The world of wheels and castors makes universal use of symbols, which represent the most useful dimensions of the product. See table below:

Swivel castor Wheel diameter-Ø

Fixing bolt diameter-Ø

Brake swivel radius 
Tread width Hub width
Overall height Bore diameter-Ø
Plain bearing Offset Load capacity
Roller bearing Single bolt hole-Ø 2 Taper roller bearings
1 Central ball bearing

Top plate dimensions

Fixed castor
2 Ball bearings

Bolt hole spacing