The GT ATB B/B is one of the first high end, aftermarket B/B's to require the use of the Shimano (or Park) installation tool. Although requiring the use of the Shimano style tool offers no apparent advantage, it lends to the B/B the cachet of being topical and current. This is one of two outrageously similar B/B's introduced in 1993, and both deny any involvement with the other. World Class is the other and they declare strongly that the similarities and arrival to the market are just coincidence. We do know that at least parts of this B/B are made in Bob Hadley's cousin machine shop in Claremont, California.
This bottom bracket offers pleasing minimalist construction. The threaded rings that hold the B/B in place are machined from aluminum rod by a CNC milling machine. At the Interbike trade show held in Las Vegas in September of '93 it became nearly impossible to find a small to medium sized manufacturer or designer that hadn't taken advantage of the 3 or 5 axis CNC machine tools.
These cups are one more pleasant example of good numerics code turning out great parts that appear to be complexly hand crafted. The interior of the cups is smoothly machined, the indented facets for installation are perfect. There is a trough holding an O-ring as the spindle exits the cup to keep moisture and contaminants out. The spindle is machined from Ti 6Al-4V rod, drilled through, with tapped inner ends for the crank bolts.
The sealed bearing cartridges marked #61903RS are machine pressed onto the spindle, which is generally a good thing, but there is a complication. After one of the bearing cartridges is pressed on, an anodized aluminum tube with the "GT" logo is placed over the spindle before the second cartridge is pressed in place. This tube is precise fit and conforms perfectly to the outer diameter of the cartridges.
Most other B/B with machined pressed bearings have a gap between the water sleeve and the bearing that enables you to slide a piece of plate steel to support the bearing as you rap with a hammer to free a failed bearing for replacement. These pieces fit so tightly, and have no gap, replacing a bearing should it fail is likely to leave some real physical damage on the remaining parts.
At the outer sides of the bearing cartridges, on each side, is a White nylon washer, which slightly protects the bearings from contaminants. These really guide the bearing cups, as they're tightened, to hold the spindle properly aligned in the shell. The GT ATB comes with two crank bolts, that are machined from Ti 6Al-4V rod with a pair weight of 15 1/2 grams. One distinction these bolts have is that they are replicas of the "capless" design Shimano sells with their cranks, so they will serve as perfect Ti replacements. (We probably will begin seeing these bolts available separately).
The GT ATB bottom bracket is available in 107mm, 113mm, or 122.5mm spindle lengths. The weight of the 122.5 including the bolts is 195 1/2 grams.
Spindle Length-07-13-22 $Price in Catalog