Level 1
Described by Zack as one step above stock, these transmissions are completely remanufactured using improved Alto frictions and a mild shift kit. They are capable of reliably handling 300-plus horsepower and are perfect for daily drivers.

Level 2
Gear Star's most popular offering, these transmissions are also rebuilt with all-new internal parts. In addition, the torque capacity of the clutch packs is increased by more than 70 percent, and a more sophisticated shift kit is used for a firm, positive shift. Horsepower capacity is 400-plus.

Level 3
Equipped with all hardened and stress-relieved hard parts-including shafts, drums, and planetaries-these transmissions are for rods that see street and part-time dragstrip use and are equipped with high-torque, 600-plus horsepower engines. Additional modifications include three different shift kits, expanded capacity clutch drums with Alto Red Eagle frictions, and Kolene steels.

Level 4
The King Kong of transmissions, these are built to withstand the horsepower a blown big-block can put out. Also popular behind Hemis and big-block Fords, these transmissions have all the Level 3 modifications plus a more aggressive shift kit.

An Overview Of GM Transmissions
GM has produced a substantial number of automatic transmissions; these are the most popular for custom classic truck use.

First produced with cast-iron cases until 1962 and then from 1963 up, aluminum. In the 21st century these transmission have two primary applications-restorations of original vehicles and racing. The early versions are often redone for Tri-Five Chevy restorers (Gear Star routinely rebuilds Powerglides and other vintage transmissions, so call for more information) and there is practically an entire industry devoted to building Powerglides for drag racing.

The original Hydramatic was a tough but complex four-speed automatic transmission used in a variety of GM vehicles into the '60s, including Chevy and GMC pickups, as well as other makes, such as Lincoln. These transmissions were so tough that they became some of the first automatics to find favor in drag racing (the B&M Hydro-Stick was based on the Hydramatic). The Hydramatic was replaced by the much less complicated and more sophisticated Turbo-Hydramatic.

As an aside, occasionally vehicles that should have been equipped with Hydramatics are found with a different transmission. A fire (Editor's note: The 1953 General Motors Hydramatic transmission plant fire in Livonia, Michigan, is to date the biggest business interruption insurance loss in U.S. history. I thought you guys should know that.) destroyed GM's Hydramatic plant so Oldsmobiles and Cadillacs used Buick's Dynaflow and Pontiacs used Chevrolet's Powerglide.

Turbo 350
These small, compact, and affordable three-speed transmissions were the most popular custom classic truck transmission before the availability of overdrive automatics. Still a viable choice, Zack has seen a resurgence of interest, and they seem to be making a comeback. The Turbo 350 is a great buy at just a little more than a grand, and comes complete with converter, TV cable, dipstick and tube, dust cover, cooler, and fluid.

Turbo 400
Physically larger and able to handle more horsepower than the 350, the Turbo 400 isn't as efficient, but it is tough. These three-speed transmissions are still an excellent choice for high-horsepower applications, where overdrive isn't necessary.

According to Zack, this underrated transmission is the best four-speed overdrive automatic that GM has come up with. Because of its compact proportions, many people assume it's not particularly strong; however, they will handle 800 horsepower or more with the proper modifications. Extremely efficient, thanks to a unique bellhousing bolt pattern, it will bolt to any GM engine.