Gear box cracked - help me understand why

[valross]

The gear box in my Bagheera broke. I don´t know why and I hope someone can help me understand what happened.
When I put it in first gear there was a resistance, and when I tried again harder, there was a big bang and the clutch didn´t work. Under the car oil was running from the gear box.
I managed to start the car in second gear and could drive c a 1 kilometer home, leaving an oil trail.
When I inspected the damage I could see that the three topmost bolts holding the gearbox to the bellhousing was broken and the gear box case had cracked open.
When I removed the gear box I noticed that the input shaft was loose and had an axial movement of c a 5 millimeter. The two bearings were moving with the shaft.



I  have three old gear boxes with the same case as the broken one. But they had different ratios. And as I don´t want to change the differential I have to take the output shaft from my old broken gearbox  and put in a sound case.



So I have gutted the broken gear box, gutted another gear box with a sound case, I have cleaned all the parts, bought new bearings and seals.
Before the breakdown I had no problems with the syncromesh so I have not bought new sync components. Should I?



Before I put it all back together again, I would much appreciate any suggestions.

Lasse

[andyowl]

So the parts from the old gearbox are going into the "new" casing? I think that should be OK.

If the gearbox had moved away from the clutch when the casing broke that might explain why the clutch would not work. But I don't understand why it broke the casing.

Did the clutch look OK?

The edges of the casing break look clean and I can see no dirty signs of a crack developing.

Puzzling.

Andy

[Spyros]

Isn't it the bottom part of the gearbox that broke away ?

Did you hit something ?

[valross]

Did the clutch look OK?

The edges of the casing break look clean and I can see no dirty signs of a crack developing.

Yes the clutch seems OK.
And there are no signs of developing cracks.

Isn't it the bottom part of the gearbox that broke away ?

Did you hit something ?

No I did not hit anything. Everything was OK until  I started the engine and tried to engage first gear.
It looks like the whole gearbox has been tipped over - the top bolts broke and the case broke at the bottom.

[andyowl]

Valross wrote ....the gearbox top bolts broke..

By "broke" do you mean broke in two pieces or did the threads strip out of the diff casing? I don't suppose you still have the bolts?

Don't the gearbox top bolts also hold the left side engine mounting too?

Maybe the bolts were too tight in the past?

Failure of the top bolts would certainly place great strain on the lower bolts and that could easily explain the broken casing.

Andy Owler