The New Process 231 transfer case is one of the most common and durable transfer cases ever produced. Found under Jeep Wranglers, Jeep Cherokees, Dodge Rams, and Chevy S-10 Blazers and pickups, the NP231 has proved itself reliable under some of the most hardcore rigs.
From the factory the NP231 is fitted with a 2.72:1 low range planetary gearset. On most stock applications this low range ratio is very sufficient, as it offers enough low-speed control for crawling while not being so low that you can’t create wheelspin for hillclimbing and mud bogging.
The new Tera Low 231 HD front case comes with the 4:1 five-gear planetary gearset already
The 231’s cast aluminum housing and slip-yoke rear output have long been its downsides. Fortunately, dozens of slip-yoke eliminator kits are available from the aftermarket and protecting the lightweight aluminum case is usually as simple as building a skidplate.
So what else would one want to do with such a well-rounded case?
Once your original 231 is torn apart you’ll begin transferring over your stock drive gear
Well, once you’ve fitted your truck or Jeep with heavy aftermarket bumpers, larger knobby tires, and traction aids like selectable or automatic lockers, you may find that the 2.72:1 low range isn’t enough gear reduction. Sure, a V-8 under the hood could take care of most of your power woes, but that simply isn’t a cheap option. Take our ’00 Jeep Wrangler for instance. Powered by a 2.5L four-cylinder, AX-5 manual transmission, and an NP231, the Jeep works well but is often down on power off-road.
Like most rigs, the Jeep was fitted with numerically higher 4.88 differential gears to help compensate for the larger-than-stock, 36-inch-tall tires. Though the lower axle gears help, the Wrangler still heavily lacks in low-end torque. So to provide the Wrangler with more grunt, we opted for a 4:1 Tera Low 231 HD transfer case upgrade. The Tera Low 231HD case from TeraFlex replaces your 231’s original front half with a cast aluminum unit that comes fitted with a heavy-duty five-gear 4:1 planetary gearset.
Since our original 231 front output bearing was in fine working order we transferred it to
Increasing the low-range ratio from 2.72:1 to 4:1 effectively works as a torque multiplier, allowing the Jeep to crawl more easily in low range.
To perform the upgrade, we dropped the case from the Jeep and hit the work bench for an afternoon of wrenching. It took us roughly half a day from start to finish, and once we shifted into low range we noticed an immediate power difference. Though our 2.5L may not be a powerhouse, the extra gear reduction has given our Jeep the performance it lacked and confidence to take on more challenging terrain.
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The new Tera Low 231 HD front case comes with the 4:1 five-gear planetary gearset already
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Once your original 231 is torn apart you’ll begin transferring over your stock drive gear
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Since our original 231 front output bearing was in fine working order we transferred it to
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When you engage the transfer case shifter a shift fork moves a slide collar and engages th
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Once the gearsets align, the chain feeds power to the front output, causing the yoke to tu
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The NP231 chain is the most important part of the case. Over the years chains can stretch,
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After cleaning off all of the old gasket material from the original back half of the case,
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One area to be cautious with both before and after mounting the case halves is the oil pum
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Instead of a mechanical speedometer pickup, the newer 231 cases use an electronic sensor a
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The AA SYE kit allows you to have a stronger fixed rear output. This also gives the Wrangl
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To install the new transfer case mounting studs, we used the old double nut trick. To doub
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The new TeraFlex 231 HD setup is the same length and width as the stock 231. This means yo
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Advance Adapters
4320 Aerotech Center Way
Paso Robles
CA
93446
800-771-6171
www.advanceadapters.com
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TeraFlex
5241 S Commerce Drive
Murray
UT
84107
801-288-2585
www.teraflex.biz
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