Building the Bombproof TH400

Building the Bombproof TH400

I admit 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 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 the last time out. It was time to look up Gearstar.

I found Gearstar online (www.gearstar.net) and clicked 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 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.

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.

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.

Since this Gearstar TH400 transmission was getting the Level 4 package, it gets all the best bells and whistles, like this 300M full-billet clutch hub (left) rated to take over 1,000 hp. The stock cast 400 OE forward clutch hub (right) 300M is only rated to handle 500 hp.

The Level 3 and 4 builds use a high-capacity clutch assembly with Raybestos racing blue frictions and hardened Kolene steel plates. These racing frictions and steels have a much higher heat tolerance than OE material.

Every transmission built at Gearstar is handled by a single technician, start to finish. Here, the tech is installing the heavy-duty third clutch drum into the transmission case.

A 300M full-billet forward drum and 300M heavy-duty forward input shaft assembly are standard with the Level 4 build. Note the larger center diameter of the upgraded Gearstar shaft (left) versus the stock one (right).

Here is Gearstar Performance's heavy-duty third clutch drum with extra-wide heavy-duty 34-element sprag assembly and 300M hardened race (left) next to the stock drum with spring and roller assembly (right). A stock drum with spring and roller is only rated to 500 hp, while the upgraded Gearstar assembly is rated to 1,000-plus hp.

Heavy-duty carbon fiber bands with reinforced apply lugs in the Level 4 builds have wider lugs (left) than the stock low and reverse bands (right).

For increased power and load demands on the transmission, Gearstar adds a 300M full-billet main shaft (right) to the hi-po build instead of using a stock-style main shaft (left).

A new high-flow high-volume pump assebly with new gears is standard with every level build. Our Level build gets a hardened gear in the assembly, though.

After the vavlebody is installed, all bolts are torqued to exact Gearstar specifications.

Final installation of an extra-capacity heavy-duty cast aluminum pan puts the finishing touches on all Level 3 and 4 Gearstar performance transmission packages.

Gearstar ensures that every transmission and torque converter package that leaves their AKron, Ohio, shop is fully functional by dyno testing every unit before it's shipped out.

When we broke the crate open at South Bay Truck, there was a new Geasrtar Level 4 TH400 transmission, two Hayden transmission coolers, a new dipstick, and ATF to fill the transmission the rest of the way once it was installed.

Every Level 4 Gearstar Performance transmission comes with a 10- or 11-inch six-bolt heavy-duty torque converter that is matched specifically to the transmission it's installed in. You can order the torque converters with a 2,600 to 4,500 rpm stall.

Gearstar uses a factory housing that has all its fittings in the stock locations so it will bolt directly in place of any other TH400 transmission.

Kenny and Frank at South Bay Truck made the quickest work they could of the TH400 transmission removal and replacement. They had to deal with two drivetrain crossmembers, two transfer cases, and one rear suspension crossmember to be able to get the transmissions in and out. Luckily for us, they've seen all kinds of bad drivetrain situations and always seem to find a way to get the job done.

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 quite 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.

Archived Articles