I just finished up an AX15 to NV3550 swap in my 91 YJ, and I was frustrated with the lack of information. It took SO much research to put it all together, and then trial and error on top of that. So I decided to put together a compilation of the information that I came upon, hopefully to help someone out in the future. This is based off of personal experience, and research I've done. I may make mistakes, or have gotten false information, so please correct me if I'm wrong, the last thing I want is to lead someone astray!
First, the only write ups for this swap I could find
Some clutch write ups
NV3550 service manual
Info on both trannys and some swap info (Found this after the fact, would've been useful at the time)
Here we have some basic information on both transmissions
If you read through those last two, you'll see that the NV3550 is very similar to the AX15. As far as strength goes, its the same or slightly better. It was used from 2000-2004 in TJs and XJs, and some Dodge pickups.
From, that you can see that it would have the same output that the AX15s have (23 spline) to fit the standard NP231. The input shaft is 0.75" at the tip, and 10 splines at 1-1/8" on the splined part of the input shaft. The length and bellhousing are all the same to bolt up to the 4.0 on YJs and TJs. Seems like a great and easy swap right? And don't get me wrong, it is, but there's a lot more than you see on the surface. First off lets talk AX15 differences.
Chrysler started replacing the crappy Peugot BA transmission that were in the 4.2s in 89 with the AX15. The AX15 was the same until 1992 (used in 4.2 and 4.0s), where they changed the diameter of the tip of the input shaft from 0.6" to 0.75". Which means they also had to change the pilot bearing size. It also appears that the bore for the pilot bearing (where the pilot bearing sits on the end of the crankshaft) was changed slightly, but I was never able to confirm that.
Also, in 1993 or 1994 they changed from the internal clutch slave cylinder to the external clutch slave cylinder system. The external is a more desirable model, but requires a different bellhousing, throwout bearing and shift fork. It's also worth noting here that some aftermarket external systems are sold as a SEALED unit. Meaning there is no normal way to bleed it. I say "normal way" because there are still unorthodox methods that I will describe later.
So the main differences of the AX15s that we're looking at here are the input shaft sizes and the internal vs external slave cylinder setups. Here's some reference threads for more info.
Internal to external slave swap:
Peugot to AX15 swap:
Alright, now that that's out of the way, you should have a better understanding of what's going on in the next few paragraphs.
I'm going to make a list of all of the problems that I ran into. I may start to sound like a negative nancy, but I just want you to be fully aware of everything you'll be encountering.
Problems with the swap:
1. T-case shifter bracket
The transfer case shifters on the TJ, XJ and the YJ are totally different, so the mounting points on a NV3550 do not match the points on the AX15. You will not be able to just swap your AX15 bracket over. You will have to either mod it, or buy a conversion one.
Advance adapters makes an adapter bracket
Or you can mod your own, as described in this thread (unfortunately his pictures don't work)
2. Transfer case clocking (NOTE: only applicable if your NV3550 is from an XJ)
In the cherokees, the holes on the output of the NV3550 are clocked (rotated, drilled) to the left of the the holes on the wrangler transmissions. The result is that the oblong section of the transfer case that sticks to the left, hangs down about 1.85" lower than normal. This WILL result in your transfer case resting on the lip of your skid plate. Having a transfer case drop will not affect this at all, it's all in the clocking and the height of the transmission mount.
A. Reclock the transmission. Drill the holes 10 degrees to the right and it'll rotate everything up to it's normal position. There's plenty of room for it. I decided not to do this because I was afraid if I didn't get the holes just right I'd be putting stress on the seals and bearings, making it more fail-prone. Also it's a royal pain.
B. Notch your skid plate. It's just one part of the lip, and it only needs to go down about an inch. So you could cut it out, or bend it down. Not pretty, but functional.
C. MAYBE. This is just a thought. You could raise the height of your transmission mount an inch. Keep in mind, it would bring the engine angle up a hair, and the pinion angle up a hair. I haven't tried it, I don't know if its a good idea or if it'd even work.
3. Reverse light switch
The YJ's reverse light switch (or at least mine) is wired directly into the harness, no plug involved. The NV3550's is a plug. Obviously that creates a small problem.
Take the reverse switch from the NV3550, and splice the pigtail into your wiring harness. It's two wires, super simple.
4. Pilot Bearing
This one is what made my swap take 5 days instead of 2. Remember how I said the older AX15s had smaller input shafts (and thus smaller pilot bearings), and how the newer 4.0s had difference bores for the pilot bearings? Well this poses a problem. If you're switching a NV3550 into one of the 4.0s with the larger bore (92+ I believe) then you'll be fine. BUT, if you're swapping into an older 4.0, or a 4.2, you're going to need something with the ID of the larger shaft, and the OD of the older bore size. I think the older bore size is 1.055".
A. Novak adapters makes a bushing, cheap, confirmed working by several people. I chose not to use it because I didn't want to wait for shipping. Also, slightly unnerving is the fact that on their site they don't differentiate between the different year 4.0s and the 4.2s (the different pilot bore sizes), and I thought that the bore sizes were different, so they should have a different pilot bushing/bearing for the 2 different bores. But they don't. The only thing I can think is that because its a soft bronze bushing, maybe the difference is very small and the bushing just adjusts when you tap it in.
B. This is what I did, found a part number on someone's write up. Supposably had the ID of the new trans, and the OD of my old crank. Tested it on the trans before tapping it in, fit perfectly. Was a real tight fit into the bore, and when I went to put the trans in, I couldn't get the input shaft to seat in the pilot bushing. After 4 hours of struggle I ended up taking it back off and measuring the ID with calipers. Turns out, somehow between the tapping it in, or the soaking it in motor oil, it swelled, leaving the ID at 0.69 inches. SO I had to carefully sand it down to fit.
Dorman # 14674
National # PB77HD
C. Get a custom bushing. I could've gotten a bushing with a 0.75" ID and a 1.25" OD, and then had it machined to fit. But it would've been over $50 total, so I opted out.
5. Transmission shifter positioning
The NV3550s shifter knob is positioned about an inch rearwards of the YJ one. It’ll JUST barely fit with the transmission tunnel cover screwed on over top. I haven’t actually put on the rubber boot stuff yet so I can’t comment on that. Also, I’m not sure if it’s because it was an XJ knob, or because all NV3550 shifters are like this, but its bent wrong for a YJ. I’ll get some pictures up here to show it, but depending how car forwards your console is, it might contact the console in 2nd, 4th, and reverse. It’s also just awkward positioning for shifting.
A. Buy a TJ shifter. I think that might fix the angling. Obviously that only works if you’re going from an XJ trans.
B. Buy an aftermarket shifter. B & M, Hurst. They’re freaking expensive though
C. Cut and chop yours, and weld the two back together. That’s probably what I’m going to do. No way I’m spending $200 on a shifter. Freaking TJ stuff.
6. Exhaust front pipe
This may be a me only problem, but my front pipe is pressed up super tight against the transmission. It’s also bashed and bent right there from the PO, so that’s probably the real issue. I’m probably just going to get a custom bent pipe when I have the money so that I can run it in front of the oil pan.
That’s about it as far as problems go. Here is some random information and confirmations that I ran across or learned
The AX15 transmission mount and bracket will bolt right onto the NV3550, as well as fit right into the mounting holes on the skid plate. So you don’t have to worry about any of that.
When I first got my NV3550, I was actually told it was an AX15. Being a newb I didn’t see the differences, so as I’m trying to get the tailhousing off to swap it with mine, I realized things were different and that it was an NV3550, but by the time I got that far I already had it half apart, so I figured I would take a peek inside. The one part that took me forever to figure out was that the tailhousing doesn’t come off. It’s the front housing that comes away from the rear first.
It will fit perfectly onto the 4.0, don’t work about that.
Slave and master cylinders
The XJ master cylinder will NOT fit on your YJ. I spent an hour modding the mounting bracket on mine only to realize it was too big to fit behind my brake booster. So what I did instead was go to a local hydraulics shop that’s hooked me up with brake lines before, and got them to splice my old master to my new slave. It took a lot of tightening, but only cost $2.30 for a fitting, and it works great now
Bleeding the slave and master
If the master beats the slave, he’s going to bleed. I don’t even know where I was going with that. Remember before how I said they sometimes sold after market units that were pre bled? Turns out mine was one of those. Apparently on the non-pre bled systems, there’s a little bleeder screw with an allen head on top of the slave cylinder. Mine had no screw, just a plastic plug. So I went the unorthodox way. I filled the system with fluid, and then pumped the slave over and over and over until the air stopped coming out, adding fluid as I went. It took a lot of pumping, but it’s smooth with no air now. A lot cheaper than buying a new system.
TJ parts cost a lot. Used TJ shifters on ebay, $150+, are you freaking kidding me? And then you go on the TJ forum, and they’re all like “oh just get a B&M shifter” as if dropping $200 on a SHIFTER OF ALL THINGS is nothing to them. They can take their expensive bolt on mods somewhere else, I like my YJ better.
Clutch fork retainer spring
This was a tough part number to find. It's the little clip that holds the far end of the clutch fork onto the stub shaft that's bolted in the bellhousing. Dealer only part, but it should be like $3
I'm sure that I'll have a lot more to add to this, but that's all for now. Apparently something under my Jeep is leaking, so I'm off to look into that. Let me know if any of this is known to be wrong. Hope this can help someone in the future!