Frame alignment. Gearbox sprocket and rear wheel sprocket

I have just spent considerable time aligning my 74 850 Mk11 following the “World straightest Commando article”.

It’s not that easy and a big problem is that to follow the complete method you need a very large granite plate. I had access to a 500 x 500 mm one but this isn’t big enough to bolt the frame down completely.

Anyway we did all we could and found good alignment apart from the following.

- the front rim was 4 mm off centre between the forks. Too far the the left side. This was corrected by adjusting the spokes.

- both rear isolastic tube end faces needed machining square. The front iso is a new item from CNW.

- the cylinder head steady surface was machined square

- the swinging arm was twisted and corrected

Everything else seemed OK. The holes in the cradle and frame all measured parallel, crank and gearbox parallel. Rear wheel central to the centre frame and front wheel once it was centred.

But - the gearbox sprocket and rear wheel sprocket while parallel were 4 mm out of line. Believe me I checked several times! The rear sprocket needed to move 4 mm to the right. Or the gearbox sprocket come 4 mm out on the shaft but that would have bought it into the rear primary chain case.

And before you ask the shock mounts on the frame to the swinging arm shock mounts were perfectly aligned. My initial thought was that perhaps the swinging arm had been pushed over but no. The frame shock mounts measured the same distance from the main frame tube and these were in turn exactly in line with the swinging arm mounts.

The solution seemed to be an additional 4 mm spacer between the brake plate and the left swing arm. With a corresponding 4 mm off the spacer on the right side.

The brake anchor had to be extended so we made a new longer one and the rear rim recentred to the frame and front wheel.

So - has anyone seen this before. Any other bright ideas will be considered.


This bike has always been a nasty handler with a strong pull to the left and tank slapper above 80 mph. I know the bike history for 40 plus years bare the first 18 months and it has never been dropped. But the first owner does seem to have raced it I think.

All I can say is that I've long suspected the iso tubes on my 74 are either out of line or not squared. When you run a feeler gauge around to set the iso clearance it isn't even. Entirely possible the bike was pranged at some point but equally possible it came from the factory "close enough".

I could understand if the swinging arm was pushed over but no sign of that. I would have thought the gearbox rear wheel chain alignment is the easiest to build correctly. All the components are constrained.

We squared up the isotubes but that’s only a few thou. Not 4 mm.

Its a significant amount and not explainable by the normal production line issue of jigs getting embedded pieces of dirt or swarf and throwing off all production until the jig is next cleaned, there is the possibility a jig was made 4mm wrong right at the start.

Is your frame an Italian metric frame or a UK imperial one.

Uk one. The tubes are imperial sized

It went through my mind that the entire engine cradle could be offset if the isotubes were wrong but that cannot be so because the front iso is a new CNW item that has never been used.

If anyone has a bike with the primary cases off could they measure the distance from the face of the left cradle plate to the face of the installed gearbox sprocket please

I have an Italian 74 frame so will try to check some dimensions over the weekend.

On BSA Unit singles there is a documented change in the gearbox sprocket chain alignment to the mainshaft bearing when a bearing retaining circlip was added, getting the parts mismatched misaligns the chain to the rear sprocket alignment by 70 thou.

Could be as simple as a mismachined gearbox chain sprocket.

I dont have any insight for you.

Who did your frame analysis? I had F1E do mine, and all the parts were straight. Didn't have them do the fully assembled analysis tho.

The frame was done by a 70 plus year old mechanical engineer and toolmaker who has been a mechanic for world super bikes and raced his own commando for years. I have done all the subsequent assembling measurements sharing a workshop with another engineer who has classic bike IOM lap records with his machines

kommando said:

I have an Italian 74 frame so will try to check some dimensions over the weekend.

On BSA Unit singles there is a documented change in the gearbox sprocket chain alignment to the mainshaft bearing when a bearing retaining circlip was added, getting the parts mismatched misaligns the chain to the rear sprocket alignment by 70 thou.

Could be as simple as a mismachined gearbox chain sprocket.

Click to expand...

I’ll get hold of another sprocket to check

Make sure you have the ~1/8" thick spacer on the top transmission mounting bolt inside the cradle.

Could the rear cradle and or the swing arm be bent? The rear end has had a good knock at sometime in its life if the swing arm was twisted.
Did you centre the front wheel relative to the fork stanchions?

Looks like around 33.5mm on my 72 Combat, 19t.



https://andover-norton.co.uk/en/shop-drawing/178/rear-wheel-drum-bearings
Part 21 is a washer that fits between the brake backing plate and axle.
In my original parts book it lists the quantity AR. Don't know how thick they are.
Somebody, somewhere knew there was an issue with alignment.

I recently added the thinnest hardware washer I had sitting around .090, to clear the chain guard.
Haven't really taken the time to check what it affected.

Really interested on how all this turns out.
Keeps us informed.

johnm said:

This bike has always been a nasty handler with a strong pull to the left and tank slapper above 80 mph. I know the bike history for 40 plus years bare the first 18 months and it has never been dropped. But the first owner does seem to have raced it I think.

Click to expand...

This situation described above alone should indicate there's some significant problem that you wouldn't need a granite surface plate to find. Maybe you have a bad wheel bearing? Bad steering head bearings?? broken headsteady?? My bike handled horribly until I fixed the swingarm play with the Kegler clamp modification. Now it feels so solid and predictable, I'm tempted to push it harder in the corners... (but I crashed hard as a young man and was lucky to have lived)

batrider said:

Make sure you have the ~1/8" thick spacer on the top transmission mounting bolt inside the cradle.

Click to expand...

Yup that’s there.

cash said:

Could the rear cradle and or the swing arm be bent? The rear end has had a good knock at sometime in its life if the swing arm was twisted.
Did you centre the front wheel relative to the fork stanchions?

Click to expand...

yes the front wheel was centred. The cradle was checked on a granite table and the swinging arm on v blocks after it was straightened

MichaelB said:

Looks like around 33.5mm on my 72 Combat, 19t.
View attachment 13348 View attachment 13349


https://andover-norton.co.uk/en/shop-drawing/178/rear-wheel-drum-bearings
Part 21 is a washer that fits between the brake backing plate and axle.
In my original parts book it lists the quantity AR. Don't know how thick they are.
Somebody, somewhere knew there was an issue with alignment.

I recently added the thinnest hardware washer I had sitting around .090, to clear the chain guard.
Haven't really taken the time to check what it affected.

Really interested on how all this turns out.
Keeps us informed.

Click to expand...

thanks for that measurement. This week I’m in Australia but will get back in about 6 days thanks

o0norton0o said:

This situation described above alone should indicate there's some significant problem that you wouldn't need a granite surface plate to find. Maybe you have a bad wheel bearing? Bad steering head bearings?? broken headsteady?? My bike handled horribly until I fixed the swingarm play with the Kegler clamp modification. Now it feels so solid and predictable, I'm tempted to push it harder in the corners... (but I crashed hard as a young man and was lucky to have lived)

Click to expand...

Wheel and stealing head bearings checked out fine. Had a Dave Taylor head steady. Swinging arm spindle and bushes were slightly worn but now renewed and Keller clamps installed. The frame measured up fine.

I have a few more measurements now so I will go back and check when I get home.

thanks John

You’re not running slimmed down 1/4” sprockets are you?