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September 2007 4x4 Tech Questions - Techline

Willie Worthy | Writer
Posted September 1, 2007

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Four Wheeler
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Question: I'm thinking of swapping the NP203 transfer case in my '75 K-10 Chevy for an NP205.

My truck has a 6-inch suspension lift, 35x14-15 Gumbo Mudders, a 350ci engine, and a TH350 transmission.

I know I need an adapter plate, but will my driveshafts have to be modified? What kind of shift linkage do I need? Where would you find an adapter plate?
Josh Henning

Answer: Yes, you will need a new adapter, and you're in luck in that the 350 transmission and 203 transfer case share the same coupler sleeve spline as the 350/205 combination. You will also need some new shift linkage. Off Road Design ( has new aluminum adapters, or they may be able to hook you up with a used factory cast-iron adapter. They also have some pretty trick shift linkage.

As to the question of needing new driveshafts, Steven Watson of ORD tells me that the front shaft will have to be 3 inches shorter and the rear 6 inches longer.

You might also want to consider keeping your 203 and combining it with a 205. This "doubler" will give you both a 2:1 and a 4:1 low range. With this combination of two transfer cases, the rear driveshaft will be the correct length and the front shaft will need to be 3 inches longer.

Question:I just opened up my own 4x4 shop, mainly dealing with the sales, and some installations. However, I've never set up a rear end and would like to learn the proper method and a source for the necessary tools. Yes, I have some service manuals but some leave a lot to be desired, such as saying to use factory tool numbers such-and-such.

I know that there have been lots of magazine articles about how to set up rear ends, but is there a book available that takes one through it step-by-step, and could you direct me to a source for the proper tools?
Name And Address With Held By Request

Answer: Up until a month or so ago, I would have told you to stick with service manuals, but Jim Allen has just come out with a 378-page book on how to rebuild, identify, and modify differentials and axles: Differentials: Identifications, Restoration and Repair. Jim wrote this book in conjunction with Randy Lyman, owner of Randy's Ring & Pinion. Jim is a well known and respected book author and the book is full of great information. I just checked, and one source of availability is Or they are available from Randy's (800/292-1031,

Question: I have an '05 GMC 1500 Sierra 4x4 Z-71, which came stock with 265/70R17 tires and 3.42:1 gears. I recently changed the tire size to 285/70R17 and also changed gears to 4:10:1. The shop that changed the gears recommended a Hypertech programmer to fix the odometer and the ABS light. I purchased a Hypertech and it did fix my speedometer, but it didn't fix the ABS light-the "Service Brake System" error code is still coming on. I called Hypertech, and the tech guy said that the programmer wasn't going to fix my problem.

After the programmer didn't work, I called GM. The guy told me I needed to change my cluster. This seemed a little strange to me, so I am seeking advice for a better solution.

Answer: Greg Hooker, parts manager for a Colorado Chevy dealership, offers the following:

"I would take the truck to the dealer and have them scan it and find out what is setting off the code. If the code was set off by false information from the tires and gears, and the Hypertech corrected that info, the computer should reset itself after so many key cycles.

"You may have another problem or possibly haven't completed enough key cycles to clearthe code. I'm not sure about the cluster exchange, as these vehicles are getting more and more complicated."

Question:I have asked several service departments at various dealerships and have gotten contradictory answers. My '02 GMC Sierra 2500 HD 4x4 has the 6.0L Vortec with about 180,000 miles. I am curious if it is possible to drop in an 8.1L with the Allison five-speed tranny when the time comes. Your thoughts?
Shawn Newton
Birmingham, AL

Answer: I don't see any reason why you couldn't make the swap. However, you'd better have a bucketful of gold coins. Not only will the engine and transmission be costly, but the transmission uses its own computer to "talk" back and forth with the engine's computer. This means lots and lots of wiring changes. It's not something that I would want to undertake, but sure, it could be done.

I think that you would be money ahead to just sell the truck and buy one with the larger engine and transmission. My second choice would be to rebuild the motor you have when the time comes, and perhaps add a few performance-enhancing aftermarket items such as headers and an exhaust system.

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