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BMX Three Piece Crank Service

This article will discuss the installation and service of the BMX three piece crankset using the 48 spindle spindle. These models of cranksets use cartridge bearing cups, a spindle with 48 small splines, a chainring and two arms.

Bottom Bracket

There are several types of bottom bracket standards for BMX and Freestyle bikes. The large unthreaded bottom bracket is the "one-piece" or "ashtabula" bottom bracket. The threaded bottom bracket is referred to as the "euro" bottom bracket. The threading is the 1.37" x 24 TPI standard, the same as most road and mountain bikes. In the euro standards there are two common shell widths of 68mm and 73mm.

The euro threaded cups will be in a left-hand and right hand threading. The left side thread of the bike is a right-hand direction thread, which tightens clockwise and removes counter-clockwise. The right side of the bike is a left-hand thread, which tightens counter-clockwise and remove clockwise.

1. Prepare the threads with grease or anti-seize. Install one cup and secure fully.

2. Install spacer (if any) into shell. Hold spacer in place with spindle place and thread on other cup (Figure 2). Secure cup. Bearing cups are designed to load against inner sleeve, but not press against it so much it binds bearings. Some frame facing may be required.


Figure 2 Install spindle, sleeve and cup.

3. Install bearing cup cover cap and spacing washers on drive side arm. Spacing washers are used to adjust chainline of front ring to rear cog (Figure 3).


Figure 3 Use spacing washers to adjust chainline.

4. Grease inner splines of right arm and install on spindle. Install bolt and pull tight (Figure 4). Spindle will be pulled into arm.


Figure 4  Pull spindle into crank.

 

5. Continue to tighten until spindle fills at least one half of space available in arm (Figure 5). Remove bolt to check spindle to arm fit.


Figure 5  Check fit of spindle in arm.

6. Grease splines of left arm. Use spacing washers to set arm symmetrically to right arm. This will take some trial and error, adding or removing spacers as necessary. Install arm and pull arm until it fully contacts spacing washers and bearing cap (Figure 6).


Figure 6   Install and space left arm.

7. Cranks can be pulled on some makes and models using a crank puller such as the CCP-22 or CWP-7 (Figure 7). If the crank bolt is the common 8mm, use the same puller as for square type spindles. However, some makes us a proprietary puller or a mallet.


Figure 7

8. After setting arm spacing, check that crank turns relatively smoothly and there is no side to side motion. If the arms do not turn smoothly, the inner spacer between the bearing cups may be too short. If there is side to side motion in arms, pull arms further onto spindle, and or add another spacing washer.