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. Popular Light-Duty GM Truck Automatic Transmissions Model 1st 2nd 3rd 4th Crawl Ratio* TH350 2.52/2.75 1.52 1.00 N/A 50.16 TH400 2.48/2.75 1.48 1.00 N/A 50.16 TH700R4 3.06 1.63:1 1.00 0.70 55.81 4L80E 2.48 1.48 1.00 0.75 45.24 *With NP205/Doubler (4.00:1), 4.56 axles CRC Performance Transmissions did several test-fittings. Clearances for the transmission and Doubler with its optional three-stick linkage were checked and rechecked.CRC Performance Transmissions did several test-fittings. Clearances for the transmission a The goal is to have the rear output yoke point about 3 degrees downward. This can mean raising the transmission’s bellhousing end higher, adding a body lift, or shimming down the crossmember.The goal is to have the rear output yoke point about 3 degrees downward. This can mean rai Lowering the crossmember is easier, but maximizing ground clearance is preferable. CRC modified the floorpan to get the TH400/Doubler’s nose higher in the hole. Here, the tunnel is bent up using a jack and a block of wood.Lowering the crossmember is easier, but maximizing ground clearance is preferable. CRC mod CRC owner Craig Calkins sectioned the floorpan to clear the Doubler’s linkage and to allow the new geartrain to be mounted without lowering the stock crossmember below the frame. The truck’s full cage and integral seat mount limited the sheetmetal’s upward mobility.CRC owner Craig Calkins sectioned the floorpan to clear the Doubler’s linkage and to allow The optional three-stick shift linkage enters the cab farther back than it did behind the TH350. CRC was able to use the factory pivot points and get the sticks to clear the dashboard and rollcage by setting them to their shortest throws. We’ll patch the tunnel during the interior part of the revival.The optional three-stick shift linkage enters the cab farther back than it did behind the The stock crossmember was reused but repositioned on the frame. We’ll measure for new driveshafts once the suspension and tires are in place.The stock crossmember was reused but repositioned on the frame. We’ll measure for new driv SOURCES TCI Automotive 151 Industrial Drive Ashland MS 38603 888-776-9824 www.tciauto.com John Kilgore racing Transmissions 818-767-4480 www.kilgoretrans.com/ Coast Driveline & Gear 2457 Palma Dr. Ventura CA 93003 800-533-8087 www.coastdriveline.com Off Road Design 970-945-7777 offroaddesign.com/ CRC Performace Transmissions 805-497-0399 « | 1 | 2 | View Full Article By Tom Morr Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!