Author Topic: SRM Dynamo Belt Drive  (Read 724 times)

Online muskrat

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Re: SRM Dynamo Belt Drive
« Reply #15 on: 18.01. 2021 19:27 »
G'day Mosin.
Speaking of tension (the belt, not you). The belt needs to be slack enough that you can twist it 90 degrees in the middle of the run, but not slack enough to hit the raised boss of the middle screw hole. Most of those have gouge marks from loose chains and will destroy a belt in no time. DAMHIK.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline Mosin

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Re: SRM Dynamo Belt Drive
« Reply #16 on: 18.01. 2021 20:06 »
Hi Muskrat,

I saw the bit about the 90 degree twist in the instructions. I am assuming that it means 90 degrees forwards to backwards (as you are looking at it while kneeling next to the bike) rather than 90 degrees left to right (as you are looking at it WKNTTB)? It isn't very clear from the SRM instructions and could be interpreted both ways.

I know that I haven't explained what I mean very well here. But clearly 90 degrees is a very different direction depending on whether you are sitting on the bike looking at it from front to back, or kneeling next to it. Hopefully that makes some sense!

Mosin
1960 A7 Shooting Star
1959 D3 Bantam
1994 Triumph Trident 900

North West England

Online bsa-bill

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Re: SRM Dynamo Belt Drive
« Reply #17 on: 18.01. 2021 20:20 »
I assumed it means to twist the belt ninety degrees from flat as in the link below

https://www.youtube.com/watch?v=9RAUtE2aztk
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Mosin

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Re: SRM Dynamo Belt Drive
« Reply #18 on: 18.01. 2021 20:30 »
Cheers Bill That was my second interpretation and the one I opted for. My first interpretation was to look at the place finger and thumb above and below the belt and rotate 90 degrees left to the left - so the belt formed a Z shape between my fingers. Clearly this would result in the belt being WAY too slack. You would think that the instructions might be a bit more specific though. Getting it wrong could result in all sorts of costly problems.
1960 A7 Shooting Star
1959 D3 Bantam
1994 Triumph Trident 900

North West England

Offline S-O Skott

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Re: SRM Dynamo Belt Drive
« Reply #19 on: 16.02. 2021 11:54 »
The idler shaft thread was originally 3/8 x 26 tpi cycle.

When you have the large pulley secured in place and with the belt on both pulleys try gently tapping the dynamo pulley onto its taper, that  may grip enough to allow you to tighten the nut.

The dynamo shaft thread has a different thread,  3/8 x 20 tpi BSF.

The other way is to remove the dynamo and partially strip it so that the shaft remains in the drive end bearing and housing. Carefully grip the armature in a soft jawed vice and then fix the pulley.

On my bike (BSA A10 plunger 1954) the tread on the idler shaft wasn't 3/8 x 26 tpi, if I remember right it was 3/8 x 27 tpi. Had to go to a friend who have a lot of old and strange tools to make a new nut because I couldn't find a nut to buy anywhere.