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GM Truck Four-Wheel-Drive Conversion - Part 1

Posted in How To on May 1, 2006 Comment (0)
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GM Truck Four-Wheel-Drive Conversion - Part 1

Millions of pickups have been built without transfer cases. Many of them share similar architecture with their 4x4 siblings, and their owners often ask us what they need to convert their stock 4x2s to four-wheel drive. The conventional wisdom here has always been, "You'll be time and money ahead in the long run if you sell your truck and buy a 4x4 instead." But is it really true nowadays? For this article, we'll delve into the conversion process to better understand what's necessary to transform a two-wheeler into a 4x4 trail machine-and we'll add up the costs.

Off Road Unlimited of Burbank, California, recently developed a kit to simplify the process of converting two-wheel-drive GM pickups to four-wheel drive. Designed specifically for '88-'98 fullsizes, this kit includes all the necessary hardware to mount a solid front axle, leaf springs, and a transfer case. Lucky for us, ORU's owner, Maurice Rozo, gave us full control over this particular project, a conversion that he claimed was gaining curiosity among many would-be 'wheelers. Thanks to an impressive production run, it's quite probable that you or someone you know owns a 4x2 GM pickup and would rather have a 4x4. This might include that hand-me-down ranch pickup that doesn't get used during winter months, or perhaps a truck that one of your resourceful buddies scored a deal on at a local auction house. Either way, the vehicle likely has a venerable 350 V-8 under the hood, and probably has a decade's worth of good years left. Sound familiar? Follow along as we highlight two very different ways to make a pavement-pounder work in the dirt.

We started with a used '92 two-wheel-drive 1/2-ton pickup Maurice purchased for $2,500. Ideally, you would already own one, but we'll include the cost of the truck for reference.

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For the budget-minded individual, a good option for the front axle is the somewhat rare six-lug Dana 44 with flat-top knuckles. However, a more common equivalent is the Corporate 10-bolt (shown) out of a '77-'87 GM 1/2-ton pickup. The rear axle out of the same pickup-usually a Corporate 12-bolt (or 10-bolt, after '83)-would be the best option for the back. The average used price for a decent pair of 1/2-ton axles can vary, but as a general rule; don't pay more than $1,000 for both axles complete, and in good shape. You can save yourself a lot of headaches by checking things like the condition of brakes, bearings, and gears. Also, make sure both axles have matched gearing, which will save the cost of a new ring and pinion.

Due to the fact that the two-wheel-drive pickup transmissions won't mate to a standard New Process transfer case, it's necessary to locate a sibling TH700R4 out of a four-wheel drive. Luckily, these are readily available in most American salvage yards. From the GM dealer, these are priced right around $1,800 with a 3-year warranty. We've found these transmissions used for around $500, or rebuilt for around $1,200. For a transfer case, look for the NP205 or NP208 out of a '67-'87 1/2-ton Chevy pickup. Most of the wrecking yards that we contacted charge around $500 for one of these. We advise taking the time to pull off the PTO cover to have a look inside. Avoid cases with gears that appear rusty, have excessive wear, or chipped teeth. Be sure the transfer case you purchase is equipped with a passenger-side drop for the front driveshaft. If lifting the truck is part of your plan, consider an NP205 out of a '75-'76 GM 1/2-ton because the front output shaft will accommodate the higher-angle Spicer CV-style front driveshaft. Otherwise, any NP205, NP208 (up to '88), or NP241 ('89-'91) should suffice.

Here you can see how the Switch Kit looks (see arrow) when installed between a 700R4 and an NP205. Here you can see how the Switch Kit looks (see arrow) when installed between a 700R4 and an NP205.

Fitting a new or rebuilt 700R4 transmission to an NP205 without the proper OE adapter can be a headache. For this reason we picked up a simple Switch Kit adapter (PN TP7Q-35A) from Auto Matic Kings of Riverside, California. This adapter was built by Teckpac Fitzall, and provides the mechanical connection between the transmission and transfer case. This is nice because it adds valuable inches between the transfer case and the transmission. This helps provide additional clearance for a higher-angle CV front driveshaft if desired. Without one of these, the CV driveshaft would interfere with the transmission pan. It's important to note that a 1/4-inch section of the transmission's output shaft must be removed with a die grinder to make the Switch Kit fit properly. Detailed instructions come with the kit.

Once you've bolted the transmission to the transfer case, you will need to have a new pair of driveshafts built. We don't recommend trying to salvage used shafts from a wrecking yard unless you're a serious bean-counter. If you are, your time is best spent searching for shafts that came in the 4x4 version of the same model-year truck that your transfer case came out of. We didn't find any shafts that would fit our test mule, but we suppose if time were not an issue the correct shafts could be found. Most salvage yards will part with a pair of used driveshafts for under $50. Keep in mind that modifications may be necessary to make them work.

One area we strongly recommend using brand-new parts is the shocks. Used shocks found in a wrecking yard will deliver unpredictable results. An inexpensive set of new shocks will run somewhere in the neighborhood of $120. Leaf springs depend on what you plan to do with the truck. If you don't plan to lift the vehicle, a pair of used stock leaf springs off a '73-'87 Chevy truck will work. Typically, these leaf springs will run you less than $60 a pair from a salvage yard.

Depending on the condition of the truck, a different set of wheels and tires might be required because of axle lug pattern changes. We searched online and found a decent set of used aluminum wheels with 31-inch mud tires mounted to them. We were stunned to find that they still had about 70 percent tread left on them. If editors' salaries weren't so low, we might have grabbed them at the bargain asking price of $200. Consider checking out the classified section of your local newspaper, or local garage sales to find the best deal.

Don't forget little details like the transfer-case shifter boot. Pesky expenses like these can add up fast. We scored a transfer-case shifter and linkage along with our junkyard transfer case. However, flexible brake lines and additional breather hoses had to be purchased brand new. The ORU conversion kit did include the proper hard steel relocation brake lines, yet a few small pieces were required to make the system function properly. We categorized these as miscellaneous items, which for budgetary concerns we allocated $200.

Adding It Up Adding It Up
ORU conversion kit $989
Used Dana 44 front axle $500
Used Corporate rear axle $500
Used NP205 transfer case $500
Switch Kit $60
Used leaf springs $120
Driveshafts/modifications $300
Used six-lug wheels and tires (4) $200
New front shocks (4) $120
Miscellaneous parts $200
Total cost (minus labor) $3,489

As it turned out, we would have spent considerably more money for the parts we would have needed for the conversion than the purchase price of the truck. For would-be converters, whether or not the added expense is worth the effort depends on the size of your budget, your parts-scrounging ability, and your love of a challenging project. If you're still not deterred, we'll show you how to put it all together next month.

Sources

Off Road Unlimited
www.offroadunlimited.com
Auto Matic Kings
www.teckpak-fitzall.com

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