Not sure which mountain board bearings to choose ... ? There are currently three designs of mountain board truck available... - 10mm Skate Trucks - with a 10mm diameter axle.
- 12mm Skate Trucks - with a 12mm diameter axle.
- 12mm Channel Trucks - with a 12mm diameter axle.
And two designs of mountain board hubs (wheels) available... - 10mm Hubs - with a 22mm internal bearing slot.
- 12mm Hubs - with a 28mm internal bearing slot.
This leads to the availability of 3 different types of mountain board bearings... - 10mm Bearings - for 10mm Skate Trucks and 10mm Hubs.
- 12mm Bearings - for 12mm Skate/Channel Trucks and 12mm Hubs.
- Reducer bearings - for 10mm Skate Trucks and 12mm Hubs.
Hubs designed to accept 10mm bearings cannot be used with 12mm Skate or Channel Trucks since the hub bearing slot is too small. However, hubs designed to accept 12mm Bearings can be used with 10mm Skate Trucks by inserting Reducer Bearings to accommodate the 10mm (instead of 12mm) diameter truck axle. 
Check the chart below for full compatibility... | Truck Type | Hub Type | Wheel/Truck Compatible | Bearing Type | | Skate 10mm* | 10mm (22mm**) | Yes | 10mm Bearings | | Skate 10mm* | 12mm (28mm**) | Yes | Reducer Bearings | | Skate 12mm* | 10mm (22mm**) | No | - | | Skate 12mm* | 12mm (28mm**) | Yes | 12mm Bearings | | Channel 12mm* | 10mm (22mm**) | No | - | | Channel 12mm* | 12mm (28mm**) | Yes | 12mm Bearings |
*Truck axle diameter.
**Hub internal bearing slot diameter. Text and images Copyright © Session1 | 
