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The Transfer Case - Tech

Posted in How To on July 1, 2009
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Just as Dana 60 axles have found their way into small and mid-size 4x4s, heavier-duty transfer cases are also appearing in these vehicles in greater numbers. Heavy-duty, however, is a relative term. For example, some Suzuki Samurai owners upgrade to "heavy-duty" Toyota drivetrain, which is certainly a heavy-duty option when paired with the 1,300cc Suzuki engine. The NVG241OR used in late model Jeep Wranglers is finding its way in to earlier model Wranglers as a replacement for the NP231, and even the Toyota crowd is exploring use of later-model transfer cases found in Tacomas and FJs. Branching even further toward heavy-duty are the many aftermarket transfer case offerings, such as those available from Advance Adapters, JB Conversions, and STaK, to name a few. The new is merging with old and many 4x4s are reaping the benefits. Whether your 4x4 is a Jeep CJ-5, Toyota 4Runner, Suzuki Samurai, or whatever, you can build a competent transfer case to sustain your vehicle through countless trail adventures. Take a look at this overview of some of the more popular transfer cases -- both new and old -- that we see on the trail these days.

Aftermarket T-Case Guide

Tera Low231
The Tera Low231 transfer case is a complete replacement case half with the appropriate input shaft already in place. New mounting studs are also included. The Tera Low231 4:1 transfer case is equipped with a true 4:1 low ratio that operates through a heavy duty five-pinion planetary gear set. The Tera unit bolts directly to the factory front case half and adds a good degree of strength to the vehicle's 4WD system. The Tera case offers a higher input torque rating than stock and features thicker case walls to withstand cracking or fractures. Teraflex also offers its Tera 231 Extreme Short Shaft (ESS) kit and Tera 2Low231 kit to further increase strength and capability.

Advance Adapters Atlas
Advance Adapters offers its Atlas II and Atlas 4SP transfer cases for use in most 4x4s. The Atlas II transfer case is available with six different low range gear ratios, including 2.0:1, 3.0:1, 3.8:1, 4.3:1, 5.0:1, or 6.0:1. The high ratio is 1:1. This broad range of gearing allows the Atlas II to meet the needs of many applications, even in vehicles where excessive driveline length can create rear driveshaft issues. The Atlas II features a CNC-machined one-piece case crafted from 356-T6 heat-treated aluminum. The Atlas 4SP is a two-piece unit that includes a main case that it is similar to the standard Atlas case and a reduction housing that sits in front of that. The main case is actually the same raw casting as the Atlas II but features a variety of machining differences to adapt it to be 4-speed. The reduction housing uses a planetary assembly with a low gear ratio of 2.72:1 and 1:1 in high range. Available low range ratios include 2.0:1, 2.72:1, and 5.44:1, or 2.72:1, 3.8:1, and 10.34:1.

Advance Adapters Orion & Orion HD
The Orion and the Orion HD are designed to replace the factory transfer cases found in Toyota Land Cruisers and are offered with two different input spline choices to mate it to the 10-spline three-speed trans or the 16-spline four-speed unit. Though the Orion case is slightly larger in diameter than the stock TLC transfer case it is exactly the same length and the bolt pattern is the same. The Orion kit includes a new cast-iron case, four new gears with a 4.0:1 Low ratio, a new larger cluster pin, and a complete gasket, bearing, and seal kit. It also requires some parts from the factory transfer case to complete the installation. The Orion HD is a complete version of this transfer case that is fully assembled and ready to bolt-in. The HD unit features larger 1-3/8-inch diameter, 32-spline front and rear output shafts, a new shorter front output housing, an electronic speed sensor, and an adapter plate option that allows the HD unit to be installed in early Jeeps. A 3.0:1 low gear ratio offering is also available for the Orion and Orion HD transfer cases.

STaK 4x4
STaK 4x4 transfer cases are CAD-designed and CNC-built using 356 heat-treated aluminum. Each of the available units is designed to be consistent with factory t-case offerings so it would satisfy many applications. STaK cases can also be clocked 360 degrees in four degree increments so it will bolt up to most transmissions. All Monster Box units are offered in left or right hand drop configurations; use a heavy-duty 32-spline front and rear output shaft; can be equipped with an electronics speed sensor; and are available with 1310, 1350, or 1410 CV-style yokes.

The gears are constructed from heat-treated 8620 alloy and ride on 8620 carburized shafts and Timken and SKF tapered roller bearings. Multiple STaK 4x4 transfer cases are available to choose from. The Monster Box is a three-speed t-case with gear options of 5.44:1, 3.05:1, and 1:1; 4.33:1, 2.43:1, and 1:1; and 3.75:1, 2:10:1, and 1:1. The Monster Box 2 is a two-speed unit that offers a choice of Low gear ratios of 3.75:1, 4.33:1, or 5.44:1.

The Monster Box B is a three-speed transfer case designed for vehicles with narrow frame rails, such as early Ford Broncos and custom rock buggy applications. There is also a Mini Monster three-speed `case that can be fit with TJ-specific shifters.

JB Conversions' LoMax 205
The LoMax 205 from JB Conversions uses a nodular iron casting machined to exact tolerances taken from the original New Process drawings. The LoMax case also offers more internal ribbing and wall thickness than the original NP205. It offers a 3:1 gear set that is stronger than the original New Process gears and with a wider tooth width. The LoMax 205 uses 32-spline front and rear output shafts with 1350 yokes. The LoMax is available completely assembled and ready to bolt in to your vehicle and also as a kit version that requires assembly. It offers a GM six-bolt or GM figure-eight (race track) mounting pattern and can be clocked for repositioning.

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Stock T-Case Guide

Usage: 1984-1987 Jeep Cherokee, 1987 Jeep Wrangler
Notes: Chain-driven unit with aluminum case, used both 21- and 23-spline input shafts, drivers-side front drop, 2.61:1 low range.
Weaknesses: Many. Factory slip yoke is excessively long and prone to failure and leakage. Weak case, low gear ratio is lacking.
Strengths: None. Replace with NP231 for best results.

NP231 transfer cases' can be trusted for difficult to extreme trail use but it fairs much better when equipped with any of the available popular upgrades. The unit shown uses a JB Conversions Super Short SYE. This alone is enough to greatly enhance its trail prowess and reliability but the Tera Low231 half case with 4.0:1 low gears lends it even greater capability.

Usage: Jeep Wrangler YJ (1988-1995), TJ (1997-2006), and JK (2007-present), Cherokee, and Grand Cherokee.
Notes: Chain-driven unit with aluminum case, also known as Command-Trac. Uses both 21- and 23-spline input shaft spline counts in short, medium, and long shaft lengths. Driver's side front output, centered rear output. Planetary gear sets use a three-pinion assembly.
Weaknesses: Factory slip yoke is excessively long and prone to failure and leakage. Factory 2.72:1 low gear ratio is somewhat high for extreme usage. Shifter is problematic and tends to slip out of gear. No 2WD low ability.
Strengths: Versatility and parts availability. When outfit with a SYE the NP231 is a sturdy 'case. It's even better with 4:1 low gearing. This is a great T-case to build and use but after adding up cost for the best built unit it may be cheaper to install an NVG241OR or an Atlas or STaK purpose-built unit.

The NVG241OR Rock-Trac transfer case is the crme de la crme of modern Jeep transfer cases. With a 4.0:1 low gear ratio and short rear output it is a major factor that aids the Jeep Wrangler JK's off-road capability.

NVG241OR Rock-Trac
Usage: Jeep Wrangler TJ (1997-2006) and JK Rubicon (2007-present).
Notes: Offers 4.0:1 low gear ratio and a fixed yoke rear output. Aluminum case with built-in strengthening ribs in the rear half of case to withstand increased torque of low gears. It is chain-driven and uses a 23-spline input gear.
Weaknesses: Not many. This 'case is purpose-built for increased strength and ability.
Strengths: Many. This is an awesome T-case for all Jeep applications.

Usage: Many GM, Ford, Dodge, and International full-size truck applications from late 1960s through 1993.
Notes: Heavy duty gear-driven unit with cast iron case. Used circular six- and eight-bolt mounting bolt patterns and also a circle-eight or "racetrack", eight-bolt mounting pattern depending on year and vehicle model. Both drivers- and passenger-side front drop configurations. Used both male and female inputs with various shaft spline counts depending on year. 1.98:1 low gear ratio
Weaknesses: Low gear ratio is lacking, heavy unit for smaller rigs.
Strengths: Very heavy-duty. Excellent for heavy 4x4s with V-8 powertrain.

Dana 300
Usage: '80-'86 Jeep CJ-5 and CJ-7.
Notes: Jeep and Scout versions are available and are not interchangeable due to bolt pattern. Short and long versions also exist but the short version is considered stronger. Offers 2.62:1 low gear ratio and an aluminum tail housing, a 1--inch intermediate shaft, and a 23-spline input shaft. It uses helically-cut gears in a cast iron case. The Dana 300 uses a passenger-side front output.
Weaknesses: Weak output shaft in all versions, tail housing is also prone to cracks, failure. Low gear ratio is lacking.
Strengths: Versatility, strength, and parts availability. A great transfer case for many applications. Can also be flipped and used for driver-side front output applications. Twin-stick capable. Aftermarket cases of greater strength are available as are low gear sets and front and rear output strengthening parts.

The Dana 18 is the granddaddy of Jeep transfer cases. It was used in many Jeep models from 1940 to 1971 and was also used in some Scout vehicles. We like the Dana 18 for its factory twin-stick operation and available PTO, but it's also a simple gear-driven assembly that can be serviced and repaired by most users with moderate mechanical skill. Many variations of the Dana 18 exist with differences in intermediate shaft length and input gear tooth count.

Dana/Spicer Model 20
Usage: Jeeps vehicles '62-'79, Ford Bronco, and Scout.
Notes: 2.0:1 low gear ratio. Uses a cast iron case with a six-spline, 29-tooth, 1--inch intermediate shaft. Passenger-side drop in the front and center output in rear. Variations exist in shift linkage with some early models equipped with twin-stick shifters. Ford Bronco version is driver-side drop at both outputs with 2.46:1 (early models) and 2.34:1 (later models) gear ratios.
Weaknesses: Low gear range is lacking. Tail shaft strength is questionable.
Strengths: Versatility and parts availability. Durable, compact, and lightweight. Twin-stick capable. A simple yet strong transfer case that has proven its ability in 1-ton Jeep J-trucks.

Dana/Spicer Model 18
Usage: Jeep vehicles up to 1971, Scout
Notes: Cast iron case with passenger-side drop in front and rear. Early versions use weaker -inch, six-spline intermediate shaft with later models upgraded to 1-1/4-inch 10-spline shaft. Variance in locating bore when mated to Buick V6 engine. Early Scout versions were equipped with twin-stick shifter. Gear-driven transfer case with 2.46:1 low ratio. Accepts PTO or overdrive. Strong and versatile transfer case. Will also accept Dana 20 gears.
Weaknesses: Low gear range is lacking.
Strengths: PTO capable, lightweight, and durable. Twin-stick capable. Easy to work on. Legendary.

Toyota transfer cases are extremely versatile and can be used in a variety of configurations. Marlin Crawler sets up Toyota 'cases in single, dual, and triple configurations to further reduce the low gear range and create the ultimate low crawl ratio.

Toyota Pickup/4Runner
Usage: '79-'95 Toyota Trucks and 4Runners.
Notes: Many variations between years. Both gear- and chain-driven units exist. Gear cases use seven bolts in rear case half; chain cases have five bolts. As a general guide, '79-'83 cases used top-shift gear-driven units and '84-up rigs used forward shift gear and chain-driven units (depending on engine and transmission). Later models also use an oil cooler that is plumbed to rear of case. Early models are passenger-side front output; later models use driver's side front output. Gear ratios include 2.28:1, 2.57:1, and 2.66:1.
Weaknesses: Low gear range is lacking. Front and rear input/output shafts are of questionable strength. Weak die-cast aluminum case.
Strengths: Versatility and parts availability, twin-stick ability, dual, triple, and crawl box capable.

Toyota Land Cruiser
Usage: Toyota Land Cruiser
Notes: Many variances in case design and internals. Low range ratios include 2.31:1 (pre '74), 1.99:1 (1974), 1.96:1 ('75-'80), 2.28:1 ('80 -'83/'84), 1.96:1 ('83 and up). Aluminum one-piece case version uses both 10-spline (early) and 16-spline input shafts. Strongest models are 1980 and up units with two-piece split aluminum case with 19-spline input shaft.
Weaknesses: Pre-1980 aluminum case is weak and prone to failure. Low gear range is lacking.
Strengths: Aftermarket cases of greater strength are available.

This Suzuki transfer case is shown with a Mighty Kong mount available from Trail Tough. A rear driveshaft disconnect is also in place. Numerous Low gear options are also available for Suzuki 'cases.

Suzuki Samurai
Usage: Suzuki Samurai
Notes: Minor changes to overall design through years. Low gear ratio of 2.268:1 for all U.S. models. Gear-driven unit uses cast-aluminum three-piece case with 26-spline input shaft. Outputs are passenger side at front and rear.
Weaknesses: Mounts are weak and prone to failure. Low gear range is lacking. Case strength is questionable when lower gears are installed.
Strengths: Parts availability. When outfit properly the Suzuki T-case performs rather well in Suzuki vehicles.

Nissan TX-10
Usage: 1986-2004 Nissan truck and Xterra, 1988-1995 Pathfinder.
Notes: Part-time, chain-driven two-speed transfer case, three-piece aluminum case, 30-spline input shaft (four-cylinder trucks use a 22-spline input shaft), 2.625:1 Low gear ratio, 32:1 crawl ratio. TX-10 mated to automatic transmission featured an oil sill around the input shaft to maintain lubrication. Differences in wiring harnesses and speedo sensors exist through years of use. Most 'cases are interchangeable but can only swap a T-case behind an auto transmission with another auto trans T-case due to oil sill.
Weaknesses: Low gear range is lacking, lower gears may weaken case assembly
Strengths: Versatility and parts availability

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