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Nuts & Bolts: Overdrive Square-Body

Posted in How To: Tech Qa on October 7, 2018
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I have a 1977 Chevy K20 with an SM465 and an NP205 transfer case. I would like to replace this setup with a Tremec five- or six-speed transmission to gain an Overdrive ratio. Could this be done using an older divorce-mounted transfer case? I know the rear driveshaft would be much shorter than it is now (not a bad thing). The front driveshaft would be substantially longer and might require a support bearing similar to the one used on the current rear driveshaft. I’d also have to fab up crossmembers to support everything. Has anyone ever done a conversion like this? Any guidance is appreciated.
Larry A.
Via nuts@4wor.com

Tremec transmissions have a solid, well-deserved reputation in the car world. They’re strong, lightweight, and versatile. Their short-shifting patterns are awesome, but they aren’t well suited for trucks for several reasons. First, their gear ratios are less than ideal for trucks, with the lowest available First gear ration of 2.97:1. Compare that to the SM465 6.55:1 First and 3.58:1 Second. This high gear ratio doesn’t do anything to get a heavy truck with tall tires moving, nor does it do well with low-range crawling. Second, trucks are heavy and they are often used to haul and tow heavy loads for a long time. The Tremec transmissions weren’t designed for that. Third, we’re not aware of any way to attach a transfer case to a five- or six-speed Tremec tranny other than Rockland Standard Gear (rsgear.com), a company that at one time was advertising adapters for later-model transfer cases like an NP231 and NP241. However, this was some time ago and there is no information on the company’s website now, making us suspect the project was abandoned—not surprising because once again, the ratios are pretty far off for a truck application.

All that said, you could do exactly as you propose and run a T-56 with a divorced transfer case. You would be on your own when it comes to fabricating mounts for the case, and since Tremecs are pretty long (up to 27 inches), you might run into driveline issues if your truck is lifted. You would have to find a divorced NP205 because you won’t be able to use your own, and those don’t exactly grow on trees in the passenger drop you need. The intermediate shaft also means an extra driveshaft to maintain.

If we were in your shoes and wanting a transmission with Overdrive that didn’t shift like a dump truck, we’d take a hard look at an NV4500. Used in GM and Dodge trucks in the 1990s to the early 2000s, they have the proper gearing (including Overdrive) and torque capacities for your 3/4-ton truck, and they have a substantially shorter throw. They can’t be shifted fast, but we’d consider them halfway between a traditional truck tranny and a car tranny. They can also be easily adapted to your existing NP205 with help from Advance Adapters (advanceadapters.com) or Offroad Design (offroaddesign.com). Oddly enough, finding a donor tranny from a Dodge is better than a Chevy in terms of adapting, and the Dodge version is much more common. For all but maybe drag racing, the NV4500 is really the better and most likely less expensive way to get Overdrive.

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