Fretted internal threads in mast

[Hudsonian]

My mast has a cut out to allow the use of mast slugs as well as the bolt rope. To retain the slugs in the groove and still allow the slides to go by without jamming the manufacturer provided (2) specially contoured SS plates. The plates measure 1 1/4" wide x 3 3/4" high with 2 horizontal slots on 3" centers. These plates are held in place by (4) SS thumbscrews(#10-32).

Twenty years of wear has fretted the internal threads in the mast so that the screws are not properly retained. I tried drilling out and filling the screw holes with epoxy and tapping new threads. These threads have fretted away in less than a year. Do any of you have a suggestion for a more permanent solution?

I have considered trying to install a pair of backing plates with predrilled and tapped holes but the backing plates would need to be located approximately eight feet from the mast butt and my arms aren't that long.

[US27inKS]

Helicoils are a wonderful thing.

[Hudsonian]

Thanks for your prompt and helpful response. I was vaguely familiar with helicoils but have no personal experience. I am concerned that they may not be appropriate given the thin wall thickness of the mast. The load is not very great but I'm afraid that the mast wall is so thin that only one or two threads would contact the mast wall. The helicoil recommendation led me the wikipedia article on the helicoil where I found references to "rivnut" and 'nutsert". They appear that they may more applicable to thin wall structures. Does anybody have any experience with them?

[US27inKS]

I do. You have to be very careful to install them properly. If the bolt seizes in the nutsert or rivnut, there is a very real possibility of having the nutsert or rivnut spinning in the hole and it will be very difficult to repair. I would try the helicoil first. You can cut down a helicoil insert by just snipping off what you don't need. Any burr created can be trimmed with a dremel. Nutserts and rivnuts tend to need larger holes than helicoils, so if the helicoil doesn't work you can still go with the nutsert.

[cacique1977]

I have a 33 year old Vancouver 27 that had a similar problem. My Kemp mast uses a rivnut as standard. I drilled a fresh hole in the mast and used a rivnut, they are supposed to be fitted with a special very heavy duty rivit gun, but if you are careful you can use the rivnut's own thread to rivet it in place, I did this repair about 2 years ago and I am hopeful it will last another 28 years. Obviously you need to get a marine rivnut.