Commando Swing Arm Spindle Length

1974 850 Mk1A
Going to attempt a repair in situ by fitting an oversize spindle and reaming the cradle and bushes to suit. Getting conflicting parts info on spindle length, Andover Norton 6.96", other suppliers suggest 6.5". Also, what is the nominal spindle diameter please?
Obvious way to go is to strip and measure but cant afford to have an immobile bike in the garage whilst I strip and then wait for the correct parts to turn up. Anyone know if there are differences other than the differing fixing methods twixt Mk3 (twin cotter pins v single bolt) and earlier bikes?
Regards,
John S

It should be 6.96 inches.

JTBS666 said:

Anyone know if there are differences other than the differing fixing methods twixt Mk3 (twin cotter pins v single bolt) and earlier bikes?

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Yes, the Mk3 spindle is shorter.

https://andover-norton.co.uk/en/shop-details-2/16353/swing-arm-spindle-850mk3-06-5053-6-32-long-

There are two length's, original long and a later short one to suit the welch plug ends. With the introduction of aftermarket cradles with the later cotter pin holders then both versions got the cotter pin slots. If you have the 2 flat end plates and the long thin bolt then you need the long one.

Reaming the bushes needs to be done correctly with a fresh new cutter or instead you will not cut but deform the bush, this will close up the pores required for lubrication. Better to bore them on a lathe with a sharp HSS cutter. They do close up slightly when pressed into the swing arm, don't be tempted to use sand or emery paper to do the final sizing as the grit will enter the oil pores and come out later to grind the spindle.

JTBS666 said:

Also, what is the nominal spindle diameter please?

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7/8"

JTBS666 said:

1974 850 Mk1A
Going to attempt a repair in situ by fitting an oversize spindle and reaming the cradle and bushes to suit. Getting conflicting parts info on spindle length, Andover Norton 6.96", other suppliers suggest 6.5". Also, what is the nominal spindle diameter please?
Obvious way to go is to strip and measure but cant afford to have an immobile bike in the garage whilst I strip and then wait for the correct parts to turn up. Anyone know if there are differences other than the differing fixing methods twixt Mk3 (twin cotter pins v single bolt) and earlier bikes?
Regards,
John S

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I'm no machinist but reaming an oblong hole round and larger doesn't sound feasible to me and if the hole is not oblong, then why are you even thinking about this?

Norton provided oversize spindles and RGM still do and possibly AN.


It was found that reaming out to the next spindle size up was just the right amount to remove any wear in the cradle from a spindle even if out of round.

If you look at a normal twist drill it cuts at 2 points, put the drilled hole under a shadowgraph in a metrology lab and that round hole will look more like a threepenny bit.



You then use a reamer to get a rounder hole and more accurately sized, the reamer works by only being designed to take a small cut but on a lot more cutting edges that the two on a drill.

So depending on out of round a hole is a reamer can and does make them back into bigger round holes. Where that hole is centred on is the problem, not that a reamer cannot make a wobbly hole round.

kommando said:

Norton provided oversize spindles and RGM still do and possibly AN.

SWINGING ARM SPINDLE LONG +.005"

RGM NORTON 064077

www.rgmnorton.co.uk

It was found that reaming out to the next spindle size up was just the right amount to remove any wear in the cradle from a spindle even if out of round.

If you look at a normal twist drill it cuts at 2 points, put the drilled hole under a shadowgraph in a metrology lab and that round hole will look more like a threepenny bit.

You then use a reamer to get a rounder hole and more accurately sized, the reamer works by only being designed to take a small cut but on a lot more cutting edges that the two on a drill.

So depending on out of round a hole is a reamer can and does make them back into bigger round holes. Where that hole is centred on is the problem, not that a reamer cannot make a wobbly hole round.

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Since worn cradle spindle tubes are not normally worn in the middle, if you have nothing to center the reamer to start with, I can't imagine success. Now if the cradle was out and, in a fixture, and the reamer was in something stable, and progressively larger reamers were used or maybe a tapered reamer, I could see that work. But just sticking a reamer in the hole and turning it with the cradle in the bike and probably the primary still installed.... Again - I'm not a machinist.

Why are you worried about the centre being off by 2 or 3 thou max when the tolerances on the cradle and swing arm etc are much greater.

kommando said:

Why are you worried about the centre being off by 2 or 3 thou max when the tolerances on the cradle and swing arm etc are much greater.

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The cradles I use have no discernable play and certainly I don't build bikes with play. If there was slight play then I would install the clamps, but the drilling would be done with the spindle out, a new spindle with flats would be used, my reamer would cut off the burrs caused by drilling for the clamps and not cut the tube otherwise, and the clamps would be tightened evenly. Just did this last week for the umpteenth time.

3 thou wobble of the spindle gets you a lot of rear wheel wobble! If you start 3 thou off you will end with a total of 6 thou off most likely.

A new spindle in a good cradle is a snug slip fit, not a sloppy fit!

I am not talking about the play but the hole being off centred after reaming. When you ream a hole 5 thou oversize which has ovality the most the centre will be moved by is 2 to 3 thou. That is not the play I would allow in a cradle, sliding fit or it gets reworked until it is.

kommando said:

I am not talking about the play but the hole being off centred after reaming. When you ream a hole 5 thou oversize which has ovality the most the centre will be moved by is 2 to 3 thou. That is not the play I would allow in a cradle, sliding fit or it gets reworked until it is.

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OK - understood. As long as the hole is perpendicular to the cradle, being off a little does not matter.