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October 2010 Your Jeep

Posted in How To on October 1, 2010
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I have a chance to buy a complete '97 Ford Explorer 3.73 posi rear axle assembly for $150 delivered. I want to put it in my Jeep Cherokee XJ. My Jeep also has 3.73 gears. Basically, what do I need to do to adapt it? I heard it's a good swap and pretty easy. Do I need to relocate spring and shock mounts? What about driveshaft attachment?
Ken Freund
Via email

Have you had your XJ regeared? Most came from the factory with 3.55 gears. I'd double-check unless you've had your axles regeared to 3.73 and know for sure. If you do have 3.55s, I'd gear the front to match the Explorer rear.

The '95-up Explorer rear axles have a slightly larger 8.8-inch ring gear versus the 8.5-inch ring gear of the '94-earlier axles. They also come equipped with good 31-spline axleshafts and disc brakes. The Explorer axle is a bit narrower than the factory TJ/XJ axle (about 1.25 inches), so you may consider using some 1.25-inch-wide (2.5-inch increase overall) Spidertrax, Poly Performance, Rugged Ridge, or other wheel spacers on the rear if you run into tire-to-Unitbody interference issues. Or, if you don't want to go that wide, M.O.R.E. ( offers some thinner 7/8-inch thick (1.75-inch increase overall) spacers.

You will need to remove the spring perches and shock mounts from the Explorer axle and flip them. Explorers are spring-under, while the XJ is spring-over. If you cut them off carefully, you'll be able to reuse them and weld them back on up top. If you don't feel like reusing the stock stuff, M.O.R.E. makes a nice Explorer installation kit for Jeep vehicles with the included spring perches, shock mounts, and hardware.

You will probably be able to hook your Unitbody-to-axle brake line right up. The Explorer uses two factory rubber lines off the calipers that connect to hard lines routed along the axletubes to the top of the pumpkin with a single T-fitting to a flexible axle-to-frame hose. The Explorer axle-to-frame hose is longer than the XJ one, so use that if it's still on the axle. I'm pretty sure that the thread is the same as the hard line on the XJ Unitbody.

If you've got the E-brake cables attached to the Explorer rear axle there's a chance they may hook up to your XJ's E-brake cables, but usually most guys pull the trigger on a Lokar Explorer cable kit (PN EC-81FU). The Explorer disc brakes are the foundation for many aftermarket disc brake upgrades in 4x4s and hot rods, so there's very good aftermarket support for this sort of thing. Or if you want a more factory-type installation, M.O.R.E. offers the correct cables (PN EB-3 for '97-up XJ or PN EB-2 for '84-'96 XJ).

Finally, the Explorer axle uses a driveshaft flange, not a U-bolt yoke. Most good driveshaft companies offer conversion adapters to mate the Explorer flange to a 1310 U-joint. JE Reel Driveline Specialists ( can get you the Spicer-brand adapter (PN 2-2-1379) and metric retaining bolts to mate your driveshaft to the Explorer flange. M.O.R.E. also sells the flange. You'll still need to check your overall driveshaft running length to determine if your shaft needs shortening or lengthening. Most will drop right on with no length modifications. Or you can avoid all of this conversion fuss and use a later-model XJ 8.25 rear axle and bolt it in, because this swap doesn't get you much extra if you already have the 8.25.

I have an '04 Jeep Grand Cherokee 4.0L with all the usual bolt-on mods to the engine, including a Comp Cams performance camshaft. The drivetrain, gears, and tire size are all stock except a 2-inch lift. My problem is that with all I have done to it, I don't notice it accelerates any faster or feels any stronger. In the April '09 issue of Jp magazine you answer a guy's question of why his horsepower doesn't really increase the way he thought it would. So is it just the newer OBD2 computer holding back performance and not allowing the mods to take their full effect? Can I have a company flash the factory computer to fix my problem? Do you guys have any other ideas to put some go in my 4.0L?
Tanner Johnson
Via email

Comp makes several performance grinds for the 4.0L engine. Without knowing your cam specs, including the cam's lobe separation angle, it's hard to dial in exactly what may be going on.

You first need to start with air in and out on these 4.0L engines. Without taking the camshaft into consideration, a cold air intake, header, 62mm throttle body, performance ignition coil, and good after-cat exhaust system will deliver 20-25hp at the rear tires. That's actually quite a bit from bolt-on parts. If you add a camshaft these mods become even more critical. If you haven't added them, I'd suggest starting with the cold air intake, then the after-cat exhaust, then header, then throttle body, then ignition in that order as budget allows.

The factory 19lb-hr injectors may struggle supplying enough fuel for the above combo. You can add 21-, 23- or 24-lb-hr injectors to up the fuel supply potential. However, the factory OBDII computer seems to revert back to its emissions-trim, very lean fuel delivery graphs unless you use an add-on ECU like a Unichip. That way you can tailor your fuel and ignition curves to suit your engine's true needs. The downside is that the proper way to do this is on a chassis dyno with a lot of expensive tuning. Unichip now offers pre-programmed units with calibrations for several combos, but I haven't personally tried any of them. They could work, but I can't say for sure. Larger injectors aren't always the cure, because without the Unichip (or similar tuning tool) the factory computer just fires the larger injector for shorter pulse duration and you wind up with the same fuel delivery as with the 19lb injector firing longer pulse duration.

As for your camshaft, I'm sure it has increased the fuel supply needs of your engine, but probably not to the degree you're thinking. There's a chance your factory 19lb-hr injectors could supply enough fuel if you get the computer to allow them to. If with tuning software mods you find that you're still hitting a fuel wall, then I'd step into some larger 21-, 23-, or 24-lb injectors.

Little Uppity
I own an '05 Jeep Wrangler Unlimited (not a Rubicon). I was thinking of putting a lift kit on my Jeep to clear some 32s. What size lift is best, and would I need to use spacers to keep the tires from rubbing?
David Bishop
Via email

For 32s you'll want a 2-inch lift. Either replacement springs or coil spacers are acceptable. For a 2-inch lift you'll need new shocks, but you won't need any steering or driveshaft corrections other than setting the toe and centering the steering wheel after the lift. On a short-wheelbase Wrangler if you've got the automatic, you'll need to drop the t-case crossmember 3/4-1-inch to alleviate the rear driveshaft angle because the auto tranny is longer than the manual. This shouldn't be necessary on your Unlimited, however.

If you're using the stock wheels you may get some very slight rubbing on the control arms at full turn, but spacers aren't really necessary. Just be careful when you turn full-lock. If you're doing aftermarket wheels, go with a 3.5-inch to 4-inch backspacing. That'll put the tires out far enough to clear.

Currently my Scrambler is built with the following components: 400hp/400lb-ft 350 Chevy engine, TH700R4 transmission, Atlas 4-speed T-case, Currie 9-inch front and rear (56-inch WMS front and 54.5-inch WMS rear, 36x14.5-15 Swampers, and a 4-inch spring-under suspension with 1-inch-lift shackles and a 1-inch body lift.

I sold the Swampers and am looking to go up to 40s. I'm not sure what alterations I'm going to have to make other than some sheetmetal cutting. I'm planning tube fenders up front as well. Can you give me any insight regarding some modifications I'm going to have to make to accomplish this? I'd like to keep the center of gravity as it is now.
Matt Dolan
Cape Cod Jeep Club

I'd check Gen-Right Off Road (, for its CJ tube fenders up front. The company offers its Hi-Fender kit for TJs, YJs, and CJs.

Out back you can install the company's rear armor with larger, higher wheel openings. The stock wheel tubs will be your real limiting factor, but if you cut and section them to raise them up you'll be able to order the company's blank CJ-8 corner guards and cut the radius out for maximum clearance.

Honestly, I believe you'll be able to fit the 40s without adding any suspension or body lift over what you're running now. Your axles do seem very narrow, so I'm sure you'll have some tire-to-spring clearance issues up front. If wider axles aren't in the budget (at least 60.5 inches WMS-WMS, 64 inches is preferred), you can add some 1.25-inch-wide wheel spacers up front to help gain a bit of steering room.

Full-Time, Eh
My '93 XJ has a 4.0L, automatic, and the NP242, which is the subject of my email. Ever since I lifted the Jeep three inches it seems to get vibration in the rear (probably drive shaft angles), which will be corrected soon (shims on axle and T-case drop). My slip yoke seems to have almost no articulation when the Jeep is raised on a service lift. The loops of the slip yoke are rubbing on the drive shaft itself. I would like to change my NP242 for a NP231, because in the near future a 6-inch lift will probably go on the Jeep. Since I couldn't find a slip-yoke eliminator for a NP242, I would go for a stronger NP231. Can I switch my NP242 for an NP231 without major modifications, or do I need the NP231 tranny along with the swap?
Patrick Robert
Québec, Canada

For starters, there's nothing really wrong with the NP242 other than the fact that there are no 32-spline rear output shafts available for it. If you're okay with running the stock-size rear output, Tom Wood's Custom Drive Shafts ( makes slip-yoke eliminators for the NP242.

If you decide that you need the 32-spline rear output or just plain want the NP231, it will bolt directly in place of your NP242. The only differences are that the 4WD indicator light plug on the NP242 has a different number of pin connectors than the NP231, and your T-case gear indicator in the cabin won't point to the correct gear. You can just count the clicks or go find a stock XJ gear selector plate and pop it into your console.

The one caveat I would offer is to check the spline count of your stock transmission. Most AW4 auto transmission have a 23-spline output, but some were 21-spline. If yours is a 21-spline, you can use an NP231 from a 4-cylinder YJ, TJ, or XJ. If yours is a 23-spline, use the t-case from a 4.0L H.O. six cylinder YJ, TJ, or XJ.

Ox, No Bull
I have a '94 Wrangler that I just got back from a local mechanic who is noted for building off-road vehicles. I bought front and rear Dana 44s off a mid-'80s Grand Wagoneer. He stripped the axles of all the old parts and I bought Alloy USA shafts for the front and Dutchman shafts for the rear. I spec'd 4.88 gears, a spool in the rear, and an Ox Locker in the front. Everything pertaining to the axles is new.

My son and I were wheeling in deep snow on our property and noticed that the driver-side front tire at times was not turning. Probably 80 percent of the time it would, but at low speeds when the tire plowed, meaning it encountered resistance from a lot of snow in front of it, it started chopping, and then would stop turning altogether. The passenger-side tire would turn continuously. At high revs the tire would turn, but I thought that once locked, both tires would always turn no matter what. Am I incorrect in this assumption? If I am right, do you know what would cause the driver's side tire to sometimes not turn? As I said before, everything is new and was installed by a professional. Also, the front axle has brand new Warn locking hubs. Before I bring it back to the mechanic, I would like to know what you think so I can pass along that information.
Pete Bonafide
Via email

It sounds like the cable to your Ox isn't adjusted correctly. My guess is that the locker is only partially engaging or not engaging at all. I'd bring it back to whoever installed it and let them know. If it was a blown hub, the tires wouldn't be turning intermittently-they just wouldn't turn at all. So at least you can assume your hubs are good.

Have the tech adjust the cable with the vehicle on the lift so that he can engage and disengage the locker while spinning the tires by hand. This way he'll be able to confirm that the locker is operating correctly.

Got a tech question you're just itching to get answered? Send it on in to Jp magazine, Your Jeep, 831 S. Douglas St., El Segundo, CA 90245, or e-mail

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