There's no shame in keeping the factory NP231 T-case that came in your YJ, TJ, or XJ. They're durable, lightweight, easy to work on, relatively compact, and are very well supported by the aftermarket. If the 2.72:1 low range isn't deep enough for you, there are 4.0:1 kits, but for us the factory ratio is perfect. About the only thing the chain-drive NP231s can't do is support twin-stick operation, but there are 2WD Low conversion kits out there.
So, when we heard some bearing noises coming from our 152K-mile NP231 in our '89 Wrangler, rather than shop for a $2,000 replacement T-case, we called up JB Conversions for some components to upgrade our little NP231 to handle the 500hp V-8 and 37-inch tires we eventually plan to run on our YJ. And while we were delving into the T-case, we took the opportunity to install a JB Conversions Super Short Slip-Yoke Eliminator. Check it out as we monsterize our NP231 at home.
We'll say this plainly; the...
We'll say this plainly; the JB Conversions parts are of the finest quality. The casting on the slip-yoke eliminator housing is super-thick and nickel-plated inside and out. The super-short rear output yoke is already clearanced for no binding of the CV. For about $1,040 we got a PN NP231PMRBLD partial rebuild kit ($125), a six-pinion HD planetary gearset ($250), a PN 2032 wide chain kit ($325), and a PN 1206 Super-Short Slip Yoke Eliminator for a Chevy-style VSS since we plan on running a Chevy LS engine ($340). Naturally, the company stocks Jeep versions as well.
The wide chain kit replaces...
The wide chain kit replaces the factory 1-inch wide chain and sprocket assembly with upgraded 1 1/4-inch parts. The kit includes the wider chain, mainshaft sprocket, and front output shaft gear. The wide chain kit can be installed in a stock T-case without the planetary or short shaft kit, so if your chain needs replacing it may be a good time to upgrade.
On the left is the factory...
On the left is the factory three-pinion planetary and on the right is the six-pinion unit. New Venture Gear rates the three-pinion planetary at 600lb-ft and the six-pinion at 1,400lb-ft. Be aware that there are two different gear cuts on the pinion teeth, with the '94-earlier referred to as thick teeth and the '95-newer as thin. Despite the names, there really isn't any difference in strength, but you'll need to know the year of your case when ordering.
The new 32-spline mainshaft...
The new 32-spline mainshaft included with the JB Conversions Super-Short Slip Yoke Eliminator is a full 9 inches shorter than the factory shaft and can accommodate just about any driveshaft yoke or flange. The heavy-duty tailhousing is heat-treated to T-6 specs and features a thicker cross section than some other units on the market and includes a support boss for the oil pump. In short, it's about the best you can do.
The only specialty tool you...
The only specialty tool you need to fully rebuild an NP231 is a nice set of snap-ring pliers. After removing the front yoke, rear tailhousing, and separating the case halves we stripped the T-case of its guts. You'll need to remove the input shaft snap ring and gently tap the planetary out with a dead blow since the input shaft is a slight press fit into the case bore. The input shaft is inserted into the new six-pinion planetary and then can be installed into the case.
The new 32-spline mainshaft...
The new 32-spline mainshaft can then be loaded with the synchro assembly and new 1 1/4-inch chain sprocket. A nice trick is to use the yoke to hold the shaft assembly upright to make installing the snap rings a little easier. New snap rings and bearings are installed from the rebuild kit at every step where applicable.
On '88-'89 YJ T-cases such...
On '88-'89 YJ T-cases such as ours, the shift fork rod must be cut down from 10.20 inches to 9.38 inches to fit the new tailhousing. The cut is made on the end that points towards the yoke, not the input shaft. We measured carefully and made the cut with an angle grinder with a cutoff wheel, then smoothed the cut end with some Emory cloth.
Here's the finished product...
Here's the finished product with the new six-pinion planetary, 1 1/4-inch wide chain, fresh bearings, and new seals. We applied a very thin skim coat of RTV to the case halves, and then bolted it all together. The blue tape in the lead photo is where the GM-style VSS sensor is inserted, which hadn't arrived as of press time. We ordered PN VSS224 from our local NAPA, but the JB Conversions NP231J kit uses the factory '92-up Jeep VSS sensor.