Gearbox case AFU

[Testarossa]

Constant readers may recall that my 850T gearbox began leaking into the bell housing a year ago. I parked it (I also had a broken piston ring) and bought the Mille.

I finally dropped the lump out of the frame.  Here's what I found at the back of the gearbox:




"Cap" over the front end of the output shaft broke off, and some of the ground-up bits were in the bottom of the bell housing.

What would cause that kind of failure?  Is it worth repairing? There's no endplay in that exposed shaft.

About 60,000 miles on this box.

[jmac851]

Is the nut lose on the rear of that shaft. That would allow it to move forward during deceleration, pushing the shaft forward into the front case.

[Testarossa]

Is the nut lose on the rear of that shaft.

Nut is on solidly.  No endplay in the shaft.

[Antietam Classic Cycle]

IIRC, that happened to JB one time, but in his case the (left-hand-thread) nut at the front had backed off and chewed through the case.

I have a bare early 5 spd. (probably Eldo) case laying around, yours for shipping cost if you want it.

[Idontwantapickle]

That's a drag!
That particular failure is sometimes caused by rear bearing failure or one of the nuts loose. But humans often cause this by tapping on the shaft when removing the rear cover. That part of the case is easily damaged by use of force from the other end. It would be possible to damage it and have it break out later I suppose. But it does look like the nut is not staked properly in that photo.
On the upside it's not too difficult for an experienced welder to fix.
Hunter

[pressureangle]

I just emptied a 5 speed and sent the internals to Pete Roper.

The case is wondering what I'll do with it. PM me if you want it.

[jabberwocky]

I wonder if that nut backed off some and got stuck there, far enough to chew through the case. If you look, the nut is staked, but the staking is not into the slot in the shaft like it is supposed to be.

[Testarossa]

I wonder if that nut backed off some and got stuck there, far enough to chew through the case. If you look, the nut is staked, but the staking is not into the slot in the shaft like it is supposed to be.

Same thought occurred to me, but the nut doesn't project beyond the end of the shaft, and there's at least a quarter inch clearance from where the end of the cap should have been (I think).  If the nut did come loose and grind on the inside of the cap, why did it wind back into place?  My understanding is that when this nut comes loose, the end of the shaft can migrate right through into the bell housing and shifting goes wonky, then locks up. But shifting remained smooth throughout this episode.

When I took the thing apart I never disassembled the output shaft. I measured everything carefully, found it to spec, and put it back together with a new seal at the back. Never took that front-end nut off.

It was the first time I've opened a gearbox, so I suppose my own ham-handed reassembly is what killed it. I'd just like to know how, so as not to do it again.

[jabberwocky]

Is there excessive end float in the output shaft? It could have jumped forward and knocked out the case if there was.

[Testarossa]

There is NO repeat NO endplay in the output shaft.

[jabberwocky]

Sorry.There's probably nothing wrong with your transmission. Except for the big hole in it produced by absolutely nothing.

[Testarossa]

That's exactly right. There's nothing wrong with the gearbox except a big hole. It still shifts smoothly and always has.

I'm trying to figure out what made the hole it so it doesn't happen again. I stated at the beginning of the thread that there's no fore-aft motion of the shaft. So it must be something else. I was hoping someone in the group had seen this caused by something other than the shaft moving forward.

[tris]

Is the"cap" like the old core plugs used on car engines?

If yes could you simply replace it after checking there's no debris from the old one found its way inside the box

[MotoG5]

Just fishing here. Are you the original owner? With a bike this old maybe previous work done on it caused failure later. Possibly some foreign object got into the housing at some time. Any teeth missing from the starter or starter ring gear? Possible casting flaw from the get go? If it were me I would take one of the generous offers of a replacement housing and carefully reassemble. At this point its all speculation any way. You have covered all possible explanations from mechanical issues that would normally have caused this type of admittedly rare damage.   

[Antietam Classic Cycle]

Is the"cap" like the old core plugs used on car engines?

No, it's part of the transmission housing casting, not a separate piece.

[Wayne Orwig]

Did it get knocked out in one piece? If so maybe that piece can help tell the story of why.


And looking at the photo, the staked nut HAS turned.

[Testarossa]

Yup, the staked nut has turned. But it's tighter, not looser, than it ought to be.

The cast-in aluminum "cap" was reduced to small bits, nicely chewed up by the hard steel starter-ring teeth. No forensic value. No damage to anything that rotates in the bell housing.

I bought the bike in 1983 after the first owner wrecked it. It needed a new frame and fork. It's possible that the gearbox was somehow damaged in the accident, but I had no reason to open the box until after the flood of Sept 2014.

After the flood, without turning anything, I opened the box, cleaned everything (it was in pretty good shape) and replaced the return spring. Didn't take any gears off any shaft. Reassembled and closed it. After the rebuild, put 3000 miles on it before the hole happened.

I'll open it up and see if there's any evidence of damage inside, then reassemble with one of the used cases on offer.

[Wayne Orwig]

With the nut turned tighter and everything, I have to believe that the shaft floated forward somehow. Or I guess a piece of something is loose in there and got jammed in it.
Hopefully it is obvious when you go in.

[pressureangle]

My WAG is that the housing was either a faulty casting, or cracked (or both) by an overzealous bearing installation. I mean, perhaps hammering the bearing into that bore broke, or propagated a crack from a poor casting, and took some time to enlarge and finally break off. It's hard to tell by the photo, but you might be able to tell by witness marks how long the crack was propagating. If it broke off all at once, the edges should be consistent and clean with no rubbing or polishing.

[jacksonracingcomau]

Yup, the staked nut has turned. But it's tighter, not looser, than it ought to be.

IMPOSSIBLE
You do realise it is a left hand thread, don't you?
Was very common scenario in 70's, luigi had a bad torque wrench
Every box I ever took apart was not tight from new, torque to spec, stake, will never happen again. I'd buy a new nut.
Press the other shaft back together while you're at it

[Testarossa]

Opened the gearbox.  Here's what we have:
First, a close-up of the front of the box (bell housing side) showing the end of the driven (output) shaft. Note that the left-hand-thread locknut is tight but not staked. With no endplay, the end of the shaft remains clear of the plane of the fracture. Fracture looks clean -- not chewed up.




Next, similar view with the shaft out. Pretty clean in there.




Finally, the inside of the box. No sign of damage other than the missing "cap."




Remember, this box shifted smoothly when parked. Thoughts, advice, admonitions?

[RinkRat II]

 Greetings, Looks like the Staked part of the nut is long gone. Why? anyones guess but likely when it turned it sheared off. Any magnetic parts in with the swarf?
   Also looks to be small amount of fatigue cracking on the thicker part of the boss, possible crack from the accident . If you know a really good Heliarc guy see if He would make a cap and Heliarc it on as backup as it's not structural and only needs to keep oil in and bugs out. Just my $.02. 


     Paul B

[Testarossa]

Thanks, Paul.

When I opened it after the flood I didn't see anything wrong, but I may have been too naive to see that the stake was missing and the nut turned. On the other hand, none of the workshop manuals or parts books I have shows a stake -- can't find it listed on the Harper's site, for instance.

No loose ferrous stuff. Maybe the stake was never in there.

[RinkRat II]

 This is what it shoul have looked like when done correctly.

    Paul B

[Testarossa]

Thanks, Paul. That's not even close to the way mine was done.  I appear to have a different (earlier?) locknut design. Perhaps it was changed because my version had a propensity to slip?

[charlie b]

Since I've never seen the inside of a tranny, take this with a grain of salt.  ;)

Those last two pictures look like the same kind of setup, ie, the same type of nut.  The difference is the place on the nut that should have been 'staked' is gone, ie, that's what that 'notch' in the locknut in the second picture looks like.

Not sure what could have cause that.  Could it have broken off by itself and then the nut tightened?  Or did the PO remove that nut by breaking off that piece and then not staked it in a new place when reassembled?

If it did break off, where did that little bit of steel go?  And still no idea why the 'cap' is gone.

[nc43bsa]

I've seen the staked nut on a car CV axle that looked exactly like that after someone used an impact gun to remove the nut without un-staking it.  I suppose that's why a reman CV axle often comes with a new nut.

[rodekyll]

That's a drag!
That particular failure is sometimes caused by rear bearing failure or one of the nuts loose. But humans often cause this by tapping on the shaft when removing the rear cover. That part of the case is easily damaged by use of force from the other end. It would be possible to damage it and have it break out later I suppose. But it does look like the nut is not staked properly in that photo.
On the upside it's not too difficult for an experienced welder to fix.
Hunter

I agree -- the 'cap' could have been broken off by a blow to the shaft or perhaps by the end plate being screwed on with the shaft/bearings not seated.  I had identical damage to my Convert timing cover when I had too long a drivepiece in the atf pump.  When I tightened the perimeter bolts the 'cap' popped off. 

Did I read in one post that the bike had been in an accident?

[Testarossa]

There was an accident. The original owner somehow bent the frame, which was replaced when I bought it at about 20,000 miles.

[rodekyll]

See the pump showing?  The damage is identical to yours.  The cause of mine was pressure on the casting applied by that round part in the middle of the gear.  It didn't take much -- I hadn't gone to final torque on the timing cover when it popped.