Broken Transmission...again
 

Hey Everybody,

I give up and need some help on this one. I have now broken and replaced the NP 435 transmission in 78 F250 3 times. Took it out and welded the cast iron once, replaced the housing and now broke it again. No offroading, no racing, not even towing. In fact, the last transmission with a brand new housing (used obviously) lasted 3 weeks. I had a bad u joint that was causing vibration that I replaced but it doesn't seem like a vibration would break cast iron, three times. Anyway, the vibration is gone. The tranny and the transfer case are newly rebuilt and the engine is less than two years old. The truck is straight and I've done a lot of work on it. It's in great shape, other than the fact that it goes through transmissions like candy. Any questions, any advice???

Much appreciated.

Where is it breaking? Has it been the same spot?

Yes, same spot. It is breaking vertically from the top on the drivers side about two inches away from where the married transfer case meets the tranny toward the front of the car.

Is this where it's cracking?

Man, breaking a case is crazy.
Is the transfer case mounted using the rubber or is it metal to metal?

Heh, heh, Marc, you're gettin' ahead of me:-X04

That's exactly where it is breaking. I have purchased brand new (not used) mounts, poly for the ones that are sold, for all the mounts for the transmission and transfer case and installed them the last time i put the new tranny in.

This is strictly a guess.
I take it you're using the same tailshaft adapter for the transfere case.
Maybe, and this is strictly a wild a$$ guess, the front face isn't machined correctly and isn't "square", stressing the case along the mold line (visible in the picture) when you bolt it down.

Mike, yes this is a good guess and I am using the same tailshaft adapter. I'll take a peek. Makes sense to me.

Thanks a ton.

Are you using the Clutch? Haha Just messing with ya
How old is the brand NEW one?
Are ya Reving it High?

Nope, not especially high, it's my daily driver. Definitely drive it long distances but have put the work in to be able to do so. The tranny mounts and mate points are all flat, yanked it today. Is it possible the damn thing broke from drive shaft vibration from a crappy u joint mating the rear diff to the driveshaft? Seems pretty implausible.

Anybody else? Still at a loss here. Nobody around my area can figure it out either.

Are you still having some vibration issues? I thought you said that was cured before this latest?

If severe enough .... might be. Depends .... but that TC is heavy and hangs off that extension and the only mount on the TC is as much to limit torque reaction in low rangees under load. There's a trans mount to support the weight and then two engine mounts.

It's not so much vibration breaking stuff as it is the oscilations of heavy parts caused by vibration.

I don't know how bad your vibration is .... if you still have one, so I'm just speculating.

tranny issue
 

hey i had this problem too when i bought my truck from the previous owner,
now i know this sounds stupid but... first make sure that the yokes on the drive shaft are lined up that could be why it took out that u joint,

when they're running twisted a spot out on the front spline the u-joints start to work agaist each other sending vibration through every thing making stuff come apart. mine spit out the drive shaft shatterd the tranny to t-case adapter and cracked the tranny just like yours.

now if that doesn't help another option is to have your drive shaft balanced to make sure its not bent causeing more vibration to go through the drive train, a machine shop could probebly help ya with that,

anyway hope this helped:-drink

tranny issue
 

oh and if your dding it and want a liitle more usable first gear try a t-19 out of a 80's diesel it not a five speed but its geare a little higher in first and second bolts in if you get the t 18 tranny to t case adapter and works like a charm