Mystery nut

Yup. The nut has an insert, nylon I think, crimped into the nut that creates drag on the bolt to resist loosening itself. They have 'em in the hardware fastener drawers at my local Home Despot as well as the better hardware choice, Yardbirds.

Dana

I've seen nuts like this used > Yup. The nut has an insert, nylon I think, crimped into the nut

If you look carefully at the pic, this is not a nylock nut.

That wasn't very obvious from the photo, but I see what you mean - and I suspect Leon would recognize a nylock nut in the first place. Leon - did this nut come from Sebring originally?

Dana

Yes. In fact, the replacement from Lowes I put on is a nylock nut. Rather not have this happen twice.

Don't know. I didn't install it originally. I wonder whether the corrugated part acts as a lock washer of some kind, and I should have put it towards the inside instead of outside.

Leon

Leon,

May be one of these:

Press-In Panel Nuts: Also known as spline nuts and insert nuts. These broach-type fasteners have strong wear-resistant threads and are easy to install. Simply press the knurled shank into a hole using an arbor press or similar pressure tool. When the shoulder or head makes contact with the board, the fastener is securely mounted. The knurls cut into the mounting material to resist torque.

Info at

HTH

Grant Ziebell