What is thread size of bolt on Mercury 75 HP 1993

[udoittwo]

I am switching from a 4 cyl.1986 75 Mariner to a 1993 Mercury. It has been setting for a few years so I am trying to change the water pump. The 4 nuts came off easily but the one brass lock nut that is up inside fell apart. Because the nut was spread apart, I thought it was a standard 3/8 16 but a 3/8 16 doesn't fit.Is it metric?
Thanks for your time and any answers,
Karl.

 

[Fun Times]

Hi there, can you identify the nut you need using one of these parts diagrams?
http://www.mercruiserparts.com/Show...bnbr=200&bdesc=GEAR+HOUSING+(PROPELLER+SHAFT)

http://www.mercruiserparts.com/Show...277&bnbr=190&bdesc=GEAR+HOUSING+(DRIVE+SHAFT)

The 4 nuts that came off, are they the same size as the ones you need by chance? < For example, Item number 74 Merc part number 11-34933 in the first link looks to be 7/16 X 20, http://www.iboats.com/Mallory-Nut-S...9786989--session_id.093058620--view_id.364187

http://www.mercruiserparts.com/selectModels3.asp?type_id=MERC