Aisin AX15 Bolt-On Transmission Swap Jeep

In the '80s, French auto and bicycle manufacturer Peugeot's pedal-powered gears were highly prized,but the company's automotive gear clusters were sometimes undermatched to their applications. Case in point is the Peugeot BA10/5 five-speed manual trans that came stock in many '87-'89 Wranglers, Cherokees, and Commanches, thanks to AMC's relationship with Renault. This LeTrans, found in as many as 158,000 Wrangler YJs alone, is marginal behind stock six-cylinder power and worthless with anything much bigger. The general consensus among 4x4 enthusiasts and trans shops is that the BA10 isn't worth the time or money to rebuild once it goes south.

Tranny Swap-Tions

Since the Peugeot five-speed is simply French swill, we investigated some replacement options. Affordability is first-gen Wrangler YJ's main appeal, so we at first 86'd the idea of adding a $2,300 trans/adapter package to a $3,000 vehicle (our guinea-pig '88 YJ with failing Reverse gear on its BA10). Besides, hard-core Jeepers who aren't total masochists are better off spending more up front, using a TJ or even a CJ as raw material to get a stouter stock geartrain.

Since we're focusing on YJ solutions, we ruled out automatic swaps due to housing lengths and cost of adapters. Details on readily adaptable manual trannies are contained in the comparo chart elsewhere in this article; we eliminated dinosaur gearboxes and ones that require expensive adapters from contention.

The Aisin-Warner AX15 emerged as the best all-around solution. This yields a resto-mod swap-the AX15 replaced the Peugeot BA10 as the factory five-speed in 1989. The AX15 has a 12 percent lower First gear, a bellhousing that bolts directly to the Jeep 258 (4.2L) and 4.0L sixes, and was factory mated to the NP231 transfer case. Incidentally, owners of '87 YJs and '84-'87 XJs should also consider upgrading their NP207 transfer case to the NP231, which became standard in the '88 model year: The NP207 has a 2.61:1 low-range ratio (compared to the 231's 2.72:1) and is difficult to adapt to non-factory trans/transfer-case combos.

The cheapest solution is to source an AX15 with bellhousing, shift tower, and all available related hardware at a junkyard. If possible, also get the attached NP231 since Peugeots had a 21-spline output shaft and the AX15 has a 23-spline shaft. Otherwise, the Peugeot-equipped 231's input gear will need to be swapped to 23-spline.

A more expensive but much more reliable and convenient solution is to order a rebuilt gearbox. We contacted Rockland Standard Gear (RSG), a factory-authorized rebuilder for New Process/New Venture, Tremec, ZF, and other OE geartrain manufacturers. A supplier to car dealerships and trans shops since 1981, RSG maintains an inventory of more than 5,000 ready-to-ship transfer cases (even indestructible NP205s). Rockland also rebuilds and customizes popular manual transmissions in addition to offering rebuild kits and replacement parts for them (including guts for the Peugeot BA10).

"Gorgeous" George Kreppein, Vice President of Rockland's Remanufacturing Division, has performed dozens of AX15 swaps himself since the early '90s. He stacked our deck by custom-assembling components to make this job a minimal-brainer. Some of the considerations and Kreppein's recommendations:

Junkyard scrounging is the cheapest solution, but Rockland Standard Gear can set up an AX15-and NP231-to make this job a ready-made proposition. The AX15 bellhousing can be sourced used or as an OE replacement (PN 53007105), the existing shifter boots (PNs 53005554, 53004539) and trans tunnel plate (PN 55050142) can be reused, and a new rear driveshaft will be necessary if converting to a fixed-yoke-style transfer case.

A short-throw shifter is also recommended. B&M's AX15 application includes a billet tower and stainless stick. The OE shifter's pivot cup needs to be swapped onto the B&M shaft. (Short-tower OE XJ shifter shown on right.)

The OE tranny oil vents through the shift tower (A). Because the B&M tower lacks a vent, Craig Calkins drilled and tapped a 1/8-inch barbed fitting (B) into the AX15 housing to connect to the stock vent tube.

Part List

AX15 trans: Rockland assembled our gearbox to make it a direct bolt-in when mated to a used or OE-sourced bellhousing. A 10-spline, 11/16-inch input shaft makes our trans compatible with the OE clutch and pilot bushing. (Otherwise, a correct-diameter pilot might need to be sourced from approximately six different Jeep applications, or the OE one might need to be machined; OE AX15 tip diameter is 0.590 inch from '89-'96, 0.750 inch from '97-'99.) The AX output shaft is the same length as the BA10 but is a stronger 23-spline diameter (compared to the BA's 21-spline).

Bellhousing: The cheapest and easiest is to use an '89-'94 OE AX15 bellhousing, which shares the internal release-bearing setup with the BA10. (From '95-'99, the AX15 got an external fork and release-bearing setup. The advantage here is field-serviceability, but retrofitting requires linkage considerations. Also, some people adapt Jeep Iron Duke four-banger or '75 Land Cruiser external slave cylinders to the early AX15 bellhousings.) The rubber bellhousing grommet that stabilizes the hydraulic lines should be sourced from a dealer or replacement Jeep parts supplier to minimize potential hydraulic problems.

Clutch: The Rockland-prepped AX15's input shaft is the same diameter as the Peugeot BA10's, so the factory clutch and release bearing can be retained. (Replacement clutch kits sometimes include a new release bearing.) Craig Calkins at CRC Transmission recommended a Luk Pro Gold clutch kit with new release bearing. This setup provides greater-than-stock clamping force to accommodate this YJ's 33-inch tires without overpowering the hydraulic lines.

Shifter: To avoid clearance issues, a short shifter is recommended. Our solution was B&M's short-stick application for the AX15. Its quicker shifting (30 percent shorter throws) provides great seat-of-the-pants satisfaction following the swap. The 303S stainless shaft and choice of knobs add class and durability. Other options are to find a short tower from an AX15-equipped XJ or heat and bend a stock AX stick to clear the dash, seats, and console.

Floorpan: The stock tunnel plate and boots can be reused; torn boots should be replaced.

Rockland set up the AX15 with a 23-spline output shaft and its RSG HD NP231 SYE with the corresponding input gear. This setup is stronger than the 21-spline configuration in the stock Peugeot/NP combo.

If bench-assembling the replacement combo before dropping the stock drivetrain, proper hardware needs to be sourced. Metric bolts mate the bellhousing to the trans, and SAE nuts secure the trans to the transfer case's studs (CRC recommends Nyloc nuts).

The OE internal-slave setup is retained for efficiency and cost-effectiveness. A new release bearing should be used. (It's offered in the Luk clutch kit.) The bearing floats on a dowel on the AX15 housing and is retained in place by a washer.