mercury 75 hp starter problems

I have a 1989 mercury 75 hp. The nut that's on top of the starter holding the gear that engages the flywheel will back off after about a dozen times starting the motor. Can't get it to stay on. I've tried two different nuts but eventually they back off. I'm not very good repairing engines but can do some minor repairs. Any idea what will cause this nut to back off?

Re: mercury 75 hp starter problems

Assuming that the threads in the nut and the end of the armature shaft are in good shape, and assuming that you're tightening down the nut enough to start with, I'd try some blue loctite, after thoroughly cleaning the threads of any oil or grease. If that doesn't do it, then I'd go for the red.

Yo, and if that didn't work, I have MIG over in the corner . . .

As for the any idea question, I do wonder if you're tightening the nut enough, or whetehr you've omitted a lockwasher or perhaps substituted a nut d'ordinaire for a locking nut. BTW, the MIG suggestion isn't entirely in jest. If used judiciously, it's not irreversible. If used rashly, when you eventually need to rebuild the starter (if you ever do) a new starter ain't much more than a rebuilt one anyway.

Re: mercury 75 hp starter problems

That nut is supposed to be a locknut, maybe it was changed with something else. A regular nut will work if its locktited as suggested, I might use red. Not the permanent red, but the high strength red.

Re: mercury 75 hp starter problems

that should be a nylock nut, I would try a new one from a dealer. try do wear out.