[reg45]

51mm is more like 2" (2" = 50.8mm).

[robertpaulsen]

"51mm is more like 2" (2" = 50.8mm)."

Correct. A 2" radius. What I call a 4" circle.

[HIDEK6]

A 2" radius = 4" diameter.

Due to the widespread ignorance concerning the difference between radius and diameter, I think he was simplifying the statement for the masses.