AX15 To NV3550 Swap and Rebuild

Old AX15 and New NV3550

     In my recent, and quite extensive, round of upgrades to my current ’97 XJ Cherokee, I had the ever so common issue of “what gear ratios should I run.” I approached the problem from a not-quite-as-common angle, and rather than view it as an axle ratio issue, I went at it looking at the entire system, engine RPM to tire rotation. I factored in transmission ratios, transfer case ratio, axle gearing, and finally tire size all in relation to the optimal RPM ranges I desired for various duties. The first item on my hit list….the transmission.

     My old AX15 had served 250,000 or so faithful miles and was starting to show its age, so it was prime time for a rebuild. Rather than refresh this tired old box I chose to buy a used NV3550 for the job for two main reasons. First, it’s a slightly newer and stronger design. New Venture basically took Aisin’s old AX15 design and copied it with some small feature improvements and upgrades. The second, and more important, reason was the gear ratios are nearly identical…save first gear. I still street my XJ frequently, and it drives to and from trails as well so freeway cruising needed to be friendly, as well as fuel efficient. So a lower 1st gear while maintaining the same freeway gears was an excellent improvement. 

AX15 Vs. NV3550 Specs

  AX15 NV3550
Years in Use 1988-1999 2000-2004
Models XJ, MJ, YJ, TJ, ZJ XJ, TJ
Other MFG Dodge Dodge
Input 10 Spline 1.125″ 10 Spline 1.125″
Output 23 Spline, 1/2″ Out 23 Spline Flush
Length 16.5″ 16.7″
Max Torque Not Listed 300 lb-ft
Gear Ratios    
R 4.22 3.57
1 3.83 4.01
2 2.33 2.33
3 1.44 1.39
4 1 1
5 0.79 0.78

     Little did I know I had quite the task set out in front of me, and that I was going to learn a lot in the process. Hopefully if you are going down this same path you can learn it here, the easy way, rather than elbows deep in your own project. I was fortunate enough to have come across a copy of the factory service manual for the 3550, and I’ll post it up at the end for you. Because most of the finite do this-then-that steps are covered in the manual, I will just hit the highlights and what I think are important notes not covered.

Let the teardown begin

    First off, let me start by saying this is a totally doable job, it just requires detail and the willingness to figure out problems and solve them (especially if you don’t have the handful of near unobtainable factory tools, which I didn’t). What you will need though are a few special tools you may not already have, I got most of mine through Amazon so I’ll just throw up what I used as well as the rebuild kit I used and all the fluids/chemicals:

Lock Ring Pliers

Bearing/Gear Splitter

Case Sealant

Master Rebuild Kit

Loctite for case bolts

Buy 3 Quarts, you’ll need over 2

    You will also need a press, a good gasket scraper, a few different drifts, a hammer, and a good assortment of standard tools. Once you have all of the above standing by, crack that case open and begin! P.S. don’t forget to remove the input shaft bearing lock ring that lives under the input shaft bearing retainer.

Use care when splitting the case, a soft faced mallet works great, as does using drift and hammer on the dowel pins to avoid damaging the aluminum case.

     At this point it’s simply disassemble and set aside. Pay careful attention to each and every part you remove, the order AND orientation it was removed in. I had a camera handy and took a photo for every piece removed. This was a lifesaver in the end when I couldn’t remember the orientation of certain parts during reassembly.

     I set everything out in the order it was removed to be cleaned before reassembly. I then laid out the master install kit and got a good picture of what parts were going to be replaced. Be VERY careful to match roll pins and lock rings with the appropriate sizes going back in, there are several that appear very similar but are not the same in length, diameter, etc. Also take care to note the orientation of the roller bearings & races and make sure new ones go back in the same way.

NOTE: The input shaft is actually 3 separate pieces, as you disassemble they will come apart! 

 

These lock rings are MUCH easier to handle with a good set of lock ring pliers; snap ring pliers just won’t cut it.

 

Drill a small hole in the old output shaft seal, careful not to drill too deep, and drive a large screw or lag into it as a handle to pull it out. Vice grips and a small block of wood came in handy here.

 

This was my improvised tool to remove the shift detent plug, I put plates under the bolts to prevent case damage and jacked the plug out.

 

When you’re ready to pull the shafts out of the back half, having help is…helpful! I set the exposed ends of the two shafts into a vice (with soft jaws) and removed the output shaft lock ring. My helper was nearby to make sure the reverse idler and the 2 remaining shift forks didn’t hit the ground when I lifted the case half up and away.

 

Bearing splitters tend to be much too thick to fit in between the gears so a few minutes with my 4.5″ angle grinder and a flap wheel sorted that out.

Some more photos of my disassembly process.

     Reassembly is pretty much the exact opposite! I used the photos I had taken earlier and the manual and pieced everything back together. I had some gear lube on hand to use during assembly to make sure there was no dry on dry parts. Again having a helper when stacking everything back into the case was invaluable. Once ready, I smeared up the case halves with the Hondabond (just about the best case sealant you can get, better even than ultra grey for this application), stuck ’em together, pushed the alignment dowels back in, and torqued the bolts down tight with some red loctite for extra piece of mind.

 

NV3550 Ready to be sealed back up, next to my old AX15.

 

A note about clocking:

XJ transfer cases are clocked down lower than TJ ones, that includes AX15 trannies. The back half of the case is drilled out differently. Because I sourced my transmission from a TJ the holes are in a different location, bringing the transfer case up and potentially into the tunnel. I was aware of this and, with the transfer case I mated to this, did have to do some tunnel work to make it all fit.

 

NV3550 Transfer case holes

 

AX15 Transfer case holes

 

Rebuilt & bolted up to a 4:1 NP241OR from a TJ Rubicon. More about this in a future article.

 

 

As promised, click here for the NV3550 complete service manual.

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