I need to replace a lock nut on the rudder lift mechanism on a Hunter 25 sailboat. The lock nut is needed for the bolt between the 2 link plates of the lift mechanism. What size lock nut is needed? I cannot remove the bolt to measure since the bolt head is welded in. A 14 mm socket wrench seems to fit the other nuts on the rudder. Are these nuts metric? I would appreciate your help. Thanks.