G: Gearbox (Twin) New Gearbox: Help Needed

ehb86

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I'd like to thank everyone again for their suggestions and offers of help. I'm happy to report I've solved the problem and I'm able to engage all gears with the gear indicator. I ran through the checks of gear fitment, etc. as suggested and all was well. So I took Ernie's suggestion to set the gearbox up in the vice in the cover plate. and check the shifting with the camplate. That is how I had initially put the gearbox together for insertion but I don't believe I tried shifting gears with the camplate. I had just slid things back and forth and they meshed so I thought all was ok. After Ernie's suggestion I set it up again and this time rotated the camplate by hand and it became obvious that the selector forks were hanging up on the selector bar. Not really hard, but enough to make shifting very difficult in first and second.

I removed the selector bar and chucked it in the lathe and it was straight so I took about a thousand plus off of it and polished it up. On reassembly I was able to shift first through fourth with the gear selector so I'm a happy fellow and I'm going to proceed with getting her back together and running. That will be a happy day.... it's been too long off the road. Thanks again for everyone's help, it's hard to overstate how great this club is.

Ernie, thanks again.

Best regards,
Mark
 

highbury731

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Is there a sample of the McDougall cam plate (and presumably a revised index plunger assembly) and would anyone make them?
Paul
 

Bill Thomas

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Having Raced Twin for 17 years, I have found the gearbox , Not too bad !, But the Clutch can make it seem
Bad. A Standard Clutch can be made to work well, But it's not easy, A Multi plate makes it feel like a different Bike. Cheers Bill.
 

highbury731

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Comparing the horizontal Norton box and AMC box is revealing. Apart from an improved positive stop mechanism, the main difference between them is the curved cam profile of the index plunger. It's a MUCH better box. So if someone can make a similar improvement to a Vincent box, I'm keen. And right now, my gearbox door is off....

Paul
 

oexing

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Paul,
I guess you are thinking of a shape like that below from a /5 series BMW box, with all these humps on the o.d. of the camplate :

Schaltscheibe.jpg


Actually that was meant to be an "improvement" to the older shape similar to the Vincent camplate. The older BMW plate was like this:
Schaltscheibe alt.jpg 2.jpg


You can see the indexing lever just above the camplate, that is where I got my idea from for my mod to the poor Vincent indexing plunger type which induces a lot of friction in its bore from side loads when shifting gear.
I said the new type of camplate on the 2 valve flat twin BMWs was a bad idea because all gears in a BMW gearbox do about crankshaft speeds , no reduction from a primary drive there. So gearchanges are widely known to be very clunky on these BMs when going down , say, from third to second gear as you are faced with big differential in gear speeds at a high reving gearbox.
With the older type of camplate, circular o.d. , you have the chance to find a "false" neutral between all gears so you can perform truck/ tractor changes like on unsynchronized boxes.
I got my 1965 R 69 S in 1972 so am used to its gearbox quite well. So it is second nature to do downshifts with false neutrals and extra clutch action so real SILENT gear changes on a BMW for sure. I had clutch Bowden wires snap once or twice far from home so this came very handy to do gear shifts without clutch at all BUT using false neutrals for silent changes nevertheless. No way to do that with the humped camplates !
Well, this is no argument with the Vincent gearbox, certainly. When looking at the Vincent camplate I do not see a lot of scope for extra humping: You´d have to shorten the bore of the plunger pin but same time it had to do a longer stroke into the deeper v-slots, making things even worse friction-wise with the pin-in-bore design. In that case the lever indexer would help a lot - I hope.

Vic
P1050791.JPG
 

highbury731

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I understand what you are getting at... the humped cam plate is most useful on bikes with lighter flywheels and closer ratio gearboxes. Perhaps the ideal arrangement for older bikes would be a low-friction index, a two-angle ramp to encourage the camplate into gear if it's anywhere near, and a region of flat top constant diameter in between allowing a 'shifting neutral' between ratios for synchronisation.
Paul
 

Monkeypants

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Is there a sample of the McDougall cam plate (and presumably a revised index plunger assembly) and would anyone make them?
Paul

A few years ago John showed me the AMC style camplate he had made and fitted to the 4 spd in his Shadow. At that point in time he had removed the 4 speed and installed a Quaife 5 speed in the bike. According to John, the 4 speed with his revised plate shifted as well as the Quaife, which is a slick shifting setup.

I'm guessing that the 4 speed gear set and special camplate are put aside as part of the Black Shadow, which his family has kept.

Glen
 
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vibrac

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The guy who has done a lot in this area including an external to the gearbox mod all of which transform the gear changes is Tomo ( he of the 4 valve Vincent)
 

highbury731

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The guy who has done a lot in this area including an external to the gearbox mod all of which transform the gear changes is Tomo ( he of the 4 valve Vincent)
My guess is that he added an extra index system in the gearbox outer cover
 

oexing

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Hmm, I was thinking of just limiting travel of linkage in there by setting stops so as not to overselect gears beyond the proper index on the camplate. So another indexing would be a bit too much.

Vic
 
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