Fixing buggered crank threads

[frankr]

P1 said it is a 1972 25hp. That would be a 1/2-20 thread.

[lindy46]

fleetwin wrote:

lindy46 wrote:

I should have mentioned the threads are pretty much gone. I was thinking of trying to cut new threads of a lesser diameter and get a smaller nut to go on.

What happened that would take out all the threads? What brand/hp crank is it? I would hesitate to advise rethreading the crank to smaller threads….Sounds risky

I don’t know the history of this motor. I took it in trade, for $10 so a no brainer. The electric starter and bracket are worth much more than that. Maybe best to part it out. I much prefer to restore them but may not be financially the best decision.

[fleetwin]

Yeah…I surely wouldn’t risk trying to rethread the crankshaft on this engine… A used crank isn’t too tough to find, perhaps a used powerhead. Otherwise, just use it for parts…

[amuller]

dave-bernard wrote:

always put a nut on before you put on the puller. that said take a die and break it then put it on below the bad threads use a vice gip to hold it together and turn it off. may take a few times.

Using a broken or 2-piece die would be my preferred approach if only the top were mushroomed. Be sure to put die upside down. A thread file would be next.

For really ruined threads, I’ve wondered about cutting off the male threads and drilling/tapping for something like a 5/16″ grade 8 bolt. Would it be safe? I don’t know. If the crank threads have been hit really hard one has to wonder about other damage.

am

[frankr]

Actually, I have tried the drilling and tapping method. It was very difficult, if not impossible because the steel is so hard.

[opposedtwin]

Plus one on both methods. I have a 1946 mercury kd4 thst i mushroomed. Filed off the distorted portion, re-cut threads with correct size die (7/16-20) then bought a thread file and it all worked very nicely.

Be patient and good luck. And YES! always use a nut or the special plug mercury makes to protect those threads.

[lindy46]

Decided to part this one out. The flywheel key was sheared and the crank/flywheel mating surface is damaged both on the crank and the flywheel. Not worth the effort.

[fleetwin]

good decision….

[Author]

Posts