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Timing chain adjustment


billirwinnz

Well Known and Active Website User
VOC Member
I'm about to overhaul the magdyno on my A twin. I'm surprised to find a rather slack timing chain which is tensioned by two strips of rubber bonded to the inner case. Is this a bodge for a worn chain or should there be some more sophisticated form of adjustment?
 

A_HRD

Well Known and Active Website User
VOC Member
Bill,

Coincidentally, I've been struggling with the Twin mag-mounting for a few weeks now. After looking at endless photos and sketches, rubbings and samples, I made my own. Great, progress at last. NOT. Finally, I came to the only possible conclusion that the 1/2" slot is wrongly positioned on the original brackets. This limits any movement of the mag-mounting, (which needs to be in line with the two sprockets) and actually makes it cant over at an angle. So adjustment is severely limited. (I accept though that it could be an issue with the jigging of my frame at the front!).

Finally, I made a mock-up with the offending slot re-positioned and it works like a dream. So then I bought a piece of 1/4" alloy channel, 4 inches across, and marked it up as per the photo. I've since got it roughed out and hope to have it all fitted-up soon.....

I'd be interested to know if you've got full range of movement (in line with the sprockets) on your original mag-mounting bracket?

Peter B

P1080520.JPG
 

A_HRD

Well Known and Active Website User
VOC Member
Bill,

Not wishing to hi-jack your thread, but if you still have the chain cover off, would you mind measuring the thickness of the sprockets please? And also the the length of a roller in the chain (ie the inside dimension of the chain where it goes over the sprocket).

Reason is, folklore has it that the chain is 1/2 x 5/32". But I am not even sure that size ever existed; it's certainly not been available for decades. It appears that the sprockets are nominally 1/8". Therefore a 1/2 x 1/8" chain (available everywhere for bicycles) would be appropriate.

Peter B
 

vibrac

Well Known and Active Website User
VOC Member
I think by memory thats the same as the Scott magneto chain and they recommended a certain type of superior chain when I built my Scott I cant remeber what it was but could look it up if required certainly it has never slackened in use and its a long long way from central sprocket to mag sprocket on a Scott and its lubricated by air and road dust.....
 

A_HRD

Well Known and Active Website User
VOC Member
Tim,

Thanks, that would be useful if you can find the info reasonably easily. Certainly, if it is ordinary bicycle chain, there are all sorts of price/strength/manufacturer variants on the web.

The A Twin between-centres dimension is a nominal 6" plus scope for a further 1/4" of adjustment either side of that (sprockets are 12T). Lube is via whatever gets by the rear camshaft outer bushing - then it seeps out of a union and pipe at the bottom of the housing. Some people simply grease it liberally every so often and do their best to seal the rear camshaft oil source... Of course, if your name was Neal Videan, you would make the whole thing into a silent belt-drive. :) And if you owned the first production A Twin, you would be using the housing as a route for the one-off 'light spring and washer valve' breather - discontinued by the factory for the rest of the production for reasons unknown.

Peter B
 

vibrac

Well Known and Active Website User
VOC Member
This is what I got from Roger Moss via the Scott Forum (2016) it looks like 1/2 pitch but its cold and the Scott is wrapped up (NB Roger uses a mag belt drive on his racer)

However much later "The Chain Man" suggested I recommend a 3/16" wide chain by IWIS that was intended for use on the final drive of a 125cc Honda racer. He assured me that it would work fine and the extra width would give no problems in operation. I have recommended it to customers over a few years now and nobody has come back and said that this does not work. I can only pass on this information in good faith, as I have not used a mag chain for over 30 years, but believe this is very likely to be a quantum advance on rebadged Chinese bicycle chain. (I have never seem a bicycle with pedals revolving at 4500 rpm or more)
So to information
The Chain Man
Andy Forsdick
11 Gloucester Way
Bewdley
Worcs. DY12 1QF
 

billirwinnz

Well Known and Active Website User
VOC Member
Bill,

Not wishing to hi-jack your thread, but if you still have the chain cover off, would you mind measuring the thickness of the sprockets please? And also the the length of a roller in the chain (ie the inside dimension of the chain where it goes over the sprocket).

Reason is, folklore has it that the chain is 1/2 x 5/32". But I am not even sure that size ever existed; it's certainly not been available for decades. It appears that the sprockets are nominally 1/8". Therefore a 1/2 x 1/8" chain (available everywhere for bicycles) would be appropriate.

Peter B
Hi Peter

The chain on my bike has been changed at some stage with new sprocket rings welded to the old centres.

I see that you have received far more useful answers to your question than I could provide. Useful to me too as the extra rubbers were to compensate for the chain being at it's tightest adjustment yet still loose. I need a new chain.

Cheers Bill
 

Sakura

Well Known and Active Website User
VOC Member
Bill,



Reason is, folklore has it that the chain is 1/2 x 5/32". But I am not even sure that size ever existed; it's certainly not been available for decades.

Peter B
1/2 X 5/32" certainly is a size but quite probably doesn't exist new any more. I recently fitted a used length on the primary chain of a 1922 2 stroke Levis.
 

A_HRD

Well Known and Active Website User
VOC Member
Thanks for that Sakura. Interestingly, I am advised that Maughan's made a batch of A Twin mag-drive sprockets not so long ago. They are 1/8" thick.

Peter B
 

Sakura

Well Known and Active Website User
VOC Member
Obviously 5/32" is 1/32" wider than 1/8". I wonder if the 5/32" chain is meant to run on 1/8" sprockets to allow for slight misalignment on short run timing chains?
 

A_HRD

Well Known and Active Website User
VOC Member
Thanks Dinny,

What you refer to is part of the folklore to which I refer above. Richardson says much the same.
Bob also says in MPH 630 :- 3/16" x 1/2"!!

I too have considered Sakura's point while researching this issue for The Book.

Either way, given that 5/32" seems to have been unavailable for decades - and I have yet to find anyone with an A Twin sprocket greater than 1/8" - it is safe to say that there is little alternative now but to use 1/8" - regardless of what the original was.... (and check alignment carefully!).

Thinks I'll go for a belt conversion!

Peter B
 

Marcus Bowden

Well Known and Active Website User
VOC Member
Just goes to show how a couple of days away in Paris with the kids things certainly get a head of me.
Bill make sure you fit a spring/lipped Gaco seal into the inner chain case as I used nearly a gallon going to sea Proff Higgins in Sweden and then he gave me another gallon to get home with Got the sizes from Bob Stafford as he was always helpful to me also he congratulated me in using the "A" so extensively as he used his traveling back and forth to Germany when in the forces and they cam be got better with today technology . Or there is another mod that Robert Watson passed onto me and that was connecting a drain line to a "T" banjo bolt directly into sump, mine had to be a "T" as there was another from the "COFFIN" oil separator (threw away the coffin lid. ! )
FRONT CAM BEARING INTO CHAIN DRIVE TO MAG/DYNO
IMGA0748.JPG

WRONG PHOTO BUT IT IS ACTUALLY OUT SIDE STENA LINE HOOK OF HOLLAND 18th October 2013 PRIOR TO AGM THEN HAD TO LEAVE EARLY TO CATCH FERRY AT PORTSMOUTH FOR FRANCE.

IMGA0746.JPG

Peter I do like your mag/dyno platform and certainly looks right, may I pass on a little tip that I had done when Colin Jenner did the restoration on Old Harry EAR 2x3=6
Quoting,
Drill 1/8" drain holes from what was the mag base to the side of the mag pick/ups I think they emerge at an angle as Alace Leney who rode HJO 500 to the NZ annual last year found to his frustration water in the mag armature housing, also silicone rubber in the original drain hole. The bikes were parked in the rain for nearly two days while we drank beer and ate.
That goes for you too Bill.
 

billirwinnz

Well Known and Active Website User
VOC Member
Thanks Marcus

Alice warned me about the hole and I blocked mine but I've since just fitted a new magdyno body and drilled a hole in what will be the underside.

As far as chains go I have been using this handy calculator to check chain loading.

http://www.renoldchainselector.com/ChainSelector I was happy to find a similar result to my own crude calculations. As you can see from the attached result the maximum force (assuming about 200w load) is only about 20lb which is well within the strength of a typical cycle chain.
 

Attachments

billirwinnz

Well Known and Active Website User
VOC Member
Bill,

Coincidentally, I've been struggling with the Twin mag-mounting for a few weeks now. After looking at endless photos and sketches, rubbings and samples, I made my own. Great, progress at last. NOT. Finally, I came to the only possible conclusion that the 1/2" slot is wrongly positioned on the original brackets. This limits any movement of the mag-mounting, (which needs to be in line with the two sprockets) and actually makes it cant over at an angle. So adjustment is severely limited. (I accept though that it could be an issue with the jigging of my frame at the front!).

Finally, I made a mock-up with the offending slot re-positioned and it works like a dream. So then I bought a piece of 1/4" alloy channel, 4 inches across, and marked it up as per the photo. I've since got it roughed out and hope to have it all fitted-up soon.....

I'd be interested to know if you've got full range of movement (in line with the sprockets) on your original mag-mounting bracket?

Peter B

View attachment 25457
Hi Peter

The 1/2" slot on my mag mount doesn't allow full adjustment either. It's not an original but was perhaps copied from one. I have had to remove 1/16" from the top rh corner in order to get full adjustment. I'm hoping that along with a newer chain will restore correct tension.

Cheers Bill
 

A_HRD

Well Known and Active Website User
VOC Member
Bill,
Thanks for that info - exactly as I was experiencing - only more than 1/16".
Peter B
 

Robert Watson

Well Known and Active Website User
VOC Member
Mine was a similar experience. I tried to fit the mag mount bracket that came with the bike and thought I must be doing something wrong. I called the previous owner and had a conversation. He told me it didn't have one so he had this one made by a very experienced metal fabricator.

I made a new one that worked properly. I had a very used chain but sprockets that were quite usable, so looked all over for a chain. The local bike shops had nothing but Chinese chains. I found some very nice German chain but really didn't need the 25 meters they wanted to sell me. Explaining my woes to another guy restoring an A twin, he said he had a nice new spare in hand which he would send me. When it arrived it seemed that someone had got a nice new spare, put it on the bike and diligently put the old VERY tired one in the box. Worse than the one I had. In the end I ordered one from Conways........ They assured me it was a quality chain. It duly arrived. Chinese bicycle chain...... I gave up and put it in, which reminds me, I should look at it.

And A-HRD, this was the other thing I was going to talk to you about!
 

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