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Norton 650SS late model gearbox sprocket


Hi All - a question for anyone who has replaced final drive sprockets/chains on a post-64 650SS which has the 3/8" chain. Should the gearbox sprocket be the 19 tooth 3/8" wide version as fitted to the Commando, or the earlier 1/4" wide version. I'm asking as my bike has always had the wider chain but having replaced the gearbox sprocket for the 3/8" version, I'm now getting chain rub on the inner primary case. The gearbox sprocket I pulled off the bike seems to be the 1/4" wide version, but it's a bitsa (it had a Commando rear brake drum/sprocket 42t on it) so this does not necessarily mean anything. I'm starting to think the offset on the two sizes of gearbox sprocket might be different and that I need to go back to the 1/4" version. Did Norton fit a 3/8" chain to a 1/4" gearbox sprocket as standard??

1964 parts list has a 16 tooth sprocket here which seems completely wrong. Andover list only a 1/4" version for Featherbed models. RGM, similarly, seem to be listing 3/8" sprockets as Commando only and the 1/4" variety as AMC. 




The gearbox sprocket width changed from 1/4" to 3/8" after 1964. Many owners upgraded their drive set-up to the wider sprockets as these seemed to last longer. The 3/8" x 5/8" gearbox sprocket will fit both the Dominators and 750 Commandos. The only significant change being the screw thread for the nut locking plate changing from 2BA to a  28tpi UNF item..The Dominator Service Notes say this thread originally was 3/16" BSF. The difference between BSF, BSC and BA for this size is quite small.

The most likely cause of chain rub is a poor quality chain beginning to exhibit side play. This unfortunately includes current versions of Renolds chain.

Unless you are planning on riding great distances and getting very throttle happy a 1/4" wide chain and sprockets would be fine for your bike.


Thanks Philip

Inconveniently, I have a brand new 3/8" rear sprocket and drum fitted, and a new chain, which as it happens, is a Renold. It looks like I'm shimming the primary case to provide clearance. I'm looking to ride to Sweden on this bike, next year, so it needs to be right. 





.. was just contacting the primary inner case on my ES2. I found it was the split link - the extra thickness of the clip and plate just made that difference. I solved it by fitting the split link from the back instead - a bit tricky getting the clip on through the spokes but doable.


Thanks Ian - yes after inserting a shim under the left hand gear box bottom nut where it clamps the inner primary (mine is slotted) I am now down to a light scrape when the split link goes through even after reversing it as you did. I think the half link needed for the required 97 links is also playing a part. Next job is to pull the whole primary side off again and shim under the centre fixing and the top spacer bar. Irritating because it won't need much more at all to be completely free running.

The cause I am assuming is a pattern 3/8" gearbox sprocket that is not quite to spec. Following my experiences with a layshaft third gear bush which was too tight internally and externally and gold rear brake shoes which didn't fit without a lot of fettling, I'm reviewing my usual choice of parts suppliers. 


Russell & Ian........don't take a chance if you are planning long journeys. In the last 20 years of serious riding, all of my ride failures were down to Renolds chains throwing the split link spring (due to rubbing/catching). On each of these the chain had so much side-play it was whacking the chainguard walls. At an MoT a few years ago even the  Tester noticed the side-play on a chain that only had around 500 miles of use. In recent years the quality has got far worse and despite regular maintenance, after about 3,000 miles each chain was clapped out. The last straw was purchasing another news (Renolds) rear chain and on stretching it out on my workbench found that not only did some links wobble but the end of the chain could be pulled over about 6". I went out and bought a German 530 replacement which has now covered 1,500 miles with little obvious wear.

See the attachment showing a Renolds Primary after next to no miles of use. 


That does look dire, Philip. The chains I have fitted are both Renolds, new in packet, bought a number of years ago and never fitted as I was planning a complete gearbox and transmission refurb, just completed, with new sprockets (excepting clutch, which looks good), and Tutoro oiler for the rear. It is at least possible that it is the chain dimensions which are the problem, so I will take a look at the old English-made Renolds which came off the bike to compare. It cannot be wear, though, as the chain has never been on a bike.


Renolds do not support motorcycles as an application for their chains, they do allow one distributor to buy chain from them and box it in Renolds boxes but oddly when you approach Reynolds directly they will not recommend any of their chains for motorcycle use. Take of that what you like but my take is avoid like the plague and use chain from someone who does make chain specifically for motorcycle use, so The Chainman's recommended chains or DID as one of the other better makers.

When you look at the old brands like Lucas, Girling, Jones etc that are being licenced for use on boxes for selling new dubiously sourced products you need your wits about you to work out the trustworthy products and sources.


Just a random thought, would fitting a bigger box sprocket  move the chain  line further out and avoid it circumcising the alloy box casing?. Had a look at the Atlas (67)  and its definately 3/8".


I wondered whether the 3/8" sprocket was pushing the chain line too far outboard hence my question about whether these tin primary chain case bikes retained a 1/4" sprocket - thanks for checking. I assume that following the move to a 3/8" sprocket post '64 the chain case distance from the back of the gearbox had to be maintained otherwise the gearbox would have needed a different, longer mainshaft and sleeve gear to provide clutch clearance, and as far as I know they are all the same length on N and NA prefix boxes, the boxes on my bikes both being NAs. 

Anyhoo, the chain itself is now clearing the chain case at the moment, the problem being the spring clip, Renold-supplied, and the half link, which is an IWIS item from Norvil. I'm about to pull off the primary to shim it out a fraction which should provide a fix.

I fitted Regina chain to my 500, the old narrow pitch, which made it to Austria OK in 2018 with an oiler, and the primary chain on that bike is either an IWIS or DID, which has been fine, run in ATF. I won't be buying any more Renold branded based on John and Philips's advice, for which many thanks, but I hate the idea of binning two new chains, even if their life is likely to be shorter than other brands. I'll carry a spare split link!




Hi Russell. If on your way to/from Austria you need any help , I am in Kent ,feel free. I will PM a phone no.  BOB.


.. Renolds any more as I've heard too many bad reports. The Chain Man is a good source of advice and chains. And by the way it's not good practice to mix manufacturers as dimensions like pin diameters and lengths may vary fractionally.......


Very few modern motorcycle chains supplied for classics are to the original motorcycle spec. Look it up and you will see why, as they would need to down spec the current chain. 

Even Andy the chain man's chains are non motorcycle spec - feel free to ask his supplier at IWIS. IWIS & JWIS and Renold exceed the strength spec of the motorcycle spec, but do not conform to construction dimension, and thus the difference due to thicker side plates used in construction. Regina, DID, TSUBAKI also make motorcycle chain, but again it differs to the original spec. The reason being that in other motorcycle applications there is sufficient clearance so the extra dimension of side plates is not an issue like it is on Nortons.

To get chain manufactured to the motorcycle chain spec would be very expensive and result in a chain that is weaker (though the same as what was used originally in the 70's) than the markets current non motorcycle spec offerings. And yes, I have asked. It would also fit commercial applications where the higher spec chain is needed, so I would expect there is no appetite for chain manufacturers to produce it even if asked due to safety issues.

Considering some modern bikes use 1/4 chain in some serious on and off road bikes suggests it has adequate strength. 


Bob: thanks, I really appreciate the offer, but I wasn't clear: Austria was 2018: the planned trip is Sweden for 2021, so I'm likely to be heading North from here, but likewise let me know if you need a stop off on the way to the IOM next year as we're in Chester. 

Ashley and Ian: thanks for your comments. Re half links sometimes you have to take what you can get and the IWIS fits well. The side plates on the Renold 3/8" chain are certainly chunky and may be the source of the clearance issue. It's reassuring that the chain is if anything over spec. 

I wonder if part of the issue with premature chain wear is using the chain when locking shafts to tighten gearbox and mainshaft nuts - I've taken to using old chain for this purpose and hoping that it will extend the life a bit.

Case now shimmed out so will have another go when the bike is back together tomorrow. I have decaffeinated this bike and in removing the rear sets replaced the stand off in the middle of the chain case which butts up to the engine plates and takes the footrest rod. As my bike had rear sets this bike had a threaded rod welded in the end of th stand-off so I had to replace it. The new standoff was 40mm dead on. The old one was 43mm one side and 40mm the other, which will have contributed to the problem.





Shims did the trick. Now plenty of clearance for both final drive and primary chains. Thanks to all for their comments/suggestions.

650SS in the lanes around Chester today



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