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The standard Align T-Rex 500 main rotor feathering shaft (TT or Belt drive) is 6mm in diameter, 5mm diameter at the outside bearings, and 76.2 mm long.

Here are some exactly compatible shafts:

Align T-Rex 500 (Belt / TT) (6mm / 5mm x 76.2 mm)

Mostly compatible:

Not compatible are:

HDX-500 (SAS/SAH/HC) (5mm OD; xxx mm length)
Hirobo Lepton 500 (5mm OD; xxx mm length)

The Trex-500 uses a 4mm x 57mm for the tail rotor shaft. The HDX-500 and Hirobo Lepton also use a 4mm shaft, but I'm not sure of the length.