Is the rest of the crankshaft good? Thread my be least of your worries.
If the crank is bent or bearing surfaces are worn, a better crankshaft may
be required.
If the crank is good ,and there is enough thread to fully engage nut
cross this issue off your list.
Damaged threads can be refreshed if done with care.
Unknown thread can be identified by taking the nut to your hardware store
and trying bolts to see which one fits…
Mercury used USA standard fine thread not metric at time Mark 55
was made. Once you know the thread of nut, you also know crankshatr
thread and die to cleaned up threads.
You may use a triangle file as thread is 60 degree also, It may only be
necessary to clean up the entry point of thread where nut engages
thread on shaft…a problem with fine threads.
First examine the crankshaft to see if it is free or other issues.
Others with Mark 55 specific experience may see matters differently,
Louis
