5th gear nut

[azcummins]

I'm going to be dropping my transmission for a clutch install in a couple weeks so I was thinking of going ahead and replacing the 5th gear nut. How hard is the install?

 

[Ross]

It's pretty simple. Took me and a helper about 2 hours. You need two miller specialty tools, one to hold the rear main shaft (splined socket tool), and a special fifth gear wrench that a 1/2" drive fits. You can get by without the splined socket by putting the driveshaft back on, and torqing the fifth gear against the vehicle (rear tires on ground), but the tool really makes it easier.



5th gear went at 72,000 miles, now have 105,000 and ticking. I hope to go another 40K or so before having to do it again. Hopefully the set-screw style nut will work. I've heard good reports, although I've also heard of some failing. I did not replace my fifth gear, even though the moron at the dealer said that you "must change fifth gear. " It's only about 150. 00!!!! The gear was fine, not a mark, so I put it back on. Don't buy another gear until you inspect yours. If your nut hasn't failed, and the gear hasn't been rattling (riding) around on your output shaft, it will be fine.



Since your transmission will be out for the clutch install, it may be a real chore getting 300 ft lbs of torque on the nut (I think that's what the manual called for, but my memory is ..... ). It may be easier to do the gear nut change AFTER you put the transmission back in... . just a thought.

 

[azcummins]

Ross-

My truck has 70k miles on it now and no problems out of the transmission so maybe I'll just let it go. I don't haul anything and when I'm in 5th gear, I'm way above the 1600 rpms that seems to be stressing the nut.

 

[MikeBeggy]

After learning about the nut problem being related to higher power output and using 5th gear for towing, I changed mine at 20k or so. The nut was still tight but the washer had been really beat-up and a pending failure seemed evident. At 105k, I had the trans out for a new clutch and disassembled for inspection. The washer was again beat-up and the mainshaft under the 5th gear splines was really worn from the gear rocking on the shaft. I opted for the fully splined shaft from Standard trans... a really nice design change. Also, the new washer is a split type with a keeper ring that should help handle thrust from the 5th gear. Now I think the trans is up to towing in 5th gear.

 

[Steve St.Laurent]

I had Enterprise engine performance install the 5th gear nut fix when I had them install my clutch at 60,000 miles. I'm now at ~100,000 miles and just lost 5th gear. Here's the funny part - I put 2,000 miles on the truck this weekend towing my 35' TT to Maryland for the big rig nationals, raced friday night and saturday, then towed the trailer home through the tornado storms and 30 mph headwinds (stopped sunday night because the winds were so bad). The trans held through all of that - unhooked the trailer today and ten miles down the road unloaded at 60 mph cruising along (not accelerating or decelerating) all of a sudden I'm in neutral!! I knew instantly what it was! Let me tell you this, 4th gear with 4. 10's SUCKS! I drove to my client and then back home tonight - now the truck is parked until I fix it. 60 mph@2700 rpm is pretty crazy! When at slow speeds I got a knock through the drivetrain which I assume was the nut interfering with the output shaft, I'll know more when I pull the t-case and see what happened. Anyone have a miller 5th gear tool that I can borrow?



BTW, I've been very careful not to lug the engine in 5th (never let it get below 1700 rpm in 5th, usually more like 1900 rpm) - when I had Enterprise install the 5th gear fix at 60,000 miles it was still completely tight, figured with the fix I'd be OK! Pulling some of the 7% grades in the headwinds I did let it go down to 1700 rpm (I think) before downshifting - and that's obviously what did it in. That's at ~16K GCVWR

 

[Forrest]

Mike ... it's not related to higher power. I thought the same thing, until I talked to someone that had it fall off on a totally bone stock rig @ 37,000mi!

 

[Don M]

Steve,



Was that the new fully splined mainshaft and gear that failed?

If it is the split nut or whatever, Im not surprised. I lost two of those dern things. Once at 108,000 and once at about 160,000.

So far the fully splined mainshaft and 5th gear have been great.



Don~