Chain not Centered on the Chain Guard

jms · Mar 19, 2022

I've got sort of a perplexing problem with the way the rear brake drum / rear sprocket lines up with the chain guard on my Norton. When everything is tightened up and looking at the end of the guard from the back, the chain is rubbing on the guard on the outside left. Now that wouldn't happen if the guard didn't have that clip that holds it away from the swingarm. If I shim the brake plate over to the right so that the chain is centered, then the lower steel brake plate boss barely engages in it's slot nor does the short axle flats. Is there meant to be a thin shim between the swingarm and the brake plate arm that doesn't necessarily center the chain but keeps it from rubbing? Is it possible that I have the wrong brake drum? What am I missing here ?

Craig · Mar 19, 2022

Maybe my eyes are shot , can’t really be sure what I see , what type of bike are you working on , year, model never hurts , specially when asking …. or you could post your bikes in your profile or avatar or your signature , would love to offer opinion but …. like I say the photos so close just got nothing else to say other than it looks clean , good luck !

robs ss · Mar 19, 2022

There's something strange about your axle (for a pre-mk3)
You have a male thread protruding, from the inside of the drum, towards the wheel (RHS).
Every Commando I've seen has the dummy axle (inside the drum) with a female thread on the RHS.
So - if your axle isn't standard it may explain your problem.
Your brake backing plate and swingarm also look "different".
@Craig is right - 'fess up and tell us what you've got.
Cheers

L.A.B. · Mar 19, 2022

robs ss said:
Every Commando I've seen has the dummy axle (inside the drum) with a female thread on the RHS.

Looks like a bolt-up drum sprocket so that's one of the three threaded drum studs.

robs ss said:
Your brake backing plate and swingarm also look "different".
@Craig is right - 'fess up and tell us what you've got.

I believe it is a 1968 Production Racer Replica.

1968 frame

I recently purchased a 1968 Norton that has been transformed into a Production Racer Replica. A lot of nice work has been done here, however, I inadvertantly found out that there were problems with the frame bending, and I beleive that I have one with the problem. The frame is comprised of a...

www.accessnorton.com

Craig · Mar 19, 2022

Nice detective work Les !

Time Warp · Mar 19, 2022

The drum is early as is the chain guard, the offset to the left is normal (The chain is not in the centre)
It might need some tinkering taking for granted they are Commando parts.
From the inside of the swing arm shock mounting lug (which all looks stock) to the backing plate is 11 mm (7/16" ?) from that lug location to the side plate of a stock non O-ring chain around 15 mm.

Time Warp · Mar 19, 2022

The 850 chain is all but the same.
If the chain is contacting the guard checking it vertically might be in order.
That I noticed based on the guard mount design at the lower shock mount the guard is kicked inward closer to the chain at the rear anyway so is a fine line as far as clearance.

robs ss · Mar 19, 2022

Thanks @L.A.B. for your obsevations.
Based on what I can see there I would try a washer (spacer) on the LH end of the dummy axle, inside the swingarm.
TW above is correct, I think, as my sprocket runs closer to the LH side of the chainguard.
So you're not looking for central - just some clearance for the chain.
Cheers

EDIT: you don't show your shock/spring. It's not possible that the lower spring retainer is "leaning" on the chainguard, pushing it to the right?

ashman · Mar 19, 2022

Is the chain running true to the front sprocket, if not then a spacer might be put wrong or something.

robs ss · Mar 19, 2022

ashman said:
Is the chain running true to the front sprocket, if not then a spacer might be put wrong or something.

Ash - there is no spacer - the dummy axle fits straight to the inside of the swingarm. As I mentioned above, it might need some sort of spacer to move the drum/sprocket to the right.
Hope things are better for you down there in "wet-land"!!

Time Warp · Mar 19, 2022

The system on the earlier bolt up wheel differs to the later cush drive version including at the drive side as far as the bearing and spigot off the wheel hub to engage the drum. (Wheel bearings only, one single row and one double row)
That is why I staggered to the garage to get those two measurements so the OP can see where the chain is and backing plate as some form of reference.
Either way the chain will be to the left of centre on all Commando's unless the Mk111 differs from the drum brake models.

L.A.B. · Mar 19, 2022

Time Warp said:
...unless the Mk111 differs from the drum brake models.

It does:

ashman · Mar 19, 2022

robs ss said:
Ash - there is no spacer - the dummy axle fits straight to the inside of the swingarm. As I mentioned above, it might need some sort of spacer to move the drum/sprocket to the right.
Hope things are better for you down there in "wet-land"!!

All dry here Rob, as well I live on high ground, in fact its great weather here now not so hot and things starting to cool down, great riding weather just got to dodge the pot holes, it like riding in the twisties lol.

jms · Mar 20, 2022

Thanks all for your input. First chance I have had to respond. Learned quite bit here. This is a 1968 Fastback "Widowmaker" that I converted to a replica Proddy Racer. I welded in a top tube to rectify the frame problem. The brake plate is a vented production racer part and yes the rear brake drum is the two part axle with the bolt on drum/ sprocket. looks like I may need to simply shim the whole brake over just enough to allow the chain to clear the guard now that I know that the chain was never meant to be center of chain guard. With the vented brake plate it actually takes up more space between it and the inside of the swingarm because of the combined thickness of the vent screen and rivets that hold it on to the brake plate.

robs ss · Mar 21, 2022

Just a note after I gave my (850Mk2A) bike a thorough check over.
As I said, the sprocket is to the left by 6mm. The shock isn't leaning on the chain guard.
So.. shim/space the wheel to the right?
Wrong... the (correctly aligned) rear wheel is already 3-4mm closer to the RHS inside swingarm (front) than the LHS - so moving closer here will be wrong.
I guess that, in the interests of production, Norton just got it a bit wrong here.
Luckily it's the only mistake they made!!!
Life's full of compromises!
Cheers

BTW @jms , nice bike, but a lotta polishing!

jms · Mar 21, 2022

Yes an awful lot of polishing! What else am I going to do in my spare time? LOL

ashman · Mar 21, 2022

jms said:
Yes an awful lot of polishing! What else am I going to do in my spare time? LOL

Ride.

Chain not Centered on the Chain Guard

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1968 frame

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.......back to the 70's.

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.......back to the 70's.

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Time Warp

.......back to the 70's.

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