Crankshaft threading

[Spinner]

Hello! I am well into this motor rebuild and I have found another area that is questionable. The crankshaft on my 57 Mark 55E was rusted pretty heaving by the flywheel and I was able to get the nut and flywheel off. However, I noticed the threading was weak/missing toward the outer end of the shaft-the part the cowl nut goes onto. I am not sure if there is more metal missing as the cowling was not on the motor when I received it and the end looks kinda banged up. Is it possible that anyone would have a good measurement of the threading length from a good Crankshaft? This is the earlier crankshaft with key. Thanks!

• This topic was modified 2 years, 10 months ago by Mumbles.

[dave-bernard]

If there are enough threads to cover the nut AND nyloc you are good.

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[green-thumbs]

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

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[joecb]

Just for info… there are purpose made thread restoration “dies” , these are usually hex shaped and are NOT adjustable. These are designed to follow the original thread and not cut as aggressively as a regular thread cutting die. With a regular cutting die there is great risk that one will cross thread and cut through the original thread form. Also some risk that you will cut an undersize thread.

Joe B

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[Spinner]

Thanks everyone for the replies. I broke down the entire crankshaft to look for wear, etc…. Did find the main bearing carrier is cracked but everything else looks goodHopefully, I can source a replacement. Will see about getting the correct die and dressing the treads properly. I believe it’s a 5/8 16 but will check with a gauge. I am going to do an experimental dry fit to see if the crank has enough exposure through the top cowling before I go deeper into repair. If not, then time to source a crankshaft.

[fastjohn]

I have successfully used a thread file first and then a die to fix bad threads. Just type “thread file” in your search engine.

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