Primary Drive - Home brewed duplex conversion??

stewb · Post by **stewb** » Sat Feb 02, 2013 4:05 pm

Hi all,

new to the forum (although I have been dipping in for a while for general info capture)....

I'm in the process of putting a '63 CB72 back together after starting the project in the mid 1990s.......

One item i'm not all that happy with is my primary drive chain. It seems okish but i've been checking out as many pics as I can find & it would appear that judging by the tensioner position there is a fair amount of stretch/wear in the chain I have.

The sprokets are pretty much mint but there is minor evidence of the chain whipping & catching the casing.

Looking at the tensioner it's no great surprise as on the overrun it would be easy for the tensioner to be compressed by the natural chain tension (slack side becoming the taught side) & as such allowing the top run to whip.

I was planning to make up some form of adjustable backstop for the tensioner arm to limit the amount that it can retract. I was thinking along the lines of an eccentric bush which could bolt to the cases via one of the spring bracket bolts. It would be set to give 2-3mm of clearance normally & allow the chain to "bounce" without whipping.

Anyway....during my research I've come across various info & have measured what I have. It seems that it would be fairly straightforward to convert the primary to a standard 3/8" duplex chain which is readily available & pretty cheap also.

The only bit which isn't very straightforward is the primary drive sprocket spline. My current thinking is along the lines of using a standard sprocket with the O.D turned down to suit a stock "off the shelf" duplex sprocket which is bored to be a fairly heavy interference fit over the remainder of the original primary sprocket. This can then be welded just to make sure nothing moves.

If I'm reinventing something then accept my apols - just after some feedback from those with a greater experience of these bikes.

Also I am after a worn / damaged / used primary sprocket to use as I don't want to damage the good part I have at the moment. Ditto with the clutch basket (although I'm not so worried with the basket as any mods are reversable)

Does anybody who is UK based have one they would be prepared to sell to me for this project? I'd be happy to share the info gained in the process with the forum.

Thanks for reading!

Stew

I'll place a wanted also.

teazer · Post by **teazer** » Wed Feb 06, 2013 1:39 pm

I thought that duplex primaries were available from Nova and Oily Tom. (both in the UK)

Phil-UK · Post by **Phil-UK** » Thu Feb 07, 2013 4:22 am

Hi

There used to be an advert in Tansha ( UK VJMC club mag ) for the duplex conversion but it hasn't appeared for a few years so I guess they gave up ( due to poor sales ? ). If you are a member you might try contacting the editor to see if he still has contact details.

If you simply want to replace the chain and can't get NOS ( they occassionally pop up on UK ebay ) then "Sprockets Unlimited" have the correct size chain. Its not Japanese and has thinner side plates and is still bushed not roller chain but if thats all that is available ...
They are at most autojumbles and have a website
http://www.sprocketsunlimited.com

Phil

stewb · Post by **stewb** » Thu Feb 07, 2013 12:42 pm

Thanks for the replies guys, much appreciated.

I've seen both NOVA & MPE.....not aware of Oily Tom (unless he is MPE)??

Both routes are fairly expensive on the basis they are "bolt on" conversions (375+vat for sprockets from NOVA & 240 from MPE with all hardware).

It would appear that it would be possible to do the conversion much more cheaply with standard "off the shelf" parts with machining to suit.

The alternative chain sounds like a viable option which I wasn't aware of. I've been in contact with a few chain suppliers & manufacturers but they have all drawn a blank & aren't interested due to the very small amount of chain involved.

I soaked & cleaned my original chain & it seems to be goosed - some rollers are free but others are seized solid....I think it is a replacement part as the original wasn't actually a roller chain?? At least this is what I have read but I have no point of reference as this is the only one I have actually stripped.

Cheers,

Stew

markstertt · Post by **markstertt** » Fri Feb 22, 2013 11:09 pm

Hi Stew, I"m also new to the forum and were I considering a primary conversion I'd probably look into the feasibility of using a hyvo chain/sprockets, have you considered this route? I don't don't what hardware is available but size for size it is a much higher rated chain and used in automotive drive systems/cam drives etc., just a thought...Mark