Swingarm differences

Guys,
I bought a secondhand Commando swingarm from a swapmeet/auto jumble last year as it was nicely gussetted at the spindle/forks section. On comparing it to my 1971 750 Commando I see that at the rear axle flat plate it doesn't have a slot for the rear drum brake torque extension/tongue. It also had needle rollers bearings for the spindle. Am I right in thinking this new swingarm is for a disc brake model.

On further inspection of my 1971 swingarm I see that the flat axle plate is replicated on the right arm. My question - is the slotted area (for the brake torque extension) there for any reason or do you think Norton just used the same stamping as a cost saving exercise.

Next questions
- can I use this later swingarm on my 1971 bike by welding the torque extension bit on.
- can I use the standard bushes in this swingarm (please - no extensive discussion on needle rollers)

Bert

pics please

the torque tab slot on both sides is indeed the result of part standardization. Less chance of an ass backwards weldment :idea:

tyborg15 said:

I bought a secondhand Commando swingarm from a swapmeet/auto jumble last year as it was nicely gussetted at the spindle/forks section. On comparing it to my 1971 750 Commando I see that at the rear axle flat plate it doesn't have a slot for the rear drum brake torque extension/tongue. It also had needle rollers bearings for the spindle. Am I right in thinking this new swingarm is for a disc brake model.

Click to expand...

A MkIII 'disc' swingarm. These have a box-section cross-member and sintered bronze bushes as standard, not needle roller bearings:

L.A.B. said:

tyborg15 said:

I bought a secondhand Commando swingarm from a swapmeet/auto jumble last year as it was nicely gussetted at the spindle/forks section. On comparing it to my 1971 750 Commando I see that at the rear axle flat plate it doesn't have a slot for the rear drum brake torque extension/tongue. It also had needle rollers bearings for the spindle. Am I right in thinking this new swingarm is for a disc brake model.

Click to expand...

A MkIII 'disc' swingarm. These have a box-section cross-member and sintered bronze bushes as standard, not needle roller bearings:

Click to expand...

Hi L.A.B.
Please confirm if that swingarm in your picture also has larger diameter arms compared to earlier versions.
Ta.

needing said:

Please confirm if that swingarm in your picture also has larger diameter arms compared to earlier versions.

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Well, I don't have a pre-MkIII s/arm to measure, but the MkIII tube measures 1.635"-1.640" inc. paint.

I measured a 1973 swing arm at 1.39"

Ok so don't have a mark III because my forks are 1.382 " diameter and my gusseting is in the form of two fully welded u shaped pieces that form a shallow v so the centre line of a tyre wouldn't scrape.
Was it only the markIII that had a disc rear?
If so then this swing arm may have been modified but the welding does look factory. :|

Be careful if you opt to weld the swingarm, I have repaired a couple of the front chain guard mounting tabs on the drum brake models and the heat bent the swingarm up a bit. I think the metal is fairly thin. I solved the problem by having my welder put a bead of weld on the opposite side of the tube and it came back straight again.

tyborg15 said:

my forks are 1.382 " diameter and my gusseting is in the form of two fully welded u shaped pieces that form a shallow v so the centre line of a tyre wouldn't scrape.

Click to expand...

Like this, perhaps? If so, then it could be a modified '850' '73-'74 064539 s/arm?

2ndchildhood said:

Click to expand...

tyborg15 said:

Was it only the markIII that had a disc rear?

Click to expand...

Yes.

Yes, L.A.B. - That is the set up.
On close inspection, the torque tabs have been ground off and also the wheel alignment adjusters.

I think I will grind the right side torque tab off my 1971 bike and weld it onto the left side of the 73-74 swingarm with the gusseting. Luckily its the plate that gets welded and not the thin wall fork tube.

tyborg15 said:

On close inspection, the torque tabs have been ground off and also the wheel alignment adjusters.

Click to expand...

If it was modified to take a MkIII disc wheel, then the spindle/axle slots may have been opened out to 17mm?