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Building The Bombproof Gearstar TH400 Transmission

Posted in How To on July 1, 2011 Comment (0)
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Photographers: Courtesy of Gearstar

Iadmit it. I’m a little hard on transmissions, and if you read this magazine often enough, you might have already noticed. In fact, it’s so bad that I was dubbed “The Tranny Killer” long ago, and the name has stuck ever since. And before you start to think I have some sadistic hatred for cross-dressers, I’ll come clean and confess to at least 19 transmission murders…and I’m sure I’m forgetting a few.

Knowing about my luck with transmissions, an ex-editor of OFF-ROAD magazine turned me onto Gearstar Performance Transmissions out of Akron, Ohio. It seemed a little far to go for a transmission build, but I’m always eager to learn about shops with claims like “unbreakable” or “world’s finest.” Besides, at this point in my tranny-killing career, I’m game to try just about anything if someone thinks it might hold up.

Over the last couple years, I have been nursing a well-ridden TH400 transmission in a ’74 K5 that probably should have been rebuilt after ripping the torque converter in half. It’s been patched back together a few times and kept sort-of running until the back of the housing actually broke off this last time out. It was time to look up Gearstar.

I found Gearstar online (www.gearstar.net) and clicked on the “TH400” link. There were three levels of TH400 builds: Level 2 ($1,395) will hold up to 450 hp; Level 3 ($2,195) up to 550 hp; and the Level 4 ($4,395) package will handle up to 850 hp. They come with bottles of ATF, fluid coolers, heavy-duty aluminum pans, and new dipsticks. Did I mention that all of this comes in a crate shipped free of charge within the continental United States? Oh, and there is no core charge. Admittedly, I was a bit skeptical—the prices and the quality of the build specifications seemed too good to be true.

I got on the phone and called Mark, one of the technicians at Gearstar, and the answer to my transmission issues. Mark and I talked about the way the truck is built, what kind of powerplant is sitting in front, what I have behind the transmission, my driving style, and how I like to use the truck. Though I could have gotten away with a Level 2 build, he recommended the Level 4, since I’ll be adding some more power to the fuel-injected big-block in the future.

1. The TH400 transmission originally in my Blazer had been a problem for quite a while. It started with a torn-apart torque converter, but there were other issues, as well. The final blow came when the rear of the transmission case snapped. There was no question that a completely new TH400 was in order.

Of course, I had to ask Mark what the deal was. And he told me the tale: The company was started by Zack Farrah after he chose six top builders (many working with Zack since the ‘80s) to join him at what would make up Gearstar Performance Transmissions—an employee-owned company where each transmission is built by a single technician from start to finish. Each torque converter is built to match the specific transmission it’s going in and each transmission and torque converter is run on a dyno before being shipped out. All this makes for low overhead, excellent customer satisfaction, and no returns…and this is how I can buy a complete high-performance package shipped free of charge for a great price.

I bit. How could I say no?

A couple weeks later, a crate with “Gearstar” stamped on all four sides showed up at South Bay Truck in Hawthorne, California, where a K5 Blazer was waiting to accept the new TH400. After a fairly laborious install (dual transfer cases and three crossmembers will do that), South Bay Truck had the transmission back in the truck, and I was ready to start putting the Gearstar TH400 to the test.

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Driving Impressions
With the new Gearstar TH400 installed in my K5, I hit the freeway and headed for home. Everything felt just fine, and the shifts weren’t too firm, but were still fast and crisp with no lag between gears.

Of course, I’m trying to take it easy for the first few hundred miles on this transmission, and I’m not quiet past that point yet, but when I get some real miles on this tranny, you’ll see a long-term update on how it has been holding up.

Gearstar TH400 Level 2 Level 3 Level 4
Max Power 450 hp, 425 lb-ft torque 550 hp, 475 lb-ft torque 850 hp, 650 lb-ft torque
Matched Torque Converter 12” or 13” heavy-duty high-performance furnace-brazed, 1,600-2,200 rpm stall 10” heavy-duty furnace-brazed flanged hub with billet cover, 2,600-3,000 rpm stall 10” or 11” six-bolt heavy-duty furnace-brazed flanged hub with billet cover, 2,600-4,500 rpm stall
Shift Kit Transgo shift kit Transgo reprogramming shift kit with Shift Command Transgo reprogramming shift kit with Shift Command or full-manual forward pattern OR Gearstar full-race shift kit
Bands and Internals Master overhaul kit with high-energy frictions and all new steel plates High-flow filter New kickdown solenoid Adjustable modulator New front band New Borg-Warner low-reverse band New Torrington bearings Bushing kit with flanged no-walk-out case bushing Thrust washer kit Master overhaul kit with Raybestos Stage 1 racing frictions and hardened Kolene steel plates High-flow brass filter New kickdown solenoid Adjustable modulator New front band New Borg-Warner low-reverse band with heavy-duty lugs New Torrington bearings Bushing kit with flanged no-walk-out case bushing Thrust washer kit Master overhaul kit with Raybestos Generation II racing frictions and hardened Kolene steel plates High-flow brass filter New kickdown solenoid Adjustable modulator New Kevlar front band New Borg-Warner low-reverse band with heavy-duty lugs New Torrington bearings Bushing kit with flanged no-walk-out case bushing Thrust washer kit
Clutches, Drums, Sprags, Springs, Rollers High-capacity direct clutch drum Heavy-duty forward clutch hub New low-reverse Spring & Roller High-capacity direct 8-clutch drum with 34-element intermediate sprag and hardened race Heavy-duty forward steel clutch hub Extra-capacity intermediate clutch assembly New low-reverse spring & roller High-capacity direct 9-clutch drum with 34-element intermediate sprag and hardened race Extra-capacity intermediate clutch assembly 300M billet forward clutch hub Reinforced forward drum assembly New low-reverse spring & roller High-rev spring kit
Fluid Pump High-volume high-flow pump assembly with new gears High-volume high-flow pump assembly with new gears High-volume high-flow pump assembly with new hardened gears
Shafts Stock shafts (optional shafts available) Stock shafts (optional shafts available) 300M main shaft 300M hardened input shaft
Transmission Pan Chrome pan with drain plug Extra-capacity cast aluminum pan with drain plug Extra-capacity cast aluminum pan with drain plug

Sources

Gearstar Performance Transmissions
Akron, 44308
330-434-5216
www.gearstar.net
South Bay Truck and 4x4
310-219-0727
http://www.southbay4x4.com

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