Whilst watching my Rotary Aviation video, I notice that they say, "using the 2 and a half inch, or 54mm socket...." when they are talking about the flywheel nut. However, 2.5 inches is 63.5mm. Now, 54mm is 2.125 inches...so is it a 54mm socket?

 

i believe it is 2 1/8th.

 

54 is right

 

54mm is correct.

 

2 and 1/8 is correct too

 

When you get into bigger sizes, you can get away with using standard sizes sometimes. I have a Craftsman 2 1/8" socket that I've used for ages - fits and works great. A 54mm socket is just much harder to come by.

Dale