Transfer case woes...HELP

[Smallz72project]

So I have a 99 GMC Sierra 1500 extended cab short box its lifted 9 inches. 3 inch body and 6 inch suspension. Shift kit, beefed up motor, and 4.56 gears with a locker running 35-12.50-20s. I keep tearing the transfer case out of it. This is the third time in 2 years that the tcase has gone to crap. It cracks around the adaptor plate. This time its really bad though...cracked completely around and about ready to fall out. Anyone ever experienced this or have some solutions on how to strengthen it. Believe its happening due to natural vibrations that come with driving a vehicle. Second ? is how do I tell what tcase is in it, did they have multiple options or these years only came with one option for tcase. Thanks in advance, I am really getting tired of fixing this all the time. Keep on truckin

[skchevota]

Multiple options on the t- case, depending if you have auto track or not. The will be a red rould tag on the back of the t case that will say the modle such as "NP 242" .
As for the cracking, Do you wheel it hard? are you running it in 4x4 on dry pavment? Is the U Joint angle maxed out, meaning is it almose bound up when at ride hight so that is the suspention un loads a bit the yokes will toutch as the shaft is spinning. To ckeck this, disconect the shaft from the rear end and see how much lower you can drop it before the upper joint binds as you spin the shaft. If it is only an couple of inches that is not enough.

[special-K]

i`m not sure why this would happen.I expexted to see your case was breaking at the output and was prepared to suggest a slip-yoke eliminator.I`d expect the tail to break before the input.It`s a rigid union there.Chevota`s suggestions are the best I can think of.Or,the mount bushings are bad?Is the t-case dropped?Were either drive shafts lengthened?

[Smallz72project]

Its a NP 246. And no I dont wheel it hard is the part. I very rarely need to run it hard when wheeling....but that aside it does it when I have to start running on the highway for any amount of time. I dont use the truck for trips but have had to every once in awhile and thats when I start noticing that its dripping atf and then just keeps getting worse, and clearly the 4:56 gears need to go for one.

Drive line angle is fine, personally think the rear could be lengthen'd maybe an inch. It stays in 2Hi unless I absolutely need 4wd. I pulled the tcase the other day and its actually the adaptor thats shattered. But last time I had a shop look at it they had said that the case half was cracked as well and needed a bearing and the chain replaced....I split the case halfs and have yet to find a crack although agree that it needs a rebuild. I am going to try and fab a braket to help stabalize the rear of the case since the adaptor is what holds the weight of the case and mounts the rear of the trans. Basically thinking I need a stronger adaptor from the OEM cast aluminum. I called Advanced Adaptors, and they said that they didnt have ANYTHING!!! for my truck to fit a 4L60E to a 246. So thats pretty much where Im at with it...any help or suggestions are greatly appreciated. Im tired of paying someone else to just fix the broken part and not figure out why its braking...so Im just gunna do it myself. This is the third time!!!

[skchevota]

If this is the third time, there is some sort of binding issue going on here, check the shaft very carfully. I like the Idea of a slip-yoke eliminator, with a double cardanan upper u-joint on the new shaft. That would eliminate any ange issues and remove 8" or so of leverage from the back of the t- case.

[justcuz]

Does it have a metal rod that goes from the t-case to the back of the transmission? I agree that a slip yoke eliminator may be your best bet here. You have some kind of leverage or harmonic set up by the rear driveline that is leveraging the t-case and cracking the adaptor. Is you rear drivshaft balanced? Is it dented or bent slightly? All of these things will set up damaging vibrations and take out the weakest link. if you stay with the slip yoke I would lube the splines on the output shaft and inside the yoke with grease and double check your u joints and make sure they are not bad.

[tallhoedude]

man i beat the hell out of my tahoe,on and off the pavement with the same trans and t/case.i never broke a case,had 373,456,and then 513's.i went thru trannys every time i went out but never a t/case.one time i did break the end of the trans case off,just ahead of where you did.one of my engine mounts was cracked and i guess allowing to omuch flex,double check engine and trans mounts.
triple check your driveshafts too,even if theyre not too long,if they bind cause theres not enough flex it could stress the case.
i work in a gm dealer and have not seen ore than one or two trucks ever break a case,and a broken mount or frozen up u joint was the cause.