The crank on a bicycle holds the pedals in place on each side. The crank arms turn when you ride to move the sprocket and chain. This will be true regardless of the bike manufacturer. Diamondback makes a variety of bike styles, including mountain, road and hybrid bikes. The construction of the bikes will vary, but older models may have an adjustable crank bracket. Examining the bottom bracket will tell you whether you can tighten the cranks or if you must replace the assembly.
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Place the bike on a stable surface, such as a sidewalk. Sit down on one side of the bike and examine the bottom bracket. The bracket is a solid piece that threads through the frame to hold the two crank arms together. If the center of the crank is sealed, it cannot be adjusted. If there is a plastic cover, slide the screwdriver under the plastic and pop the piece off. There should be a bolt under the plastic.
Place the bike in a workstand to secure it before making the adjustment.
Insert a crank extractor over the retaining bolt on the left side crank arm. A crank extractor is small tool with the proper head to fit the bolt. The bolt will sit in the middle of the crank arm where it connects to the bike frame. Apply pressure to turn the extractor counterclockwise until the retaining bolt comes loose. Remove the bolt and take off the crank arm.
Place a lockring removal tool over the lockring assembly. The lockring is a threaded washer the sits just under the retaining bolt. The removal tool has a head the matches the outer edge of the lockring. Do not remove the lockring, only loosen it by moving the tool 1/2 turn counterclockwise to access the bearing cup.
Push a pin wrench into the holes in the bearing cup. There may be one or two holes in the cup sitting on the outer edge. The wrench has pins that fit into the holes to move the piece. Turn the pin wrench 1/4 turn or less to loosen an overly tight crank bracket. Turn the wrench clockwise to make a tighter adjustment. For example, if the crank arms slip when you ride, you may need to tighten the bearings, so turn the pin wrench clockwise.
Tighten the lockring as you hold the bearing cup in place with the pin wrench, otherwise your adjustment will not hold. Replace the crank arm and retaining bolt.