What the Diff??
 

I've never had a problem with my differential and I don't think anyone else has either for its some 29 odd years of its life. But out of the blue yesterday while pulling out of the garage I felt some loud clunks coming from the underside of the car and it was just while turning the steering wheel. After those two or three clunks while leaving the garage it went away. Granted, I didn't have to do any sharp 90 degree turns after that until I got to where I was going. Had dinner with my girlfriend and a couple of co-workers and everything was going great until I went to leave the parking lot.

CLUNK CLUNK POP! I heard metal pieces fall to the ground as I was trying to navigate through this parking lot. I rapidly fixed my focus to my oil pressure gauge and stared at it. To my relief I maintained a healthy 60PSI. I get out of my car to see a puddle of diff fluid behind my rear rears and along side that fluid four solid pieces of my diff rear housing.
https://cimg1.ibsrv.net/gimg/www.mia...98571837e.jpeg

I didn't know which would be more embarrassing to me being with my girlfriend, having to get my Miata towed home again or hearing those gears grind one last time before I completely destroyed them. Turns out I sported both of those options that evening. After 12 miles or so the gears had ground down so badly that power was no longer being sent to the rear wheels while just a mere quarter mile away from home. Every time I would hear the diff gears grind I would see my girlfriend cringe. It's not a pretty sound to say the least so I don't blame her. Sad day. My girlfriend officially hates my Miata. Thinks that it's time I "grow up" and ditch this project. I'd sooner see her gone than my Miata to be honest; especially with that attitude.

Anyways, it was time to upgrade my open differential. I had been meaning to for a while now. It's just a little more urgent now to find a proper Torsen LSD.
What do you guys think that would cause that clunking noise out of nowhere and eventually shatter the housing of a differential? I cannot think of anything honestly.

It's a 1.6 diff, so existing probably did it in. They are notoriously weak

Yeah I get that being a 1.6 diff on its own was a big enough crime but I'm just curious on what changes from 30 years of steady operation to detonation in one day. Basically, what physically changed to cause it to explode like that?

The only guess I have (and it's just that, a guess) is that it was already weakened, maybe from the drain plug being overtightened and causing a small crack to form (since the break leads to that area)

And then MAYBE the vent was blocked and caused it to pressurise and bust? (really unlikely though, doubt it can even build any decent pressure, and also it would leak past the axle seals long before it would bust out a housing)


I assume it IS an open diff, and a previous owner hasnt welded it? (again though, usually that will bust a CV or strip a spline before it blows a housing apart)


It's a weird break for sure, all the dead MX5 diffs i've seen have been either a snapped moustache bar, or just stripped pinions.
Not seen a housing give way.

On my old NA I had my diff explode randomly kinda like yours. I would think what happend was a piece of the pinion gear broke off, and the ring gear carried it around until it finally blew out the back of the housing.

the broken case is a result of something breaking inside the diff...

You have the right attitude here

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Yep, exactly what I said in the post above you. I bet the pinion gear lost a piece and the ring gear carried it around until it clearanced itself out the back of the housing.