Common 12-bolt failures

For those of you that've broken Moser, Strange or KTRE 12-bolts, what specifically failed. I know they are considerably stronger than 10-bolts but weaker than Ford 9" and even 8.8" rearends but I broke mine at the track Friday night on a 1.64 60ft launch with a little over 400rwhp. I haven't had a chance to take it apart and see how much damage there is or what specifically failed, I'm just curious whats commonly goes wrong with them. I have 33 spline bolt-in axles, a girdle, Eaton HD posi and Richmond 4.10s.

what "broke" in your rear end? a 12 bolt should take a LOT more abuse than that.

GM 12-bolts and Ford 8.8" rears are extremely similar. It's rumored that Ford copied GM's design. I don't have firsthand experience here, but I'm told that their breaking points are pretty much the same as well.

Both setups are generally known to withstand more than 700hp, with 60' times well below 1.5 seconds.

Not knowing what broke in yours, it's hard to really speculate, but I'd be curious to know if it wasn't assembled correctly.

The only failure I've ever seen in a 12-bolt was one that burned up the clutch discs at about the 750HP level. But that was a "standard" posi unit, with the 9-disc pack, not the HD posi with the 14-disc pack.

Eaton posi destruction happened to a local 6 speed guy last week. His first good launch with new slicks and nitrous...:) BOOOM

I've heard of the spider gears failing and wearing out of the clutches in the Eatons...I wore out my Eaton and went with a spool, no clutches, no spiders...

--Alan

GM 12-bolts and Ford 8.8" rears are extremely similar. It's rumored that Ford copied GM's design. I don't have firsthand experience here, but I'm told that their breaking points are pretty much the same as well.


You are right in a way. I worked part time at a dyno shop for a while doing gear changes, mostly on mustangs with 8.8s. The 12-bolt has a ring gear with a slightly larger outside diameter but also a larger inside diameter so the teeth a considerably shorter than an 8.8thus less gear contact so thats why I think they'd be slightly weaker. The rest of the diffs are the same. When I rebuilt my 12-bolt I used a Ford 8.8" bearing kit. Hey, it was free and there were plenty lying around the shop:D

As for what broke, I'll take the cover off tomorrow, all I know is I have excessive backlash. I can turn the driveshaft about a 1/4 turn eachway before I get gear contact. I drove home at 40mph from the track that way but it was unbearably loud.

Sorry to hear the bad news Grease. There was a guy from Cleveland who had a video of him taking all the teeth off the ring gear during a launch. I was going to buy a 12 bolt for my car, but I'm feeling better everyday about having these 9" parts lying around my room. I have read about alot of 12 bolt failures on LS1Tech.

Sad part is Darkhorse and Handyman beat their stockers to death with no problems...

-Dustin-

As it turns out, I sheared the side bearing boss right off the posi housing. I heard that this was a weak point with the 12-bolts but I never heard of someone breaking one. Its amazing that I didn't break an axle or worse. Once I took my rear cover off I could pull my ring gear back about 1/4 of an inch. Its a shame too because I just spent 200$ rebuilding the posi with carbon fiber clutches.

Anyhow, I went to a full spool and I must say its not as bad as everyone thinks on the street. I drove around most of the afternoon and would only notice it if I had the steering wheel turn full lock. Even then, its not really annoying. Plus I probably saved about 15lbs in rotating mass.

As it turns out, I sheared the side bearing boss right off the posi housing. I heard that this was a weak point with the 12-bolts but I never heard of someone breaking one. Its amazing that I didn't break an axle or worse. Once I took my rear cover off I could pull my ring gear back about 1/4 of an inch. Its a shame too because I just spent 200$ rebuilding the posi with carbon fiber clutches.

Anyhow, I went to a full spool and I must say its not as bad as everyone thinks on the street. I drove around most of the afternoon and would only notice it if I had the steering wheel turn full lock. Even then, its not really annoying. Plus I probably saved about 15lbs in rotating mass.


Imho the spool is a good call...I just went with one a month ago and I love it...I hardly know its there with the 275 60 R15 M/T radials back there, and you know for certain that both tires are pulling when you nail the throttle...I am wearing out these new M/Ts a lot faster...Oh well...I'll deal with that...

--Alan

WOW glad to see that didn't take out the rear, you could have also gone with a Detroit Locker and gotten similar results, I've heard that spools will eventually break axles. I hope I don't have similar results with my Strange Dana 60.

As it turns out, I sheared the side bearing boss right off the posi housing. I heard that this was a weak point with the 12-bolts but I never heard of someone breaking one. Its amazing that I didn't break an axle or worse. Once I took my rear cover off I could pull my ring gear back about 1/4 of an inch. Its a shame too because I just spent 200$ rebuilding the posi with carbon fiber clutches.

Anyhow, I went to a full spool and I must say its not as bad as everyone thinks on the street. I drove around most of the afternoon and would only notice it if I had the steering wheel turn full lock. Even then, its not really annoying. Plus I probably saved about 15lbs in rotating mass.

Wow, that's screwed up. I was thinking about a spool in the 9" I'm building, but wasnt sure if I should do it. Hearing you and ABA383 talk about it not being so bad makes me feel better about going that route.

Good luck and maybe I'll catch you the next time I'm home.

-Dustin-

so, kind of off topic, but not really:
on spools, why is it that people say they are "not street friendly"?
is it just that they cant stand the whining noise when turning? Ive heard about it before, but never had to experience anything.

so, kind of off topic, but not really:
on spools, why is it that people say they are "not street friendly"?
is it just that they cant stand the whining noise when turning? Ive heard about it before, but never had to experience anything.

A spool locks the left and rear wheel together 100% of the time. This means they always spin at exactly the same speed.

When you're turning corners, one wheel has to go farther than the other, which would generally involve spinning faster. A spool won't allow that difference like a regular differential will. As a result, one of the tires will hop or slide. This is particularly non-street-friendly in wet weather or other slippery conditions, as it makes the rear end very likely to kick out.

A spool locks the left and rear wheel together 100% of the time. This means they always spin at exactly the same speed.

When you're turning corners, one wheel has to go farther than the other, which would generally involve spinning faster. A spool won't allow that difference like a regular differential will. As a result, one of the tires will hop or slide. This is particularly non-street-friendly in wet weather or other slippery conditions, as it makes the rear end very likely to kick out.

alright, so in a summer only car it really wouldnt matter too much. Good thing im about to buy a winter car haha.

alright, so in a summer only car it really wouldnt matter too much. Good thing im about to buy a winter car haha.

I suppose I wasn't clear enough.

If you drive your car more on the street than you do at the strip, get a posi or a locker. You should only put a spool on a strip-only car.

Wow, that's screwed up. I was thinking about a spool in the 9" I'm building, but wasnt sure if I should do it. Hearing you and ABA383 talk about it not being so bad makes me feel better about going that route.

Good luck and maybe I'll catch you the next time I'm home.

-Dustin-

Yeah Dustin its not bad at all with the M/T radials...I can't tell that theres a spool back there at all...I have noticed that the spool (for obvious reasons) wears out the tires faster, but I'm ok with that for now..I also don't DD the car anymore, maybe 4-5000 miles a year so the spool worked for my needs...For DD and in the bad weather I would not use one...Also I think the 33 splined Strange axles should hold up to the spool...

Its nice to know that both tires are getting hit equally when the power is applied, this is something I noticed at my last track outing where both tires were not spinning equally all the time and it hurt my launch...Having a spool back there now is just one less factor to worry about...

--Alan