'm not aware of anywhere to buy spare components.

Regards Mick
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clive
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Location: LONDON UK

Re: Replacement speedo gearbox '62 31CSR

Post by clive »

I think you could be over thinking this. If you have a 1548 ratio speedo it is pretty well correct regardless of the number on the face (assuming of course that someone has not just fitted a new face to totally different works :o ) There are two styles of gearbox, those with a smaller hole for the earlier single sided hub and those with a larger hole for the spacer fitted to the full width hub. It is possible to open up the hole but I would not recommend trying to do it with a file. You need a 2:1 clockwise gearbox.
As to life changing events it is not the speedo or gearbox that cause the problems with the rear wheel locking up, its having the internal bearing parts in the wrong order that cause the problems.http://archives.jampot.dk/technical/Whe ... pieces.pdf

What causes the problem is some people put 17 the oil seal inside the cup 18 this seems logical (as it mirrors the arrangement on the other side of the hub 3 and 4) but is wrong and the diagram is correct. Of course if a worn out gearbox stops stuttering and locks up completely its a problem too, but more usually if they are worn out they just rotate without driving the cable.

Cannot help on where to get the gearbox, I am still using ones bought at jumbles 35 years ago.
clive
if it ain't broke don't fix
Andy G
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Posts: 358
Joined: Mon Jun 01, 2020 9:42 am
Location: Aberdeenshire UK

Re: Replacement speedo gearbox '62 31CSR

Post by Andy G »

Noted Clive and thanks.
That cup-washer conundrum caused some head scratching when I rebuilt the wheel...and couldn't have worked it out without help from here!
Time to get the wallet open I guess!
A
Bombling contentedly around rural Aberdeenshire
39speedtwin
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Location: ESSEX UK

Re: Replacement speedo gearbox '62 31CSR

Post by 39speedtwin »

Be very careful of Indian drives, some have nylon gears which only last a few miles, best to buy a NOS Smiths 2:1 if you can find one, the centre hole can easily be enlarged if necessary.
Andy G
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Posts: 358
Joined: Mon Jun 01, 2020 9:42 am
Location: Aberdeenshire UK

Re: Replacement speedo gearbox '62 31CSR

Post by Andy G »

Agreed, for this I'll be willing to wait for a trusted item!
Bombling contentedly around rural Aberdeenshire
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