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Old 04-11-2010, 02:59 PM
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TH350 Keeps cracking

I have replaced the tranny in my Nova twice now and this is the second time that I crack the housing on the transmission. I am getting ready to install a TH400. You guys think that the TH400 will give me these kind of problems?
Thanks,
JD

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Old 04-11-2010, 03:11 PM
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What are you using for motor/ trans mounts? Sounds like a mismatch. I had the same thing happen to me when someone put a solid trans mount in with old motor mounts.
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Old 04-12-2010, 02:43 PM
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Quote:
Originally Posted by 355Nova
I have replaced the tranny in my Nova twice now and this is the second time that I crack the housing on the transmission. I am getting ready to install a TH400. You guys think that the TH400 will give me these kind of problems?
Thanks,
JD
Either you have an engine that produces an amazing amount of power, or something isn't lining up correctly. What is the motor and is it a replacement from the original.

These problems can originate from mounts that bind things up as has already been mentioned. In addition the trans bell housing or the engine's bellhousing mount are not lining up which puts a bending moment on everything. Or the flexplate isn't straight with the crank, torque converter, and tranny input shaft. Or the converter's internals aren't on center. Or the input bearing on the tranny shaft are shot letting the shaft get off center, though that usually comes with a lot of oil leakage.

Before I'd hook up another tranny, I'd sure want to know that these alignments were proper, if that means taking the motor out to do these checks, do so.

Bogie
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Old 04-12-2010, 03:15 PM
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An out-of-balance driveshaft will break bellhousings also. So will chassis flex in a unibody car like a Nova or Camaro.

If things aren't right you will break the 400 just as easily, the bellhousing are is no stronger.
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Old 04-12-2010, 08:26 PM
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Well my engine is a 350 bored over .30 The engine has 10.5:1 pistons, new forged rods and stock crank that was redone. The engine was heavily customized to be similar to the ZZ4 roller style engine. I have fully ported LT1 aluminum heads from a 93 vet. The water ports were welded and redrilled for GENI block. Heads are also gasket matched with a holley 300-36 high rise dual plane. Rockers are 1.7 Rockers, and gasket is .026 My cam is a ZZ4 cam 208/221 - .474/.510 Engine has shorty ceramic coated LT1 headers. Everything is electric except power steering and alternator of course.

Torque peaks at around 463 trq around 4100rpm

It's a fresh build with only about maybe 40 miles. The tranny in the car only cracked when I decided to see if passing gear worked. Didn't hear it but noticed oil leaking.

Second tranny was a 350 from a chevelle same year and it cracked when I decided to see if passing gear worked.

I don't have solid mounts and the torque converter was replaced and mount also was replaced when I installed the second tranny.

Originally I had replaced the drive shaft with a new one and I thought it was that. So when I installed the second transmission I put the original back in it, and it still cracked.

Any advice would helps
thanks guys
jd
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Old 04-12-2010, 08:33 PM
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tranny

Where is the tranny cracking at?
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Old 04-12-2010, 08:42 PM
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What is the condition of the motor mounts and the front subframe-to-body mounts?? Something is allowing too much movement somewhere or something is out of alignment, HP alone does not break cases until you get to more than double the hp levels that you are seeing.
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Old 04-12-2010, 10:54 PM
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cracks towards the center

It cracks just above the tail, towards the center of the tranny. The motor mounts on the car are from summit and they are suppose to be some of the best ones. They are new. Maybe subframe? I haven't beefed the subframe up yet. You guys think I should start there?
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Old 04-12-2010, 11:57 PM
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Quote:
Originally Posted by 355Nova
It cracks just above the tail, towards the center of the tranny. The motor mounts on the car are from summit and they are suppose to be some of the best ones. They are new. Maybe subframe? I haven't beefed the subframe up yet. You guys think I should start there?
This statement leads me to the driveshaft. Just because it is a factory shaft don't think that it is automatically good, factory shafts are rather poorly made, thin material and not very true as far as runout. Make sure it isn't too long or too short also, 7.5" and 8.5" rears take different length shafts. Chassis flex could be the problem too. Worn out leaf springs will allow too much axle wrap and that can cause problems also.
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Old 04-13-2010, 05:19 PM
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Quote:
Originally Posted by 355Nova
The motor mounts on the car are from summit and they are suppose to be some of the best ones. They are new.
May be the motor mounts. They are so rigid, it is like having solid motor mounts. I bought a set of the Energy Suspension ones for my 32 3W ... ( with a Chevy ).



They were so rigid ... I felt every little shake and vibration in the 430 HP engine. I got tired of that and installed a new set from the Chevrolet dealer.

Much better ...
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Old 04-13-2010, 08:21 PM
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Subframe

How can I strenghten the subframe? I'm going to do that first before I install the other tranny.
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Old 04-13-2010, 08:28 PM
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What condition are the mounts that mount the subframe to the body?? What specific year car are we talking here?? It is not usually subframe flex but flex where it attaches to the body. I have solid aluminum mounts here on my Nova, you don't want the subrame to move different from the body.

Subframe connectors are almost mandatory on a unibody car with any power to connect the front and rear of the car, do you have them??
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Old 04-13-2010, 08:39 PM
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if your driveshaft is too long the slip yoke may be bottoming out all the way in when you hit a bump or nail it and the car squats down, this would put a ton of stress on the transmission, between that and the fact that you dont have subframe connectors it could easily see the trans seeing alot of extra stress.
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Old 04-13-2010, 10:47 PM
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mounts

subframe mounts are bad but just picked some up. I have a 1970 nova. I don't have the suspension / subframe re-enforced
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Old 04-14-2010, 06:23 PM
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check your tranny and pinion angle. if the angles are all wrong they will break your tranny all the time.
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