So I broke my diff on the dragstrip. Clean break at the notch.
4:11 torsen from an NA in a v6 powered NB.
I have a friend with a TIG. I have aluminum plate.
Can I tig the arm back on, then plate the whole diff arm on both sides to repair?
Is there something else I can do to prevent this from happening again? Solid bushings? A truss assembly to triangulate things from the centersection to the mount bushing?
Give me thoughts, feedback, and options please.
20191028_150204 by Michael Crawford, on Flickr
Titanium 8 bolt plate. Operation takes about two hours. Put the diff in a sling for a week to immobilize it. Should heal in 6-10weeks.
At least that's my recent experience. 
moxnix
HalfDork
10/28/19 4:48 p.m.
Once you weld it up and reinforce it you should not have problems with it again. With an unbroken one you can install the K-miata Differential Arm Reinforcement Plates if you are a bolt on guy or weld it up if you happen to have a welder laying around.
With that much of a break, I would almost say get an empty diff casing from the yard. It's going to have a weak spot in the metal from the welding and there is a greater chance of it breaking again. Then with the new one, do the reinforcement of the truss there.
Here is what Treasure coast has been selling for $175
just out of curiosity, what diff bushings do you have in there?
Diff bushings are stock. Front is no longer hooked to a ppf, but to a heim joint to the frame.
I was concerned about the chance of failure at the weld. Would boxing the i beam with flat plate restore the strength of the casting? Or am i overthinking it again?
It was designed to break there. That was part of the crumple zone in an NA/NB miata. That took some of the energy in the event of a rear impact. Notice that there is a notch there?
I've seen this a lot with hp HP high traction miata. Basically the shock from the launch causes the diff movement. So in mine, I have the Delrin Bushings and the reinforced plate there. With less HP than you and a tire that is not as grippy as what you ran, I'll have no issues. I learned this lesson a long time ago when I broke my first diff casing.
RX-7 diff housings (they look like uteruses to me, dangit) don't have the notch because the diff wasn't directly attached to the engine with a PPF. (Notice the notch is on the PPF side)
So you find a '86-91 RX-7 rear and use its part. The bushings have to be changed to Miata bushings, I forget exactly why but the RX-7 bushings won't work in the Miata. This only works if you are using the 7" rear (1.8l).
Or, $175 for a reinforced one is a screamin' deal considering what it costs to find an RX-7 part and then modify it to work.
In reply to Knurled. :
Have any?
Also, evan said you may have a temporary chaining solution for me to enjoy the last few days of fall with the top down ....
Dusterbd13-michael said:
In reply to Knurled. :
Have any?
Also, evan said you may have a temporary chaining solution for me to enjoy the last few days of fall with the top down ....
Yeah, let me scrounge pics of that. I was driving his car and I put it on the lift the day before I was to leave for a two day divisional rallycross in it and saw that the rear end was practically dragging on the ground. I ran a chain through the PPF and up over the subframe. Kind of hard to snake the chain between the fuel tank and the subframe but I got it to work. The hardest part was keeping shut up about it in case it faield tech inspection.
Then I blew the drain plug off the oil pan on the second to last run of the weekend, which is another story altogether.
Four days later.
Week later:
So that was 2015... the diff housing lasted four years before it broke again (this summer). Not too bad, really.
amg_rx7
SuperDork
10/28/19 7:32 p.m.
That is a common break in Spec Miata from idiots that "bump draft" too hard. Here's the reinforcement plates.
https://www.miatacage.com/differential-braces
If you need a spare broken diff I have one sitting around. I also have a RX7 diff from an 87 GTU.
You can ignore what I said about welding cast stuff if you use those reinforcement plates- they're huge and overkill enough that it won't be an issue. I still maintain that just welding the break back together would be silly, though.
I say plate it and weld it.
For now, you could drill some holes through the top on either side of the break and put a big plate on the top and just bolt it back together.
I called one of the local Miata Parts guys today and was able to Source an unbroken housing with fresh bushings for a very reasonable price. I got to figure out how to press the bushings out before I plate it and install it. Is there anything else I should do while it's out and apart other than replace the seals and gaskets?
In reply to Andy Neuman :
Is it an 88 GTU diff? If so, it has the clutch LSD in it- they're fairly sought after. If it was an '87, I think it was the "Sport" model in that year.
Dusterbd13-michael said:
I called one of the local Miata Parts guys today and was able to Source an unbroken housing with fresh bushings for a very reasonable price. I got to figure out how to press the bushings out before I plate it and install it. Is there anything else I should do while it's out and apart other than replace the seals and gaskets?
There's not much else to do, really. Be careful not to gouge the housing if you replace the axle seals, it's soft. I put a pretty good groove in mine but was able to get it to seal with some RTV (so far). Maybe replace the breather on the diff too if it feels like it's sticking, they're cheap.
I've had good luck driving the bushings in with a dead blow once you get them started if you're using OEM style. Maybe consider poly though, you'll get more NVH but it will help your wheelhop issue.
Brett_Murphy said:
In reply to Andy Neuman :
Is it an 88 GTU diff? If so, it has the clutch LSD in it- they're fairly sought after. If it was an '87, I think it was the "Sport" model in that year.
I have an 87 GXL(still should be the LSD) for some reason GTU sticks in my head. Replacing it with a turbo 2 unit.
In reply to Andy Neuman :
The trick with GXLs is limited slips were not in automatic cars. In fact I don't think any automatic got a limited slip.
In reply to Knurled. :
Good it was a manual, I'll put it on the shelf.
Knurled. said:
In reply to Andy Neuman :
The trick with GXLs is limited slips were not in automatic cars. In fact I don't think any automatic got a limited slip.
Strangely, some did actually.
I had a 3.9 lsd in the roller I sold to furiousE on here. 3.9 only came in autos.
So, since necrobump from canoe
Bought another housing, had it plated. Destroyed it and the diff in spring.
Just got my 3.63 housing back from the welders being plated. If i kill diff number 3, im really going to be pissed.
The root cause is likely a clutch that doesn't slip enough. Excess torque capacity in a clutch not only increases the clutch's potential to inflict damage on your drivetrain, but it will also cost you ET on the dragstrip. Not a situation where too much is just right.
Grant
