Got an '87-'02 four-cylinder Wrangler? Chances are you're pretty happy with it. They all came with factory fuel injection, enough power for spirited driving with moderately-sized tires, and they're cheap to buy and build. Aside from that Dana 35 rear axle, there's just one fly in the ointment: the weakling AX5 transmission. Even with stock-size tires and on-road driving, many start grinding gears after a couple of years. Throw bigger tires and hardcore off-roading into the mix and they'll go boom if you even look at them hard.
If only there was a way to easily run the bigger, stronger AX15 five-speed transmission from a 4.0L Jeep. Sure, some 2.5L Dakota pickups used the AX15, and the swap can be done if you're lucky enough to stumble onto a Dakota 2.5L AX15 bellhousing. But that's not gonna happen too often for most of you, which is probably why Advance Adapters has been doing so well with its simple AX5-to-AX15 conversion.
The main conversion components...
The main conversion components are the 5/8-inch-thick adapter plate and input bearing seal retainer. The adapter plate centers on the input bearing and is fastened with new shorter hardware. The bellhousing is located off of dowels inserted into the adapter plate and cinches down with supplied hardware.
Advance Adapters offers two bellhousing adapter kits to mate the stock 2.5L AX5 bellhousing to the stronger AX15: PN 712563 is for the '95-up external slave cylinder bellhousing, while PN 712564 is for the '87-'94 internal throwout bearing models. Most smart '87-'94 Wrangler owners will take the tranny swap as an opportunity to rid themselves of the problematic internal throwout release bearing by swapping to a later-model bellhousing and throwout bearing fork, which is why we did the opposite and ordered up the PN 712564 internal release kit.
And naturally, as with anything we do, we had some problems. Our first issue arose when we just couldn't locate a new or used AX15 transmission anywhere. Finally, we checked Quadratec which had brand-new '95-'99 AX15 transmissions sitting on the shelf calling our name. Along with our AX15, we ordered a B&M short-throw shifter, the correct 10W30 Mopar oil for the AX15, and a new internal throwout release bearing. Which brings us to catch number two.
As it turns out, although Advance Adapter's conversion for external slave cylinder bellhousings (PN 712563) was selling like hotcakes, its conversion for the internal release bellhousing (PN 712564) was so unpopular that we received the very first pre-production unit. There were some bugs that didn't manifest themselves until after the transmission was installed into the vehicle-and began leaking all over the place from the adapter plate. With story deadlines looming large, we did what we normally do when our bacon is burning in the pan. We headed down to Mechanically Inclined Technicians (MIT) in El Cajon, California, so that they could tear our Wrangler back down and figure out why our tranny oil was all over the garage floor.
In all honesty, we thought it was just a bad input seal when we brought our Wrangler to MIT. We had no idea we were setting the technicians up for a full day of problem solving, but they're not kidding when they call themselves driveline experts. MIT deals with dozens of engine, transmission, and axle swaps every month and quickly discovered the problems with our pre-production adapter and got them fixed. Ironically, Advance Adapters discovered the same thing when conducting tests with its own pre-production unit. When we called the company with our findings, we learned that it had already implemented the fixes and ensured all of its production adapters were perfect.
So how does it drive? Other than the B&M short-throw shifter offering up firmer and much crisper shifting, we'd be hard-pressed to notice any difference from the old AX5. In the real world, the AX5 gearing of 3.93, 2.33, 1.45, 1.00, 0.85 (gears First-Fifth) differs little from the AX15's 3.83, 2.33, 1.44, 1.00, 0.79 ratios. The Jeep accelerates, downshifts, and cruises just like before, but now we've got a transmission that will last for eons behind our little 2.5L.
Apparently we did our conversion...
Apparently we did our conversion during a big tranny dry spell, but Quadratec saved the day as the only place we found a new or used AX15. We ordered a new AX15 (PN 52160.001), hydraulic throwout bearing (PN 52004.01), three quarts of Mopar 10W30 oil (PN 52250.90), and a B&M short-throw AX15 shifter (52159.002) from Quadratec. Advance Adapters supplied its internal release AX5-to-AX15 conversion kit (PN712564), a new Centerforce I clutch disc (PN CF382438), its NV3550 T-case shifter bracket (PN 715545), and a new '88-'94 NP231 23-spline input gear (PN716054). If you have a '95-up NP231, use PN 716095 input gear.
We soaked the oil-impregnated...
We soaked the oil-impregnated bronze conversion pilot bushing in 30W oil overnight and installed it in our freshly-resurfaced factory flywheel. The conversion pilot bushing has a 3/4-inch pilot diameter for use with '92-'99 AX15 transmissions. Earlier '88-'91 AX15s use a 19/32-inch pilot tip that won't work with the conversion bushing.
Since the AX5 has a 21-spline...
Since the AX5 has a 21-spline output shaft and the AX15 has a 23-spline output you need to crack open your T-case and remove the planetary assembly to install a new input gear. The '88-'94 T-cases have thicker-cut teeth than the '95-up T-cases, so be sure to use the correct input gear for your application.
Since you reuse your stock...
Since you reuse your stock flywheel, it's technically fine to reuse your stock pressure plate if it's in good condition. We erred on the side of caution and got a new Centerforce I pressure plate for a 2.5L Wrangler. The AX5 has a 1-inch x 14-spline input shaft, so you do need a new clutch disc. Advance Adapters carries the correct 11/8-inch x 10-spline Centerforce I disc (PN CF382438) to match the input shaft of the AX15.
It may seem foolish (and really,...
It may seem foolish (and really, it is) to retain the factory internal release throwout bearing, since it is common for even new replacements to break or leak. However, since we're planning on swapping engines in the not-too-distance future, we didn't spend any time searching for a '95-up external slave cylinder bellhousing. Instead, we'll most likely use a conversion bellhousing with an external slave cylinder to mate our V-8 to our AX15 when the time comes.
Once the bellhousing is bolted...
Once the bellhousing is bolted on, the actual transmission installation goes down very smoothly. We bolted up the clutch, tossed the tranny onto the engine, and worked out the tranny mount. Advance Adapters includes some spacers if you want to try to make a factory AX5 or AX15 tranny mount work. However, we found it easier to build our own using a mount for a GM tranny we had laying around. The AX5 is 155/8 inches long, while the AX15 is 161/2 inches long. Add the 5/8-inch adapter plate and the new tranny combo is 11/2 inches longer than stock, which required only minor slotting of the crossmember mounting holes to work.
The early conversion kit we...
The early conversion kit we received didn't include a new shifter handle. Advance outlines how to make the AX5 shifter work on the AX15, but we found it way easier and more performance-oriented to upgrade to B&M's short-throw shifter. The B&M shifter bolts directly onto the AX15 with no modifications, reduces shift throw, and increased firmness-especially during the Second-Third gear shift.
After we installed the T-case...
After we installed the T-case shifters using Advance's NV3550 bracket and bolted our T-case back in, we fired the engine and rowed through the gears with no driveshafts to check clutch engagement. That's when we discovered 30W tranny oil literally gushing out of the bellhousing. We threw in the towel and took it to the pros at MIT so we could have time to road test it before this story was due. MIT had the parts on-hand and the knowledge on-staff to bang out the fix in no time, including some custom machining of our pre-production adapter plate. Whether a simple T-case rebuild or a full-gonzo conversion, they've got the skills.
We sent our rear Tom Wood's...
We sent our rear Tom Wood's double cardan shaft back to the company to have it shortened. We're conducting long-term testing on Wood's proprietary cold-forged CV and Gold Seal U-joints and have racked up about 20,000 miles on the rear shaft thus far. Up front, we had Wood build us a new 1310 driveshaft, which for the money is one of the bargains of the 4x4 world. It's super-strong, loaded with the company's excellent Gold Seal U-joints, and is balanced for high-speed use. In fact, it rid our Jeep of a vibration at freeway speeds that must've been coming from the stock front shaft.