49 C.F.R. § 230.114 – Wheel centers.

(a) Filling blocks and shims. Driving and trailing wheel centers with divided rims shall be properly fitted with iron or steel filling blocks before the tires are applied, and such filling blocks shall be properly maintained. When shims are inserted between the tire and the wheel center, not more than two thicknesses of shims may be used, one of which must extend entirely around the wheel. The shim which extends entirely around the wheel may be in three or four pieces, providing they do not lap.
(b) Wheel center condemning defects. Wheel centers with any of the following defects shall be removed from service immediately and repaired:
1. (1) Wheels centers loose on axle;
2. (2) Broken or defective tire fastenings;
3. (3) Broken or cracked hubs, plates, bolts or spokes, except as provided in paragraph (b)(4) of this section; or
4. (4) Driving or trailing wheel center with three adjacent spokes or 25 percent or more of the spokes in the wheel broken.
(c) Wheel center repairs. Wheel centers may be repaired by welding or brazing provided that the defect can properly be so repaired and, following the repair, the crankpin and axle shall remain tight in the wheel. Banding of the hub is permitted.
(d) Counterbalance maintenance. Wheel counterbalances shall be maintained in a safe and suitable condition for service.