Gearbox crack

Onder · Jun 28, 2010

This is hardly news, the crack twixt the two drive side
bearings in the gearbox . It doesnt seem to go further than
the web. So is it considered okay to press on with the box
over haul or need I do the wise thing and order up a RGM
improved shell?

79x100 · Jun 28, 2010

I've replaced shells for this on projects and that would still be my policy. Welding is rarely satisfactory here and the trouble will usually return.

The most usual cause of the crack in the first place is layshaft bearing failure so if you build a box with a new shell and a decent bearing, it's not something that should happen again.

nortonspeed · Jun 28, 2010

Onder said:
This is hardly news, the crack twixt the two drive side
bearings in the gearbox . It doesnt seem to go further than
the web. So is it considered okay to press on with the box
over haul or need I do the wise thing and order up a RGM
improved shell?

As long as both bearings have a tight fit (without loctite) you're ok. But buying a new beafed up shell isn't a bad idea either!

Hobot · Jun 28, 2010

When ever you do go shopping for Commando gear boxes,
you will find them on the pricey rare side.
Exact same gear box is used in Atlas models and are fairly
common and at least half the price.
Hi end gear boxes are always a good move if affordable.

The only difference is no notch cut out that requires the
bothersome thick spacer washer placed in Commando versions.
Supposedly to make installs easier once washer knocked out.
But I found its so so much easier w/o the notch since Atlas
version is same width top and bottom and moves just
fine once clamp force released.

There are piles and piles of Commando gear boxes with
too loose bores scattered around the world I found
and none of them worth recovering d/t thermal distortion
to compensate and predict for.
I'd bet hi end epoxy and line boring might re new the
supply.

hobot

Onder · Jun 28, 2010

Thanks for responses, I pretty much thought as much.
If any one has the RGM improved box, does there
appear to be any thing done about the thin area between
the bearings? How could there be?
Oddly, all the bearings were serviceable and both shafts
as well. Of course, I plan to replace all bearing and all
bushes.
The gears were all lovely and new looking with 15k on
the clock. I was surprised.

chris plant · Jun 28, 2010

hi onder,i have a quaife strengthened gearbox shell,and as you sugest there is,nt any more meat between the bearings [how can there be]but there is more thickness around the outside of the bearing locations,and two strengthening ribs around the back of the box,contact mick hemmings for full spec

Onder · Jun 28, 2010

This is a four speed, not a five speed?

chris plant · Jun 28, 2010

hi onder,4 or 5 sp,it dose,nt matter,the shell is the same

willh · Jun 28, 2010

I changed the whole box when mine cracked. Didn't want to risk it.
But was the crack the cause of the broken main shaft, bent lay shaft, etc. :evil: or because of?
Paid 400$ for complete used tranny in great shape.

lcrken · Jun 28, 2010

I have some extra heavy duty shells Kenny Dreer had made when he couldn't get the Quaife shells. The only thing different about them (aside from being sturdier) is that he used 1/4" dowel pins to locate the inner cover. You have to open up the holes in the cover with a drill to take the larger pins, and also have to slightly enlarge the corresponding holes in the gasket. They are unused, and I would let one go for $350 plus shipping, less than half of RGm's price for their heavy duty shell. Pictures below. The shells now have bushings and dowel pins installed, not shown in the pictures.

I'd offer you one of my used stock shells, but they are all cracked, just like yours! Some of them put in a lot of racing miles that way. I suspect that yours would last a lot longer without any problems, as long as it still has a tight fit for the bearings. If it has a lot of mileage, the chances are pretty good that the sleeve gear bearing has already spun in it's bore, and you need to replace it anyway.

Ken Canaga

Keith1069 · Jun 29, 2010

Exact same gear box is used in Atlas models and are fairly
common and at least half the price.

For some reason and I know not why, it's reported the early shells are stronger, better material? I have a '58 shell (Nxxxx) in my bike with no cracks and have a spare '59 also with no cracks. You could say they were not stressed in the 30-40 hp Dommies but mine was in the bike when I got it and has at least 48k miles on it.

willh · Jun 29, 2010

I have some extra heavy duty shells Kenny Dreer had made when he couldn't get the Quaife shells. The only thing different about them (aside from being sturdier) is that he used 1/4" dowel pins to locate the inner cover. You have to open up the holes in the cover with a drill to take the larger pins, and also have to slightly enlarge the corresponding holes in the gasket. They are unused, and I would let one go for $350 plus shipping, less than half of RGm's price for their heavy duty shell.

That,s the way I would go if I needed one. Hell, if I had the money to spare right now I'd buy one.

Flo · Jun 29, 2010

Hmm, racking my brains here. Can somebody enlighten me as to why the casings need to be beefed up.
As far as I can see, it is the internals that need beefing up.

lcrken · Jun 29, 2010

Flo said:
Hmm, racking my brains here. Can somebody enlighten me as to why the casings need to be beefed up.
As far as I can see, it is the internals that need beefing up.

Good question. I've seen several stock shells come apart in racing, besides the one I blew. Hard to say whether they broke because a gear or shaft gave up first, or vice versa. Mine split when I downshifted at too high a speed going into turn 7 at Riverside. I was using a stock 4-speed at the time, and I'd been used to downshifting there with close ratio 5-speeds. The stock shell is quite thin in places, and there's plenty of force trying to push the shafts apart and flexing the drive side end.

If it's like other Commando aftermarket castings (crankcases come to mind), its very likely that the aftermarket ones use a higher strength alloy, which means the bores for the sleeve gear and layshaft bearings won't get loose as quickly, but that's just my guess. Kenny wasn't builidng these to a production price limit, so I'm confident he didn't cheap out on the material.

It could all be bragging rights too (hey, look at my trick gearbox everybody), but I don't think so. Well, maybe a little.

Ken

ML · Jun 29, 2010

Flo said:
Hmm, racking my brains here. Can somebody enlighten me as to why the casings need to be beefed up.
As far as I can see, it is the internals that need beefing up.

Its a function of the torque and flex of the shafts captured at each end. The stress causes fatigue and fatigue accumalates until cracking initiates. The thicker the shell walls at the load paths will tolerate stress longer. In simple terms, the shaft assemblies are stronger than the shell, so beefing up the shell absorbs the stress better. In all probabilty, there is a finite stress life on the beefed up shell it just will last longer.

Mick

slimslowslider · Jul 2, 2010

Hello, had some issues with my box some year and a half ago, and did a last desperate attempt to solve it as per pictures attached.
Runs ok now for 6000 kms, crossed the Alps (EU) with it (Stelvio).
This is my first posting so don't know if it works out with attaching the pics.
Big Hurray for the people bringing this forum to us!
Cheers.

swooshdave · Jul 2, 2010

Best first post ever.

Big pictures, excellent fix and you are welcome to stay. :mrgreen:

grandpaul · Jul 2, 2010

Welcome!

I echo dave's seniments exactly.

swooshdave · Jul 2, 2010

grandpaul said:
Welcome!

I echo dave's seniments exactly.

ALthough is that to fix a crack or to fix a spinning bearing?

Hobot · Jul 2, 2010

5 years ago my world wide shopping for AMC Commando shell
averaged $450 a pop. Altas shells, which I like even better, $250.

I've a Qualfe beefy case that one of the mount bolts boss was
busted at one end and bearings obviously spun in the bores.
Ken had mount welded and cleaned up. So hope to solve
the bearing bore by something a bit less robust-complex
than slimslowslides tedious machinist intensive example.

Someone mentioned a stronger main shaft available.
Where would I shop for that?

hobot

Gearbox crack

Onder

79x100

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Hobot

Onder

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Onder

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willh

lcrken

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