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Old February 26th, 2006, 11:53 AM   #1
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Default TH400 case failure

I hear this is a common failure, especially with dual cases....

Here is how I have the setup. the crossmember suports the trans/cases just after the trans (the first crossover bar, you can see the 2 heads of the bolts).



Here is the crack.
Drivers side- the adaptror to the right of the crack and left of the orange silicon is the plate that runs down to the crossmember



pass side



So should I add a second mounting point near the rear of the 300? Maybe on the output shaft housing that crosses over the second part of my crossmember bar
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Last edited by Bones; February 26th, 2006 at 11:59 AM.
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Old February 26th, 2006, 12:03 PM   #2
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if I were you I would add the reinforcing to the bolts and make a drop rod that runs from the bell housing bolts down to support the extra weight. seems like you have so much weight bouncing againest the case it can't absorbe the shock of when you going hard at it. the other thing I see you missing is the torque bars and bottom bell cover, they are there to support the trans from flexing while under load. there should also be a bar on the right side from the bell bolt to the T case to absorbe torque there also. sounds like for the most part you just need to brace everything correctly
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Old February 26th, 2006, 12:22 PM   #3
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Quote:
Originally Posted by Yetti
if I were you I would add the reinforcing to the bolts and make a drop rod that runs from the bell housing bolts down to support the extra weight.
Down to what?

Quote:
there should also be a bar on the right side from the bell bolt to the T case to absorbe torque there also.
not sure that I have ever seen that... any pics of what it looks like?


Quote:
the other thing I see you missing is the torque bars and bottom bell cover
If you're taking about the bars that bolt in behind my motor mounts, then yes, I am not running them. I originally welded in my mounts with a carbed 350 that didn't have those. When I swapped to a TBI motor, I tried to put them in, but then my mounts didn't line up (pusshed the motor mounts outboard too far).

It seems to me that it is bouncing as you said. I think the wieght of the dual cases past my mounting bolts caused the failure, thats why I think by making a second mount back on the D300 would stop the bouncing....
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Old February 26th, 2006, 01:10 PM   #4
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the second mount will help, but you are still going to have some serious flex. I have an idea do you know what a walking beam is? if so you could make a walking beam style mount that would let both trans mounts move at the same rate and it would elliminate all the flex. the only thing you have to do is make a stronger cross member.

as for the drops on the front mounts the new style ones sandwich behind the mounts and have an ear the drop rods bolt to. you can find some if you look through some of the GM parts books from the late `80's early `90's.
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Old February 26th, 2006, 03:15 PM   #5
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Quote:
Originally Posted by Yetti
the second mount will help, but you are still going to have some serious flex. I have an idea do you know what a walking beam is? if so you could make a walking beam style mount that would let both trans mounts move at the same rate and it would elliminate all the flex. the only thing you have to do is make a stronger cross member.

hmmm, no idea...
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Old February 26th, 2006, 04:11 PM   #6
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Maybe something additional at the back of the T-case..

This is my setup on my Toyota dual t-cases:

The mounts aren't in yet, but there are rubber body mounts that go between the frame mounts and the arms of the t-case mount.

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Old February 26th, 2006, 06:01 PM   #7
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How do you get your trans out with a crossmember like that?
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Old February 26th, 2006, 07:08 PM   #8
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Talking to me? To get it out, I'd jack up the drivetrain enough to get the arms of the mount clear, then unbolt the whole thing from the t-case, then lower the whole shebang..
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Old February 27th, 2006, 09:52 AM   #9
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Bones... is the t-cases hard mounted with or with out a rubber mount? Is your motor mounted with or with out a rubber mount?
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Old February 27th, 2006, 09:54 AM   #10
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both wear rubbers all the time
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Old February 27th, 2006, 10:55 AM   #11
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Another option would be to consider making some kind of cradle.

Picture the cradle being shaped kinda like a U

One side of the U would support the trans at the stock mounting spot.

The other side of the U would go up and support the transfer case, at a couple of the bottom bolts somewhere.

Then have the middle of the U bolt to a rubber mount on your crossmember.

Shouldn't be to difficult to fab up...
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Old February 27th, 2006, 05:45 PM   #12
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the rubber mounts may be the issue allowing it to flex to much. is your front shaft binding up at full droop? that could cause it to bind and snap the case like that.
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Old February 27th, 2006, 05:49 PM   #13
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i would have to think thats a little more weight bouncing out further than the general really wanted. I would suport it back further.
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Old February 27th, 2006, 05:51 PM   #14
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just noticed are those hocky pucks as mounts? If so those dont flex.
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Old February 27th, 2006, 07:28 PM   #15
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Quote:
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the rubber mounts may be the issue allowing it to flex to much. is your front shaft binding up at full droop? that could cause it to bind and snap the case like that.
nope, neither shaft will bind up.

I agree with user name.... just a lot of weight bouncing on that hill
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