Cracked C4 Case

[Joe Grippo]

Well I had the race car out the other weekend for the first time this year and on the 3rd pass it seems I cracked the transmission case and promptly sprayed type f everywhere. I found a very small crack about 2 inches from the tail shaft on the main case right at the top along the casting seam. I have a front engine plate, stock motor mounts and the trans uses a poly mount. I also have a really beefy JW Ultra Bell. The car wheelstands every run and I have had this set up for about 10 years and have never had a problem. This past off season I did some chassis work and added a full cage, dash bar and bars off the dash bar to the rear of the shock towers. The car leaves hard and square with no twisting. This was a new trans with less than 25 runs on it. So I am wondering if its just a coincidence that the case cracked or do I have to rethink the transmission mounting based on the more rigid chassis? It should not be solid mounted I assume. Or am I missing something else that could have caused this situation? I should add that a mid-plate would probably be a good idea but it's not in the budget this year.


Here is a shakey video of the second pass of the day...

[Bamfster]

I love the way your car leaves Joe!

I broke a 400 case a while back. I was told by my trans guy that it was the solid motor mounts w/the poly trans mount. When I swapped to the 700, I added a solid trans mount and so far (knock on wood) no problems.

[Joe Grippo]

Interesting. I figured having everything mounted solid would not be the way to go but maybe I need to rethink that. Thanks.
Those wheelstands are fun but I will be calming it down some when I get this trans deal straightened out....

[BBR]

Weird. I always thought a motor plate + solid mount was a recipe for busted trans cases. Almost sounds like a freak deal. Was the trans case the same one you have used for 10 yr or was it a different one? If different, it may have had an existing fracture that your wheels up launching exacerbated.

[milner351]

My trans guy is totally emphatic about using stock rubber trans mounts, regardless of what is on the engine - he and lots of his customers run bracket cars in various classes and have good luck with it.

It could have been a stress riser in the case that was just waiting to show itself.... but it wouldn't be the first time a solid or poly mount spelled disaster for a trans case.

Love your car Joe!

[Joe Grippo]

Weird. I always thought a motor plate + solid mount was a recipe for busted trans cases. Almost sounds like a freak deal. Was the trans case the same one you have used for 10 yr or was it a different one? If different, it may have had an existing fracture that your wheels up launching exacerbated.

That is what I always assumed, that it should not be solid mounted. Perhaps a full frame Chevelle requires something different from the unibody Mustang.

This is a case that I have been using for about 2 years. it has a line of casting flash right down the top of the seam. I always wondered if I should have "de-burred" it. I thought about it having an existing flaw that just now decided to rear its head. While I would love that to be the cause it's hard to be sure, that's why I was asking about the mounting.

[Bamfster]

The way it was explained to me, and I know it goes against everything that has ever been written, if the engine and trans are solid mounted and can't flex or move around, it greatly reduces the chance of breaking things .... kinda like attaching the 'cage to the frame vs the floor pan. When you add a non-solid mount to either the trans or engine, it will allow something to flex/move around and finding the weak spot. 8 seasons on the 700 case in the car now, no leaks or cracks ( I know, I just jinxed myself!).

[BBR]

Crack runs parallel to the output shaft? I'd suspect a driveline vibration or bottoming yoke maybe. If the crack went around the case, I'd think it was caused by a twisting force.

[STINEY]

In my opinion even poly mounts are too stiff...... I'd run a stock rubber mount on the trans tailshaft, everything else can be solid but not there.

[Joe Grippo]

Crack runs parallel to the output shaft? I'd suspect a driveline vibration or bottoming yoke maybe. If the crack went around the case, I'd think it was caused by a twisting force.

It appears to be cracked going perpendicular to the center casting seam byt most certainly started at the seam. I'll get some pictures up tonight.

In my opinion even poly mounts are too stiff...... I'd run a stock rubber mount on the trans tailshaft, everything else can be solid but not there.

I don't disagree but I have run the same poly mount for many years with no problems....until now, of course.

[CDMBill]

I have stock engine mounts, a poly trans mount and an adjustable torque link from the driver side cylinder head down to the frame at the anti-roll bar mount. I know I give up some torque transfer efficiency in the name of street manners and keeping the trans case in one piece. I have also stiffened the uni-body every place I can as its a convertable along with adding a trans mount that triangulates the trans tunnel.

With engine and mid-plates I'd go with a stock mount as a unibody Mustang is very difficult to make torsionally ridgid. With what you have now I think I'd try a stock trans mount when the new tail-shaft case goes back in the car. Then you might swap back to the poly mount and use some corrected passes in RaceIQ to see if it makes any difference. For what you are doing I'd definitly plan on going to the midplate when it makes sense. I'd agree with you that you that the tailsahft housing of the new trans may have been weak. A crack on the parting line could easly be caused by tortional twist as form what you describe ist directly above the mount. Do an autospy on the tail shaft housing. I'll bet that crack didn't happen all at once.

On my last trans install I went ahead and deburred and smoothed the all of the AOD's case as well as the Super Bell. To be honest my first motivation was that I cut the crap out of hand on a sharp edge, but the really rough casting of the AOD had always bothered me. I'm smoothing and deburring everything these days. Weight savings you know.

One other thought borrow someone's Go Pro or Flip camera and record a launch while filming the engine with the hood off if you really want to see what is happening. It's fun and educational.

[Joe Grippo]

I'd agree with you that you that the tailsahft housing of the new trans may have been weak. A crack on the parting line could easly be caused by tortional twist as form what you describe ist directly above the mount. Do an autospy on the tail shaft housing. I'll bet that crack didn't happen all at once...

Thanks Bill, lots of good points there. Just to be clear the crack is on the main case not the tail shaft housing and it starts about 2" from the back of the case dead center on the seam, perpendicular to the output shaft. I'll get some pictures up tonight after work.

Thanks for the input everyone...

[Deaf Bob]

Love our Go Pro camera in th derby cars!
I know.. Derby, Drag..2 different animals...still both run trannys...
We do not bolt our tranny down.. We use a front to rear sliding mount that does not move side to side to not break trannys..
Hopefully the floor keeps it from going up!
We also use a cradle that ties in the 2front mounts like the old chevys use to the side mounts most have.. Have not broke a tranny since..

[CDMBill]

Thanks Bill, lots of good points there. Just to be clear the crack is on the main case not the tail shaft housing and it starts about 2" from the back of the case dead center on the seam, perpendicular to the output shaft. I'll get some pictures up tonight after work.

Sorry Joe, I mis-read your post. One other less likely theory: with the added bars from the shock towers to the dash bar and the cage elements it's possible that you closed up the distance between the engine plate and stock mounts and the trans-mount creating a stressed coupling through the transmission to the mount. Now add in the impact and bounce from your awesome wheelies and viola, cracked trans case. Is the trans mount reinforced or stiffened? Are the trans-mount bolts hard against the back of the mount's hole, did the threads leave witness marks? On last thought. When I reinstalled my engine and trans last November after the post DW re-flash of the converter I noticed that the trans output shaft centerline pointed an 1.5" or so to the left of the pinion centerline. I figured it didn't need that kind of misalignment so I adjusted the mount slightly and had to clearance the drive shaft loop a touch to clear the balance weight. I can't tell a difference but I feel better. We used to sweat this stuff on my E/SA car back in the day.

[Huskinhano]

My trans guy is totally emphatic about using stock rubber trans mounts, regardless of what is on the engine - he and lots of his customers run bracket cars in various classes and have good luck with it.

It could have been a stress riser in the case that was just waiting to show itself.... but it wouldn't be the first time a solid or poly mount spelled disaster for a trans case.

Love your car Joe!

I've heard the same. Makes sense to let it float a little bit. Just like the front strut rods. They sell the poly bushing to take the slop out but end up breaking the struts.