Thought I'd finish this one up because I think people would be interested in how one could weld a diff, and how it affected the car!
warning... welding a diff at all, or doing it this way, may not be the best solution. But it was fun, and effective in sno-drift.
About a year ago I had taken the trans apart to get to the differential, in order to weld it (turn it into a cheap and very heavy spool type). Unfortunately, most transverse engine transmissions require a complete disassembly to get to the differential. In this case, the differential is pretty much the LAST component to remove if you were to make the list.
This was a 'rally' build. Importantly (to me), if you just take it apart, and put the same parts back in, there's no need to worry about shimming anything to get the preload correct, it should all go back together 'rally' functional if that's how it came apart... and it did.
First: disassembly. Matt of Bentmettle helped (and supplied the odd torx I didn't have) Ripped it apart in 30minutes, and I'm thinking 'ooooh no slow down' pretty much the whole time.
Then it sat for...maybe 2 months. But then! Welding time - - the one thing I didn't want was for the welding job to fail, and take out the transmission with it. Of course, as long as the differential is stronger than the halfshafts it should be good. But I really didn't want the diff to fail.
After cleaning as well as I could (pretty poorly) I preheated it...on my old grill.
Got it up towards 450degrees and then went at it with a Hobart Handler 120 on max and flux core wire. I welded the spider gears together, and then used small squares of .25" steel to fill in the gaps, and welded the spider gears to those too. I had plenty of penetration. (hehe...)
Notice that I even left the bearings on... certainly not best practice, but it was effective. I took care to keep the gear but especially the bearing surfaces covered while I was welding to keep them clean. They certainly picked up some heat discoloration though. I really didn't want to have to set bearing preload in this thing...
Finally, I put the trans back together. Used all the old synchros and even most of the old seals. I think I only replaced the output shaft seals, case gaskets, and some of the shift lever seals, but I forget the details. I think it took... two 3hr sessions. It was my first ever trans reassembly, and I learned a lot about how a manual transmission works while doing it. I thought I already knew, but some details, like what really engages the synchros (spring tension working on a ramp, not shift lever effort...) I had no idea.
Thinking back it was interesting to note that the reverse gear was chewed up on the edge from previous owner not adjusting the shift linkage and then not correcting it, but more interesting was that the pinion has somehow lost a small chunk of one of its teeth! Maybe a piece of metal from the chipping reverse gear got in it. But judged both minor, and it did turn out that the trans was fine.
So how'd a FWD with a spool drive?
A little different than I expected. In that, at normal speeds, it does NOT feel like a limited slip. Going around corners of any kind, even with power steering, takes serious steering effort. Much more pronounced than anybody could ever complain about even with a tight and chattery clutch-type diff. The tire traction and scrub just wants to pull the wheel straight, and you can feel the tires grinding. Corner radius depends on inside-vs-outside tire traction, and which one 'wins' the battle it's in with the other one.
And then when you're heading straight, under power, and with variable traction, the car is all over the place! When one wheel has more traction than the other it kicks back through the wheel. But even if you hold the wheel straight, it'll still drive the car to the other direction.
First we rallycrossed the car, and it was pretty terrible. All tight corners, the front end would grind and scrub and push... definitely made the car a much worse rallycross car.
And in sno-drift, it was difficult to turn and pick line the same...until going sideways at speed, then you could really pull the car through with the throttle.
But maybe most importantly for this event, we never got stuck! Which is a lame problem to have, but most 2WD cars had it. Between siped altimax arctics, and the welded diff, I didn't even know that the rest of the field had some serious hill climbing touble. For us, it wasn't a problem.
So I guess the welded diff was a success. Through other means however (tree), it seems we've toasted the trans. I'm not heartbroken, I'll put the spare, open diff 020 back in for now, then later maybe a limited slip in the 02J.