Sportomatic Differential Question

[Fishcop]

I've asked this on the Early S Board, but plenty of knowledgeable people here too...

Can I use an early sportomatic (905) differential in a 901 gearbox build? Are they the same animal?

Cheers

[Bill Douglas]

Port McQuarie? Have you got a new job and moved. Hope you are having fun, Bill.

[oldie44]

I looked into this a few months ago for a guy who wanted to use a sporto R&P in his race car.
The 905 differential will fit into an aluminum case 901 housing with no problem. The bearing size and distance between the bearing seats is the same as for a 901.
However, the 905 pinion shaft is 7mm shorter than a 901/914. As a result, the splines on the end of the shaft only engage the 1st/rev guide by about 2mm. In my opinion, that is not enough.
I built up the gear packs on 905 and 914 pinion shafts to show the problem.

[Fishcop]

Thanks Ken. I'm actually looking at it from the perspective of using the differential itself for keeping the output shafts consistent. I realise I'd have to transfer the ring over and reset the differential in the case. I'm seeking to use my sporto's 108mm drive flanges in a replacement aluminium 901 with the early differential (which can't take the bigger flanges.

I have received some solid advice since posting that it's unlikely that I can make it happen - and I haven't read of anyone else successfully doing it either... But I'm hoping someone has been ingenious!

[Fishcop]

Originally Posted by Bill Douglas

Port McQuarie? Have you got a new job and moved. Hope you are having fun, Bill.

Hey Bill

Yep, got sick of city life and was successful picking up a job by the beach about a year ago. Back to more sedate pursuits such as fishing and surfing

[oldie44]

Actually, there are 108mm drive flanges that fit the aluminum case 901. Porsche changed to the 108mm flanges for the 1969 model year but did not change to the mag case 901 until the spring of 1969 (middle of the model year). I had a 1969 911S way back when that had an aluminum transmission.
Also, they contunued to offer the 905 transmission until 1971 and these had 108mm flanges. I can post some comparison pictures of the various flanges if you like.

[Geary]

This might be of help, John ..

[Jon B]

Originally Posted by oldie44

I looked into this a few months ago for a guy who wanted to use a sporto R&P in his race car.
The 905 differential will fit into an aluminum case 901 housing with no problem. The bearing size and distance between the bearing seats is the same as for a 901.
However, the 905 pinion shaft is 7mm shorter than a 901/914. As a result, the splines on the end of the shaft only engage the 1st/rev guide by about 2mm. In my opinion, that is not enough.

If I recall correctly, you cannot mount a 905 ring gear on a 901 differential, and a 901 ring gear will not register properly on a 905 differential.
I don't recall all the specific differences, but for practical purposes the two differential types are not interchangeable.

[oldie44]

When I referred to a 905 differential, I was talking about the entire 905 differential carrier and ring gear set. It will fit into a 901 aluminum case.
The 905 ring gear has a smaller diameter than the 901 ring gear and will not fit on a 901 differential carrier.

[Jon B]

I don't think that John is planning to use a 905 ring & pinion in his transmission. He was asking if he could use a 905 differential in a "901 gearbox build", and if the differentials are the same.

Originally Posted by Fishcop

Can I use an early sportomatic (905) differential in a 901 gearbox build? Are they the same animal?

For practical purposes, and for what I think he's trying to do, they are not the same and not directly interchangeable.

[Fishcop]

Originally Posted by Jon B

I don't think that John is planning to use a 905 ring & pinion in his transmission. He was asking if he could use a 905 differential in a "901 gearbox build", and if the differentials are the same.

Yes, that's it. I was trying to achieve a result using the boxes I have available - but it's clear I'll need to take a different path.

I need a late aluminum or early mag box with what I suspect is the simplified differential to use the 108mm drive flanges. Or at the very least, a differential from one of these boxes. I could custom some drive flanges, but I think that's getting carried away.

If I was keeping the car I'd use a Guard LSD and specify flanges.

Thank you all very much for the advice and diagrams -very helpful, and has probably saved me a day pulling things down