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One-piece Crank
A one-piece crank (OPC) consists of a single steel forging which constitutes the left crank, axle, and right crank in a single piece of steel. This type of crank set is found on older U.S. made bicycles, and many current low-end bicycles. It is also commonly used on BMX and freestyle bicycles.
One piece, or "Ashtabula" cranks are heavy but very strong and reliable. They are also much easier to work on than any other type of crankset, requiring no special tools. You can completely overhaul a one-piece-crank bottom bracket with just a large adjustable wrench and a screwdriver.
There are two different threadings used on one-piece cranks. Most are 24 threads/inch, but some, notably older Schwinn and Mongoose bicycles, used 28 tpi. The bearing retainers and cups are also slightly different between the two models. Although they both use 5/16" balls, 24 tpi units use a #66 retainer with 10 balls, while the 28 tpi units use a #64 retainer, with 9 balls. This is one application where retainers are essential, it is almost impossible to assemble a one-piece crank bottom bracket with loose balls.
One-piece cranks only fit in bicycles designed for them, with large-diameter, threadless bottom brackets. It is possible to buy adapter kits to allow three-piece cranks to be used in this type of bottom bracket shell.
One-piece cranks are made in a smaller diameter than other crank designs, so that the bearing cones can be installed. As a result, they have to use a smaller diameter (1/2" x 20 tpi) pedal thread.
The left side cone of the bottom bracket has a left thread, and uses a locknut and keyed washer to secure the adjustment. The right side cone has a right thread, and secures the chainwheel to the crank.
