littel trani things you did not want to know

[addictionMS]

I have been messing around now for quite some time trying to put together a trani for my race car with a 7:31 R&P. I picked up an aluminum cased trani #74F03232 so I could get the better input shaft, and thinking I wanted the aluminum (AL) case. This trani came with a 8:31 open diff, so I knew I was not going to use that.

I found a early trani #7335282 which had the 7:31 I was looking for but it was in a mag case.

SO I set about swapping the parts from one to another and selecting the best gear raitios I could get out of the two.

I also got a Quaife torque biasing diff for the race trani, fine spline, since I am running the later 930 style stub axles.

After pulling apart both boxes, and sorting the gears, I started to think about assembly and asked the question,

quaife or locked rear end

So locked rear end, Mag case, 7:31 it would be....

[addictionMS]

Funny thing happend, the input shafts are quite different, sorry I did not take any pictures, but I knew I wanted the one out of the later trani so I stacked the gear set I was not going to use on the older input shaft, and it all looked good, but the nut seemed to go a little further in than I had seen before.

Never mind, off to build mine, newer input shaft, with the gear set and bearings from the mag case. Huh, the nut does not have enough thread to get to the detent to lock the nut on.

Must be in the bearings because I can see thet the bearing for the mag case is taller than the AL case one. Bummer.

SO now I have to choose between the tow input shafts, and the two cases, the MAG case we choice #1, but I figure the input shaft is a bigger issue, so I will build the mag case into a regular trani and the al into the race trani.

[addictionMS]

press off the two bearings, one on each shaft, and swap, still not quite right, but better, the al input shaft looks normal, and the only difference is the mag case input shaft still has a little less thread than I would like, but onward, everything is press on thight, checked the stack twice so it is right.

While all this was going on, I wanted to get my diffs welded up, so I took the least desirable one, the one ou of the mag case, and did some test welding, worked great so I welded up a spare fine spline one I had. Ready to go. Still had a fine spline one for the non race trani.

So today together goes the non race trani, mag case, original input shaft, close to the same gears (only swapped second). but now with the 8:31 pinion.

[addictionMS]

everything goes together great, check the depth of the pinion, seems fine, will check later again.

get the thing all buttoned up and ready to put i the diff...but wait a minute...the fine spline diff does not go in fight, seems the distance between the bearings is different.

Seems the spacers must be different, so I pull off the bearings on the fine spine diff, and on the original mag diff (now welded into a locked rear end) and that is where I am now stuck.

It seems the two differentials are different in some fundamental ways. The al fine splined diff will never go into the mag case, and the mag case diff I have is now weled into locker, not good for the street.


Anyone have a diff out of a mag case 915 or any good suggestions....

Jim

[addictionMS]

[wevoid]

Jim,

The bearing spacing in the two final drive housings is the same.
The early differential you have does not have provision for the magnet ring for the later electronic speedo.
As a result the bearing mount width on the early differential is 6mm (nominal) wider than the later version.
You need to add 6mm of differential shim to the later (electronic speedo) differential to make it fit in the mag housing.

Make sense?

Regards

Hayden

[addictionMS]

OK, so I can see how I can get the bearings to be at the same place, the end to end measurements of the diffs are the same, and some careful combinations of spacers and I have got it.

But how do I overcome the difference is where the ring will be see the photo below



you can clearly see a huge (relative to what I could compensate for with the shims) difference in where the flanges are which the ring bolts onto. The diff without the ring is the mag case one and the one with the ring is the al case one.

I checked the ring off the mag case and it is the same as the one in the picture.

I find it hard to belive the cases have the pinion in different places, they have to be centered, so how is the difference possible?

Only thing I can think of is the the ring needs to be spaced differently because I am using a 7 pinion not the 8 pinion?

to get a smooth flat surface they are sitting on a slab of granite, and it may look like they are sitting on the spacers, but they are not. I have them on there loose so they don't get mixed up.

any and all ideas welcome.

Jim

[addictionMS]

bumping looking for help

[TimT]

see my post on the racing board LOL

[addictionMS]

there is a conclusion to this on the racing board now...at last

the other thread on the racing board

again thanks to all who helped, I now truely know more aobut 915s than I thought there was to know

[addictionMS]

finally resolved...