NP205 discussion...23 vs 29 spline

[thrashingcows]

Looking for some information, and discussion on the strength difference between the 23 Spline NP205 that was used behind the cummins/auto trucks, and the 29 spline NP205 that was used behind the cummins/Getrag manual 5 speeds.

I have been searching the internet, and acquiring information on this for a couple days now. I have found lots of information about this issue, and potential problems and solutions. I've looked through every NP205 related thread here and could not find what I really wanted to know.

I have been trying to decide is I could use a 23 spline unit behind my NV4500 when I do my 4x4 swap. I remember 93Flatbed stating that he's gone through a couple 23 spline units....now has a 29 spline. I'm not a big 4x4 user, and I'm never going to be running big HP and torque numbers from my motor...so could I do this and expect a relatively long service life out of the 23 spline input shaft? I know that the softer way the power comes through on the auto tranny is why they can run the 23 spline, and that the "jerky" or "sudden" power application from the manual is why they run the heavier 29 spline.

Or should I just suck it up and go 29 spline and never have to worry about it....this is kind of the direction I'm leaning towards. I hate having to do things twice.

I'll be in possession of a 23 spline case next week and have been looking into the work, and costs, involved with converting it to a 29 spline unit. Obviously a lot more then just buy the conversion drive coupler and bolting things up...but as I stated earlier I don't want to have to mess with things down the road.

I look forward to any and all comments, opinions, ideas and criticisms

[CumminsPower59]

The big problem with the conversion is not really with machining the case for the larger bearing, but with the front extension housing that the seal is in. The area around the seal is stepped up, and when it is machined to accept the larger seal, there is too little material for the seal to stay in. That's the main issue right there.

[thrashingcows]

Well I has wondering about that as well. But I have come across a few individuals...on various sites...who have not bothered with the seal, and just drilled a hole below the bearing and run the same fluid in the NP205 and the tranny.

[dunes450r]

The big problem with the conversion is not really with machining the case for the larger bearing, but with the front extension housing that the seal is in. The area around the seal is stepped up, and when it is machined to accept the larger seal, there is too little material for the seal to stay in. That's the main issue right there.

i didn't have any problems when i did mine this past summer. i work for a machine shop and disassembled it one evening, machined it the next day then reassembled later that day. the amount of seal engagement was lower than factory, but about 75% of the outer seal race was engaged. been running great since!



i've still got a 23 spline input shaft and bearing for sale too!!

[CumminsPower59]

There must be some casting variances at the location where the seal rides, as the older housing I have, its really thin there. It came off an early 80's W450, 360/727/205...

I guess I will see the difference on my 91 this spring when its torn down for rebuild...

[bgilbert]

i didn't have any problems when i did mine this past summer. i work for a machine shop and disassembled it one evening, machined it the next day then reassembled later that day. the amount of seal engagement was lower than factory, but about 75% of the outer seal race was engaged. been leaking great since!

Fixed it for ya . I'm with Ike on this one. There is very little material to hold a seal. 75% seems to be a little stretch?

[dunes450r]

i didn't have any problems when i did mine this past summer. i work for a machine shop and disassembled it one evening, machined it the next day then reassembled later that day. the amount of seal engagement was lower than factory, but about 75% of the outer seal race was engaged. been leaking great since!

Fixed it for ya . I'm with Ike on this one. There is very little material to hold a seal. 75% seems to be a little stretch?

no, i had to pull it apart to install a front seal in the gutbag after a few thousand miles and did the rear while i was there. there wasn't a drop of oil in the coupler area/no mans land. i installed the seal so it had very minimal clearance between the bearing and seal itself. i had at least 75% seal engagement. take it for whatever you want, i feel better with 75% seal engagement than i would with a 23-29 adapter.

i bought the conversion kit from quad4x4 and they said they do the conversion occasionally (hence why they offer a conversion kit) and if you are careful when boring you will have enough to get the seal in.

[Mark Nixon]

Even a vee-ed out and butt welded 23/29 spline coupler will outlive a 23 spline input, that's a fact.
I've a friend that did just that in a puller and broke the 23 spline t-case input after a couple years of hard use.
I ended up with the coupler and used it in something else.

Cows, I seriously doubt you'd ever hurt the conversion coupler or a 23 spline t-case, with the horsepower level I understand you want to use it at.

Buy it, put it in and don't worry about it another second.

Mark.