China Neodymium And Ferrite Magnets Manufacturer & Supplier 
In the design of motors, many engineers encounter a question: what size of magnet is suitable for an 8mm rotor shaft? In fact, there is no fixed size because the size of the magnet not only depends on the shaft diameter, but also on the motor outer diameter, stator size, speed, power, and installation method.
For motors with 8mm rotor shafts, ring magnets with inner diameters ranging from 8mm to 8.05mm are generally chosen (with assembly tolerances reserved). The outer diameter varies depending on the size of the motor, commonly including specifications such as Φ 12mm, Φ 14mm, Φ 16mm, Φ 18mm, Φ 20mm, Φ 22mm, and Φ 25mm. The thickness is usually between 3mm, 4mm, 5mm, 6mm, 8mm, and 10mm.
In the case of brushless DC motors (BLDC), the magnets are generally radial-magnetised multi-pole magnetic rings. Depending on the control method, the number of poles can be designed to be 4, 6, 8, 10, 12 or even more; different numbers of poles correspond to different control algorithms.
Magnetic ring for motor shaft with an 8 mm hole
The material of the magnet is also a key factor in the selection process. If high torque and power density are required, neodymium-iron-boron (sintered, bonded or injection-moulded) is usually chosen; if high-temperature resistance, corrosion resistance or cost control are prioritised, ferrite (sintered or injection-moulded) may be selected.
Below are some recommended dimensions for 8mm-bore rotor magnets;
D25 × d8 × 6mm / radial 2-pole
D15 × d8 × 5mm / Y10
D14.4 × d8 × 6 / radial 4-pole
D16 × d8 × 13 mm
D18 × d8 × 9 mm / 8 poles
8mm Hole High Performance Ferrite Rotor Magnet 20x8x27mm
D12 × d8 × 8 mm / 12 poles
D12 × d8 × 4 mm / 8 poles
D13*d8*3.5mm/ radial 2-pole
1750 gauss 6 poles strong magnetic ring for motor rotor 12x8x20mm
D22*d8*5mm/ radial 2-pole
If you need an 8mm inner hole rotor magnetic ring, you can provide specific specifications or customize according to the drawings, including size, tolerance, material, magnetization direction, and number of poles.