I have seen a few TH700R4/4L60E/4L60 cases split like that. Even seen a few old TH350 & 400 cases do it. Someone made a bellhousing kit for that issue.

LS engine to 700r4/4L60/4L60E
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SBC/Vortech/LT1 reverse cooling flow (93-97)
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When a bell housing goes, even a triple tube drive shaft will self destruct. A buddy of mine got a junk transmission from advance auto-parts. Transmission output shaft locked up on the customer while driving. His 2004 Dodge 2500 4X4 waded up both drive shafts, broke the transfer-case, and came back with a bucket full of lose transmission and transfer-case parts. Truck was in 2WD on Dry interstate doing 65 with 300 miles on the transmission.

As for Stock 10 bolt GM rear ends, I have killed my fair share. Racing with stock gears, carrier and axles is asking for a fail.

the one i broke was a th400, and i drove it for a while to see if it was going to be good. the first time i tried a full rpm/power leave it twisted the shaft and cracked the bell housing. the 400 was milled for a slip yoke so i dont think it would have been alright anyway.

as far as the rear the new one is a ten bolt but its richmond 4.10, auburn posi, and a summit girdle. i think that will be fine until i get big slicks. i have air bags in the springs and adjustable drag shocks

No insult here, yet that is not a wise thing to do. Even HD TH400 cases wityh the 6 bolt tail housings came in slip yoke tail-shaft versions. Same tail-shaft is used for 203, 208 & 205 Transfer case versions. The only difference is the extension housing is put on a shelf when putting the 2WD HD in a 4WD.

The bolt on yoke version uses a different output shaft. Milling splines in to it would weaken it. it also is going to be missing the splines for full suspension drop, and at full compression the yoke could go in TOO far.

Any transmission shop can swap tail shafts in a TH400 reasonably quick & for less then $150. Probably would even drop new frictions and steels in at the same time for that price.

I'm still leaning on bearings and I'll have to shoot a video using my laptop here next weekend. Maybe this will "show" what'* going on.

When the engine is cold, the fuel trims on both banks are pretty close together. Maybe .5 point off, and pretty close to 0. But when the engine starts to warm up, the trims start to go high on both banks, but even higher on the left bank.

if by up you mean richer im thinking you have the standard poppet issues. is it worse or better if you are accelerating?

the shop that did it is a racing trans shop and the guy that built the trans both times, his only job in life is building auto racing transmissions and he is the one who suggested he should do it

I grew up knowing a lot of people who worked for GM, 1 out of 20 wasn't qualified for the work they did. GM kept them anyways, firing smarter and harder working folks.

Bad ideas know no bounds. If a transmission shop convinced me this experiment would work, I would have got a guarantee they would fix it when it came apart.

Another TH400 case and your TH400 guts would make less of a garage ornament.

i used to hang out at the track every week, and aside from making them for pro'*, there were a bunch of guys there that had transmissions made by this place, i think they know what they are doing. after they fixed the cracked case and i got the driveshaft made at fleet pride its been fine for like 5-8 yrs. they made it into a dual manual and auto operation and they made me a custom tq for my application too.

The TH400 is a simple and tough transmission. There a lot of simple mods you can do to make them tougher. I'm betting the bell housing already had a hairline crack before that probably got missed. They do make tough aftermarket bell housings that bolt to the front pump and you cut the stock one off.
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It'* also nice because they make custom bell housings that you can use a GM transmission on say a ford engine.

i have seen those before, i figured if it happened again thats what i would do, but i have beat the crap out of it and so far the only thing that has happened is the governor gear broke. stupid plastic junk. i kinda guessed there might have been a crack too but cant say for sure. i traded my 700 to this guy that has a early 70'* monte 454 and thats what the original one was from