Four Wheeler - - Techline -

QWhat would be the best truck to buy and get as many good parts to build a rock­crawler with 40-inch tires? My wish list would be a 14-bolt rear axle, NP205 trans­fer case so I don’t have to buy an At­las 2, and a Dana 60 front axle. I’m as­sum­ing I’d in­vest in heavy-duty end forg­ings and king­pin high-steer knuck­les for the Dana 60. I’d like to run 5.71:1 ra­tio ring-and-pin­ion gears to avoid hav­ing to buy a dou­bler for the trans­fer case. I’ll up­grade the drive­shafts to at least 1350 U-joints. Since my knees are shot, I’d like to have an au­to­matic trans­mis­sion with a granny-low First gear. Thanks for all you do. I’ve been an avid reader for many years.


Given your list of de­sir­able parts, it sounds as though you should start with a 1-ton truck. I am as­sum­ing the frame and body will get tossed and the parts will make their way onto what­ever rock­crawler chas­sis you have planned. Dol­lar for dol­lar, it will be pretty hard to beat an ex-mil­i­tary M1008 11⁄4-ton GM truck or any of its de­riv­a­tives. These came with a king­pin Dana 60 front axle, GM 14-bolt rear axle, and the bombproof TH400 trans­mis­sion. The only hang-ups are the NP208 trans­fer case, which can be swapped out rel­a­tively eas­ily, and the pa­thetic 6.2L diesel. You can usu­ally pick up a non-run­ning M1008 for less than the cost of the parts. The sec­ond op­tion would be the ’77-’91 GM K-se­ries 1-ton truck. This truck would have come with the same Dana 60 front and 14-bolt rear solid axles as the M1008, but could have been had with a gas V-8 en­gine. Some even have a big-block 454. Most of the 1-ton GM square-body trucks came with a TH400 au­to­matic trans­mis­sion backed up to the NP205, but the NP465 four-speed man­ual trans­mis­sion was also avail­able. The TH400 doesn’t re­ally have a granny-low First gear, it’s only 2.48:1, but TCI (tci­auto.com) of­fers 2.75:1 and 2.97:1 af­ter­mar­ket First gear ra­tios that can get you a lit­tle bit lower.

The Dana 60 front axle has gear ra­tios avail­able down to 7.17:1; how­ever, the 14-bolt only has gear ra­tios avail­able down to 5.38:1. With 5.38:1 as your deep­est avail­able axle ra­tio and a stock TH400 First gear, you’ll be look­ing at a crawl ra­tio of about 26:1, which re­ally isn’t low enough. Switch­ing to a Dana 60 or Dana 70 rear axle and 7.17:1 ra­tio axle gears would get you a crawl ra­tio of about 35:1, which also still isn’t all that low. You may need the trans­fer case dou­bler to get the crawl ra­tio into a suit­able range. You’ll want to aim for at least a 50:1-60:1 crawl ra­tio with an au­to­matic trans­mis­sion for ex­tra-heavy rock use. A low crawl ra­tio like this will give you the slow speed con­trol you need in the boul­ders and help keep the slush­box trans­mis­sion from over­heat­ing.

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