Another MK III broken axle (2018)

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Pulled into my shop just as rear tire went flat. Thought what good fortune to have it happen right there. Repaired tube and, just snugging up rear axle, it snapped!

Nothing new, classic fatigue crack - fine grain crack 2/3rds through until the final third broke.

I remembered Hobot was counting them so here's another.

Terrible design. Too bad no one makes them in a much stronger steel.

Stock axle with almost 11,000 miles on the bike.
 
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You are correct, 2 piece Norton axle is crap.
Contact Don Pender.
madass140@gmail.com
One piece stainless steel axle.
Superb workmanship.
Fair price.
Nice guy.
He is a regular contributor of good info on this forum.
 
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I was under the impression that Donald Pender does not do a MKIII axle. I've bought one of the one-piece axles from him for the '74 and older models, and it's a very nice piece. If he does make one for the MKIII, i'll be ordering one!
 
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I discussed the inferior rear hub/axle design with Andover Norton and suggested an upgrade. Unfortunately they dismissed the issue, stating there were many examples of bikes which had covered 100 K miles or more, and blamed the breakages on third party vendors. JimNH, have you been riding much 2 up and/or with panniers?

I am not done pondering how the rear axle could be upgraded to 20mm. There are some obstacles related to the bearings and the hub design unfortunately.

-Knut
 
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One piece stainless steel axle.

Stainless steel is only to be recommended if a high performance alloy steel like UNS S32550 is chosen. AISI 316L is a low performer strengthwise and is lethal in this application.
The only reason for using "stainless steel" (what a misnomer) is to avoid corrosion on those parts exposed to the environment. The factory never cared about that ....

-Knut
 
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Had a stainless one break in 2002, but no problems with the AN one fitted to replace way back then.
 

Esmerela

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Pulled into my shop just as rear tire went flat. Thought what good fortune to have it happen right there. Repaired tube and, just snugging up rear axle, it snapped!

Nothing new, classic fatigue crack - fine grain crack 2/3rds through until the final third broke.

I remembered Hobot was counting them so here's another.

Terrible design. Too bad no one makes them in a much stronger steel.

Stock axle with almost 11,000 miles on the bike.

New
I believe a lot of the failures are down to over tightening o_O Unless you are the only person removing and refitting the rear wheel you just never know when a knuckle dragger has been heaving it up super tight with a long breaker bar . I always use the torque wrench in the workshop .
 
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I have had 3 go in the last 35 years. As for "over tightening", my w/shop manual specifies 80 lbs/ft...
 
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Curious as to what happens when an axle breaks. Is it something that you can hang on and ride out, or is it more catastrophic. Everybody that reports on having a break seems to be alive and well, thankfully.
 

nortonspeed

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I have had 3 go in the last 35 years. As for "over tightening", my w/shop manual specifies 80 lbs/ft...

Most likely another incorrect specified torque setting in the w/shop manual as the general torque setting for a 1/2" UNF threaded steel bolt is 60 ft lb so 80 ft lb sure means substantial over tightening. No wonder these MK3 rear spindles break occasionally!
 
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batrider

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Using the standard tool kit there is no way you could apply 80 ft lbs.
 

Onder

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Why is it even 60 ft lbs? Even that seems too much. What amount do most of you use?
 
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I would guess around 50 . 65or is it 70 is recommend torque for ZZR and I use nothing like that
 
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I would guess around 50 . 65or is it 70 is recommend torque for ZZR and I use nothing like that
Mike, What is the diameter of your ZZR rear axle? Does anyone else have a recommendation for proper tightening of the Mk III rear axle? I see no mention of a rear axle torque in Section A of the Mk III workshop manual. Section H2 [Refitting the Rear Wheel] Step 8 says:

8 Ensure the tensioner fits snugly to the swinging arm. Tighten the right-hand axle. T i g h t e n the lower suspension [shock absorber] bolt & nut.

Looking in Sections A & H of the 1970 & up 750 manual, I don't see any mention of a torque setting for the rear axle.
 
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Chaztuna and others (myself included) are updating the MK 111 workshop manual concerning helpful revisions and cleaning up a few omissions.
I believe the 80 lbs./ft. in Section A causes potential problems. This might very well be changed to 50-60 and the word Axle placed after the word Spindle (in brackets) . Many on this side of the pond do not use the word Spindle.
 
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Section A,
"Rear wheel spindle: 80 lbs./ft."
OK, I found it. I was scanning quickly. I spotted the Norton part numbers on the left column, but didn't notice the torque figures in the right column. While the axle itself is 11/16" diameter, the threaded section is reduced. NortonSpeed reports the threaded section is 1/2" - 20 TPI [UNF] 80 Lb/Ft is awfully tight for 1/2". Could this be another case of an incorrect torque call out? [Like the gearbox main shaft nut]
 

L.A.B.

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While the axle itself is 11/16" diameter, the threaded section is reduced. NortonSpeed reports the threaded section is 1/2" - 20 TPI [UNF]

The axle is 17mm.
The metric 6203 drive-side wheel bearing is 17x 40 x 12mm which 11/16" (17.4625mm) wouldn't fit! The Mk3 axle running directly in the DS bearing.

Yes, the thread is 1/2" - 20 UNF.
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