Technical 51 Mercury steel wheel center measurement?

Anyone out there know the exact measurement for a 51 Mercury center hole? I have a caliper but it won’t fit in the rim. I also tried tracing the hole on to paper but it’s not exact. Any help would be greatly appreciated.

Thanks,
George

 

I have set of what I belive is '50 merc, and thought 50-51 were the same. Mine measure 3 5/8"

 

I’m getting 3 3/8” or 3.40

 

Maybe we are thinking of different wheels. This wheel has been is what I thought a '50 merc. I have burned an inch on the steel ruler...

 

Hmm strange. That looks like my wheel just a little larger hole.