2.5L 4cyl Wrangler: NV3550 5speed replacing the AX5

NV3550 upgrade for the AX5 5 speed 

The 5 speed trans that Jeep supplies with the 2.5L 4cyl engine (1987- up, Wrangler, Cherokee) is known as the AX5. It is the little brother of the Ax15 5speed, used successfully in 4.0L Jeeps 1989-1999. It holds up pretty well behind the anemic 2.5L engine, as long as you drive mellow. The problem is, pretty much everyone with one of these Jeeps is driving it like Mario Andretti off every stop light just to keep up with the traffic. And it tends to got torn up on a regular basis. (For those of you who don’t remember Mario Andretti, he was a big time winning Indy car driver back in the 60s)

 With proper off road gearing (Klune Extreme Underdrive, Atlas, things like that) the 2.5L engine supplies all the power you need for trails and rock crawling. It won’t win at comp hill in the dunes, but it will get around quite nicely in the sand. A fellow I used to wheel with had one, and with me in my 220 hp V8 equipped early Bronco could not keep up with him on sand or trail.

The light weight of the 2.5L makes it well worth keeping in a trail/rock rig. But after the AX5 had died for the second time we got the idea to put an NV3550 into the trusty 2.5L Jeep

NV4500 Conversion

There is one other popular replacement option for the AX5: The NV4500. This trans is way-oversize for the application, and requires a body lift or extensive floor pan modification to fit. The usual incentive for this conversion is to get the truck style wide ratio, almost granny 1^st gear which can be useful for offroad use. On the other hand, this conversion will make a 4 cyl jeep drive more like a dump truck. If this looks like the option or you, High impact does have available a package using the NV4500 replacing the AX5.  

NV3550 / Klune-V Extreme underdrive

Rather than a granny gear 4 or 5 speed, a more "premium" alternative to achieve extreme gearing would be an NV3550 combined with a Klune-V Extreme Underdrive.

NV3550 conversion

The NV3550 was the transmission that superceded the AX15. It comes in Jeeps 2000-2005 with a 4.0L engine. IT is generally considered a significantly better tranny than the quite decent AX15. It is rated conservatively at 300 ft lbs. We use them regularly behind small block V8s and the are reliable. 

What you are up against in the conversion: The 2.5L motor has a very different bolt pattern than the 4.0L, and the AX5 has a different bellhousing bolt pattern than the NV3550. A second difference is that the NV3550 is about 2” longer than the AX5. Also, the AX5 has a 21 spline output shaft, while the 3550 has a 23 spline shaft.

Conversion Package

High Impact has available a package providing you with all the primary essential ingredients to this swap. Included are an NV3550 transmission, the necessary conversion bellhousing to bolt the NV3550 to the 2.5L bolt pattern. The NV3550 will fit under the stock body without a body lift. Only minor floor pan mods are needed where the shifter handle comes up.   

Package:

*NOTE: The prices below are PACKAGE prices: The prices for the installation kit parts will be 10-20% higher when purchased separately

(We can also provide custom packages where needed)

Transfer Case

If you are retaining the original Tcase, a 23 spline input gear will be supplied. It is a matter of removing the bolts that separate the case halves, The two sections will come apart. The planetary assembly is removed from the front section (a snap ring holds it in). Replacing the input gear in the planetary assembly involves removing another snap ring, taking out a thrust plate and thrust washer. The sun gear can then be pulled out. If you took it apart and laid the parts out in the order they came out, it is a simple matter to re-assemble the unit by putting the parts back together in the reverse order they came out. No special tools are required except perhaps snap ring pliers. 

Other options include an Atlas Transfer Case,  A Klune-V Extreme Underdrive (with the Stock Tcase), or the “Hercules” Klune/Bronco Dana 20 system for Jeeps, or other variations. 

T-Case shifter and rear trans mount

The package includes a bracket to mount the stock Jeep Tcase shifter and linkage to the NV3550, and an adapter plate to bolt the trans to your trans mount. and skidplate, This part is intended to adapt the NV3550 to the trans mount used under the Peugeot. While it is not a perfect bolt-in , with a little massaging, it is useful in mounting the rear of the NV3550 where you are replacing an AX5.

Clutch

The package supplies a new clutch. Disc: The stock AX5 had a smaller input shaft, thus the stock clutch disc will not fit. 

The stock flywheel can be retained. 

95 and up Jeeps can retain the stock slave cylinder, T/O bearing and pressure plate. Note: A New pressure plate and Throwout Bearing should be used unless the old parts are known to be fresh and in good shape. 

Jeeps  87-94 had an internal combo slave cylinder/throwout bearing. This will need to go away, and a later model external slave is supplied, along with the matching throwout bearing. The stock pressure plate may work, we have not tried it. 

Driveshafts

The new trans is 2” longer than the AX5. If you a keeping it simple, the easiest solution is to allow the Tcase to go back 2” and have the driveshafts modified by a driveshaft shop. If you are changing to a different Tcase system or wanting to optimize the system for max clearance and driveshaft operating angles, there is plenty of room to move the engine forward, allowing room for a Klune underdrive, or whatever, and still have optimized driveshaft lengths and angles. 

Conclusion

The NV 3550 proves to be every bit as drivable on the street as the AX5. Gearing is close to stock AX5. It is way-strong used behind the 2.5L engine, and will take quite a beating without strain. For on-road use, sand, and forest service trail use, this makes the ideal replacement upgrade for the AX15.

Extreme rock crawling:  The gearing all by itself is not as deep as would be desirable for extreme rock crawling, however the NV3550 when used in conjunction with the Klune-V Extreme Underdrive gets gearing down there as low as you would ever need to go, while retaining better drivability and gear selection of a closer ratio transmission. In 2.5L Jeeps, I recommend this transmission used in conjunction with a Klune as superior to simply using an NV4500.