1968 gearbox needs help

ILLF8ED · Apr 12, 2020

Recently acquired ‘68 gearbox that had a layshaft bearing go. How should
I go about repairing the case? The case metal is pushed outward. Push that back in then weld? I’m concerned the case will warp, how can that be prevented?

maylar · Apr 12, 2020

Pull it apart and bring it to an experienced welder.

Onder · Apr 12, 2020

At the rates welders and turners now require it may be worth it to get another shell new or used. And don't forget to think about the crack between the two
bores.

ILLF8ED · Apr 12, 2020

Onder said:
At the rates welders and turners now require it may be worth it to get another shell new or used. And don't forget to think about the crack between the two
bores.

There is a company locally that advertises aluminum

boat and motorcycle case welding. Guess I’ll check there. No crack between layshaft and mainshaft bearings.

Fast Eddie · Apr 12, 2020

The bearing housing is obviously well FUBAR’d, as well as there likely being a crack between the two bearings. Welding that up and machining it will be a big job, and goodness knows what alignment issues you may face afterwards.

This is the answer: https://andover-norton.co.uk/en/shop-details/16256/gearbox-shell-not-mk3-06-0808-

Fast Eddie · Apr 12, 2020

Obviously it needs a better picture to be sure, but if you zoom in, I think I can see a crack where I have marked in red:

kommando · Apr 12, 2020

How after that is all welded up are you going to bore the housing out and retain the interference fit in the new bearing. If you or a local machinist can't economically then get a new box. That crack means the other bearing housing will need boring out too.

ILLF8ED · Apr 12, 2020

Fast Eddie said:
Obviously it needs a better picture to be sure, but if you zoom in, I think I can see a crack where I have marked in red:
View attachment 15470

Good catch. I looked again with magnification. There is a crack between the bearings. I wanted to safe this one as it is a very early Commando production. If 126125 was the first production Commando, this is 126177. Think I will sell off the other pieces and call it a day.

Thank You.

dynodave · Apr 12, 2020

If you believe that case is original norton OK. NONE of the way more than a dozen gear boxes I have use those style number stamps. Even back into featherbeds...
Seems to be a lot of ground away aluminum when sand cast finish is normal. The one should look like this :1
not this

Ron L · Apr 12, 2020

dynodave said:
If you believe that case is original norton OK. NONE of the way more than a dozen gear boxes I have use those style number stamps. Even back into featherbeds...
Seems to be a lot of ground away aluminum when sand cast finish is normal. The one should look like this :1
not this

Not to mention you can see where the original numbers have been ground off. Who ever did that should have at least bead blasted the case to make it less obvious.

ILLF8ED · Apr 12, 2020

dynodave said:
If you believe that case is original norton OK. NONE of the way more than a dozen gear boxes I have use those style number stamps. Even back into featherbeds...
Seems to be a lot of ground away aluminum when sand cast finish is normal. The one should look like this :1
not this

This helps me feel better about scrapping the case. I don’t currently need another gearbox so may as well sell off the parts.

Thanks

mdt-son · Apr 12, 2020

illf8ed said:
Good catch. I looked again with magnification. There is a crack between the bearings. I wanted to safe this one as it is a very early Commando production. If 126125 was the first production Commando, this is 126177. Think I will sell off the other pieces and call it a day.

If it wasn't for the cracked housing wall, you could have lived happily with a minor crack between the bearing seats. I've read this is a common fault and not a critical one.
However, just welding the case is a no-no. For one, this is a highly stressed area and welding of alluminum reduced the material strength considerably (up to 50%). To get around this, you'd need to subject the case to a heat treatment cycle specific to the alloy used. I think it's going to be difficult.
Secondly, when you weld aluminum, the material realeases built-in stresses and creates new stresses due to the rapid cooling (this is termed residual stresses) which causes every bore in vicinity of the weld to become mis-aligned. That's why all machining at the factory took place after the heat treatment cycle.
I suggest you scrap this case and obtain a new one (AN sells reinforced cases as a replacement).

-Knut

mdt-son · Apr 12, 2020

David,

In another thread, we have been discussing fretting of gears and the effect of low viscosity oils such as ATF. Now I am curious to know if there are signs of fretting on your gear cluster?

-Knut

baz · Apr 12, 2020

I remember the first commando I owned it was a very used MK2 a
The gearbox was the same as yours where the layshaft bearing must have let go
The previous owner had just stuck P38 or something similar in the crack and changed the layshaft bearing
I was riding it like this blissfully unaware
When I had the chain case off to change the gearbox sprocket I noticed the crack
I stripped the gearbox ,dug out the filler and welded it up
I was just a kid at the time (40 years ago ) with no money, the thought of buying a new shell or even getting it machined after welding never occurred to me
It was my only means of transport so it was off the road for one day for the welding and that was it
It covered many hard miles after that no problems
Well not from the gearbox anyway!!!

ILLF8ED · Apr 12, 2020

Ron L said:
Not to mention you can see where the original numbers have been ground off. Who ever did that should have at least bead blasted the case to make it less obvious.

Dave and Ron,

Since replying I’ve found a few examples of the “1” exactly as stamped on this ‘68 case. Machivmotors on eBay for example has ‘68 crankcase and gearbox case. My other two Commandos also are ground at the top where the number is stamped. My ‘69 has the same stamped “1” you said was not used.
Check this ad on eBay
Item number 362870787852

Now going back the other way as this case could indeed be original. Will see what others say.

ILLF8ED · Apr 12, 2020

mdt-son said:
David,

In another thread, we have been discussing fretting of gears and the effect of low viscosity oils such as ATF. Now I am curious to know if there are signs of fretting on your gear cluster?

-Knut

Fretting is the same as galling? Yes fourth gear layshaft and all gears on the mainshaft show this. My ‘69 and ‘72 Commandos as well. The only gearbox I’ve seen without this were on my ‘57 model 50 and ‘61 ES2. Neither of those have enough power to cause this type wear.

ludwig · Apr 13, 2020

When I was young and poor, I fixed this by simply placing a shield behind the bearing.
It held up perfectly well.
The cracked area is of no importance for the structural strength of the shell.
Neither is the hairline crack between the 2 bearings ( unless all the way trough ..)
Welding can do more harm than good.
Just make it oil tight, and you'll be fine..

Fast Eddie · Apr 13, 2020

You’re obviously braver than me Ludwig. If I did that, here’s how it would end ...

Onder · Apr 13, 2020

I find it amusing that Ludwig, who has more skill in his pinkie than I have in ten fingers, a man with turning skills that are not insignificant, is willing to
glue it up and press on!
Im not saying he is wrong at all. In fact when someone on his level says to press on you probably could.
...or not as ex-aircraft I'd order a new shell from AN in a heartbeat.
Fear is a strong motivator.

kommando · Apr 13, 2020

illf8ed said:
Fretting is the same as galling?

No, two different mechanisms of part destruction.

https://en.wikipedia.org/wiki/Galling Best seen as the wear in Amals between slide and carb body caused by both being made from the same material, cured by the use of different materials including anodising of slide.

https://en.wikipedia.org/wiki/Fretting This is the gear teeth issue.

1968 gearbox needs help

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