Tarmac Rally Stratos Build begins

[Strat Fan]

Isn't there a similar problem with Integrale boxes on uprated cars? I have a vague memory of casings being machined around bearing houses to accept a sandwiched steel plate to strengthen them.

I can't comment on the Integrale but there is the 3mm steel strengthening plate which can be added to Fiat/Alfa boxes between the bell housing and the gearbox casing which is used to help prevent the diff/crown wheel from separating from the pinion shaft. This requires 3mm to be machined from the gearbox casing to me it fit.
Neil did this to his old 164 box. That bit didn't fail but he stripped 4th gear shortly after....

[westonTB]

Thanks Colin, I have learnt a lot from watching all your video's and the 105 Alfa stuff, 2 years & on the track = Result! Me = 6 years in and still bleeding air out me brakes!!! = idiot.

More Beer needed to help I think.

[Bernard]

If I remember correctly when he machined Erics gearbox for me he took 1.5 off the bell housing and 1.5 off the Gearbox casing.

I can't comment on the Integrale but there is the 3mm steel strengthening plate which can be added to Fiat/Alfa boxes between the bell housing and the gearbox casing which is used to help prevent the diff/crown wheel from separating from the pinion shaft. This requires 3mm to be machined from the gearbox casing to me it fit.
Neil did this to his old 164 box. That bit didn't fail but he stripped 4th gear shortly after....

[Strat Fan]

If I remember correctly when he machined Erics gearbox for me he took 1.5 off the bell housing and 1.5 off the Gearbox casing.

It sounds viable, I've never had one done.

[ByrneEngineering]

The one I have seen that failed in a Stratos replica (former member on here) was much more catastrophic and the weak point was actually around the outer edge of the end casing.
The increased traction of the mid engined layout must have been too much and the thrust put back up the shaft from the crown wheel was too great for the casing.

So this was the sort of failure we were attempting to avoid with the additional strap


Now 'm not saying this is absolutely correct, but here was our thinking on the situation


- the failed section is after the gearbox mount, so it won't be subject to any loads generated between the diff output to the chassis


- the only Force that part of the gearbox casting is subject to is created from the gear shaft bearings


- As the gears are loaded up the shafts will try and spread apart, this is reacted by the bearing housings in the casting


So our thoughts were, a failure occurred in the casting between the 2 bearings, as mentioned it is very very thin in there, once a crack formed the 2 gears spread apart further and caused the carnage seen in that photo


What would be super helpful would be a photo of the failed casting parts, if the 2 bearing housing are still attached together in the end plate that would falsify our theory

[Strat Fan]

So this was the sort of failure we were attempting to avoid with the additional strap


Now 'm not saying this is absolutely correct, but here was our thinking on the situation


- the failed section is after the gearbox mount, so it won't be subject to any loads generated between the diff output to the chassis


- the only Force that part of the gearbox casting is subject to is created from the gear shaft bearings


- As the gears are loaded up the shafts will try and spread apart, this is reacted by the bearing housings in the casting


So our thoughts were, a failure occurred in the casting between the 2 bearings, as mentioned it is very very thin in there, once a crack formed the 2 gears spread apart further and caused the carnage seen in that photo


What would be super helpful would be a photo of the failed casting parts, if the 2 bearing housing are still attached together in the end plate that would falsify our theory

I haven't got any other pics of the failure so can't comment on what the other piece looked like.
You can try DM'ing the owner, Sando on here but he sold his car a couple of years back and hasn't checked into the forum since Nov 2020.
You may be right on the failure being caused by the shafts trying to separate, I was thinking it was related to the thrust on the shafts induced by the helical cut of the gears/crown wheel.

[Stableblock]

There will also be an axial thrust from the taper roller bearings tending to push the end off the casing. Your mod will help that by reducing the bending of the casing but ultimately the same forces have to be accommodated somewhere. As you say Colin more pics of the failure would help identify the cause.

[ProtoTipo]

Here's a 3.2 GT box