Fixing the 5th Gear Nut
To fix your 5th gear nut you need to remove a few things in order to get to them. This will be done on my 1997 dodge Cummins 12v NV4500 2wd. If you own a 4wd, you will have to take the transfer case off instead of the tailshaft housing. I do not have a 4wd so cannot advise how to do it.
Step 1: Drain the fluid out of the NV4500, this takes some time so you can do the next step while it is draining.
Step 2: Remove the driveshaft starting at the axle.
When removing the U-Joint, make sure the bearings do not fall off. If they do you risk having all the needle bearings fall out and they are an absolute nightmare to get back in place. The 2 red circled ones are the ones you need to be worried about. Tape them so they don’t fall off when you happen to walk by and kick it.
Remove the center bearing also. You must hold the top of the bolt, it isn’t welded or anything like that.
Now you can pull the driveshaft slowly out of the tail shaft, all as one very long shaft.
Step 3: Disconnect speed sensor.
Step 4: The oil should all be drained now so you have to jack the transmission up in order to take the tail shaft support out. So get a jack and jack it up until the trans goes up a tiny bit, just so you know it is being supported. Don’t jack it up any more than that because it is still bolted to the crossmember.
Step 5: Take the bolts out of the bottom of the spacer that goes between the trans and the crossmember. It is the thing I am holding in my hand. There are 2 bolts on the bottom then 2 bolts off to the side. The bolts off to the side should have the nuts welded, but sometimes the weld breaks. You should be able to pull it right out, if it is tight, jack the transmission up a little more. Don’t get too crazy with the jack because it is still connected to the exhaust pipe mount.
Step 6: Take the mount that supports the exhaust off. You should be able to wiggle it off of the exhaust with some time. If not, I’m afraid you will have to take the entire crossmember off.
Step 7: Take all of the bolts out of the tailshaft housing. Be careful not to hit it or anything because more oil will dump out, maybe half a quart. So be prepared when you do break the seal for the oil it will puke.
Step 8: Pull the tailshaft and wipe everything down.
Step 9: Fix the nut. There are many different methods that have been tried and as new ones seem to come up and as old methods are being tried and proven or failing I would suggest you google "NV4500 5th gear nut fix" and research a little on your own as this is a very debateable topics as to which fixes work and which cause more problems than they solve.
Here is a picture of what it should look like.
And here is a picture of what it looks like when you have “lost 5th gear.”
Mopar's Notes: Make sure you use a quality sealant like RTV Black during your re-assembly of the tailshaft housing.
Another article to a more permanent repair of the 5th gear nut.