Ultimate Revival: UA Chevy K10

We’re recycling wherever possible. Our old K20 project donated a TH400 backed by an Offroad Design Doubler, a transfer-case setup that combines an NP203 and an NP205 to produce a 4.00:1 low-range. Swapping this heavier-duty drivetrain into a regular-cab Stepside requires some finesse.

We’re recycling wherever possible. Our old K20 project donated a TH400 backed by an Offroa

Last month we introduced our latest challenge, rehabbing the Ultimate K10. The truck attracted substantial attention in 2005 and 2006 then fell on hard times. Many of its premium parts were cherry-picked for other projects. In high-on-the-hog times, we might have put Old Yeller out of its misery. After all, its high-end, high-pinion Dynatrac Dana 60 frontend, Off Road Unlimited disc-braked Chevy 14-bolt rearend, and NP205 with Off-Road Design Doubler were long gone. So were the winch, Optima batteries, seats, steering column, steering box, bumpers, and more. A saner, higher-heeled approach might be to pull the RamJet 350 and TCI TH350 and put them in a truck that’s more together.

However, this truck is somewhat infamous. We have sweat equity in it, and lean times call for making do with what you already have (makes for better stories, anyway). As much as we’d like to do a Six Million Dollar Man better/stronger/faster rebuild, we ain’t gonna attempt to fix things that ain’t broke. Also, we’re using obtanium whenever possible to get the Ultimate K10 back up and running.

The K10’s existing TCI Truckmaster TH350 (foreground) was trail-prepped with a 2.75:1 First gear and an extra-capacity oil pan. It originally had a Doubler, like the TH400 that’s replacing it (background).

The K10’s existing TCI Truckmaster TH350 (foreground) was trail-prepped with a 2.75:1 Firs

Gearing Options
On Ultimate Adventure 2005, the K10 crawled over some of the best trails in Missouri, Oklahoma, and Arkansasand got between states on the interstate. The geartrain consisted of a TCI Truckmaster Turbo 350 with a low 2.75:1 First gear (stock is 2.52:1) and a finned deep pan. It handled the RamJet 350’s manufacturer-rated 350 hp and 400 lb-ft of torque without any issues. Gear reduction came from an Offroad Design Doubler, giving a 4:1 low range. Motive Gear 5.13 ring-and-pinions yielded a 56.13:1 crawl ratio. This combo passed all its hardcore tests. The tradeoff was some high-revving whine at highway speed running 39-inch BFGs.

Since this time we want to tow more and crawl less, roadability is a higher priority (37-inch tires will be used). We benched-raced with drivetrain expert Craig Calkins of CRC Performance Transmissions. He locked onto tow and as simple as possible and suggested swapping in the Towasaurus Turbo 400 he built for an article in the Mar. ’09 issue. That transmission is mated to a Doubler in our red ’87 K20. Swapping in the TH400 (which was built with a Kilgore 2.75:1 First)/Doubler would give us a great towing and trail package.

If we were starting from scratch and budgets weren’t as tight, the optimal half-ton setup for our goals might be a TH700R4 mated to a Doubler. Stephen Watson, owner of Offroad Design and the Doubler’s creator, says, A 700R4 would be awesome to make it a little more friendly on the road. You’d want 4.88s with 37s, but I think it would be great on the highway.

The main drawback here is that we don’t have a 700R4 on hand, and in stock form the light-duty four-speed might not live a long life with a RamJet 350 upstream and an overloaded car trailer out back.

Readers should know that 700R4s can be built to take an advertised 600-plus horsepower using reinforced input drums and sun shells and a larger Fourth gear servo piston. Overall 700R4 advantages are a lower low and a higher high. First gear is more than 20 percent lower than the TH350 and TH400, Second gear is lower than both, and the top gear is 30 percent higher thanks to the overdrive Fourth.

Mating a 700R4 to a noncomputerized vehicle requires sorting out converter-clutch lockup, which is electronically controlled by the powertrain computer in some OE applications. The solution is a lockup wiring kit, such as the one offered by TCI. Also, the TV cable must be dealt with when swapping in a 700R4. (Transmissions custom-built with full manual valve bodies eliminate the lockup and TV issues.)

The electronic GM four-speed automatics and Allisons are also possibilities but are beyond the scope of this project. These swaps can require standalone controllers when swapped into vehicles that predate computers.

The TH400 bolts directly to the RamJet 350, but this transmission is bigger than the TH350 it replaces. Modifications are necessary to achieve the optimal driveline angle.

The TH400 bolts directly to the RamJet 350, but this transmission is bigger than the TH350

As we said in the Mar. ’09 issue, the Turbo 400 is the strongest of the GM light-duty three-speed automatics. Its length would be an issue in short-wheelbase vehicles, but we think we’ll have plenty of rear driveshaft space in our regular-cab shortbed, even with the Doubler added.

Performance TH400 specialist John Kilgore makes nonstock-ratio First gearsets ranging from 1.80:1 for drag racers up to 3.00:1 for maximum off-idle torque to the tires. CRC Performance Transmissions recommended Kilgore’s 2.75:1 set, mainly because the 3.00:1 can generate too much wheelspin in sand and snow. CRC owner Craig Calkins’ own K2500 tows a 13,000-pound fifth-wheel on weekends. It has a built big-block, a Kilgore 2.75 First-geared Turbo 400, 4.56 axle gears, and 37s.

Installation Notes
Swapping the TH350 for a TH400/Doubler wasn’t quite a straight R&R operation in the K10. Because the Turbo 400 is longer than the 350, the job required tweaking the tunnel and floorpan to get the Doubler’s rear yoke angled about 3 degrees downward. Our Doubler also has the optional three-stick linkage, so the trans tunnel had to be hogged out for linkage clearance. (This setup uses one stick to shift the NP203 and two for the NP205. Low-range front-wheel-drive is one of the available scenarios.)

A longer speedo cable is often necessary when using a Doubler. The system also has two breathers, which need to be plumbed high and dry to keep water out.

Once the K10 was buttoned up, CRC bolted a used NP203 to the TCI Turbo 350 and swapped it into the K20. Coast Driveline made a custom rear driveshaft so the K20 could drive out of CRC under its own power. As for the Ultimate K10, it left the same way it arrived: on a flatbed. Its next destination is its pre-magazine home, GM Truck Center.