Wheel Type 42, Type 52 - What does that mean?

In the case of wire wheels for classic cars from the 1950s onwards, a distinction is made between "wheel type" 42 and 52.  

"Wheel Type" - a designation refering to the maximum size of outer bearing, in millimeters. 

The actual hub diameter is measured across the outside of the splines on the car and not the wheel itself:

• 62.5 mm for the 42 wheel type (MG, Healey, Triumph, Lotus, some Aston Martin)
• 73 mm for the 52 wheel type (Jaguar, Volvo, some Aston Martin)

EXAMPLE:  The hub below is considered a 35 wheel type from a 1960 ASA that uses splined hubs and spinners.  Look to see where the calipers are measuring to obtain the 51.97mm reading.  See Chart below.