How to properly attach fittings to mast?

[roverhi]

Use Monel rivets if you go that route. That's what the spar makers use. Personally always drill and tap.

For aluminum cleats with ss fasteners, coat the shafts of the screws where they pass through the cleat with the Lanacote, TefGel or whatever. It's ss fasteners welding themselves into large aluminum mass of a cleat that are impossible to get out.

Disagree strongly with pcmm on using fine thread fasteners. Aluminum is quite soft and the shoulders of fine thread are easily stripped/corroded out and and ripe for cross threading. Coarse threads have more meat and strength in aluminum.

The only fasteners I've had pull out are SPOT installed self tapping screws and pop rivets.

[Cheechako]

Originally Posted by pcmm

Strongest fitting will be drilled and tapped. but remember the strength of a threaded joint comes from the number of threads to spread the load with. you want FINE thread taps for aluminum and don't over torque or you'll just strip the threads. use tef-gel to inhibit corrosion and a thin pad between the SS and Alu. roll up cutting boards from the dollar store work well here and are inexpensive.

The bearing area of fine threads is about 1/3 less than coarse, which is why coarse are recommended in thin materials. It's very easy to pull a fine thread out of thin aluminum, and just a minor amount of too much torque will strip them clean.

"Coarse threads have a greater resistance to stripping out than fine threads when the length of thread engagement is short. Coarse threads have a greater height than corresponding fine threads; therefore the coarse threads have a greater volume of material between each thread to resist stripping. The thread height of a 10-24 screw is 33% greater than a 10-32 screw. For this particular reason I believe all thread-cutting screws and thread-rolling screws should always have coarse threads. "

[Terra Nova]

Agree with using course threads.

[Joyful Noise]

drill and tap and use ss bolts . coat with tefgel

[TrentePieds]

Another nix of the fine thread!

I think this recommendation must have arisen from a perfectly correct realization that the more bearing surface there is twixt male and female parts of the thread, when the fasteners are in tension, the stronger the joint. But as others have said, Fine thread REDUCES the bearing area. If you need more area, what you need to do is increase the DIAMETER of the screws, and when, for some reason, you can go no further along that route, you increase the number of screws.

Remember also that for what you are proposing to do, the stress on the screws will be sheer stress with relatively little tension stress. Therefore the diameter of the screws is what you want to increase if you want greater strength because "fatter" screwd can take more sheer stress than skinny ones, and because the increased bearing surface of fatter screws is what gives the strength. The threads are only there to prevent the fasteners from moving at right angles - more or less - to the direction of stress, i.e. from "popping out".

TrentePieds

[Scaramanga F25]

Coarse thread into aluminium.

[Reefmagnet]

I've become a fan of loctite'd helicoils into aluminium when I want exceptional strength, but this wouldn't be needed for lazy jacks. Another vote for course threads however.

[StuM]

Originally Posted by Terra Nova

Drill & tap. Use Tef-Gel.

Beat me to it. That is exactly what I was going to post.

[Tessellate]

Anyone have other suggestions for material to use as a spacer pad? Like the original poster I'll be attaching fittings to mast/boom soon too (for a vang). But not sure where to get a rubber, nylon, or plastic sheet that I can fit as a corrosion barrier.

I saw the suggestion of a rollup cutting board at a dollar store, but I don't really know any dollar stores and could see myself wasting a lot of time driving around town to all of them, finding that none of them have the right kind of cutting board. The pad needs to be thin (<1/8") so it doesn't change the fit too much.

I've heard of people cutting up plastic milk jugs for this, but am not sure that's ideal - I need a pretty big piece (4"x8") and milk jugs have corners that will be hard to form well to the mast and boom. So if there's some thin rubber, vinyl, or plastic pad sheeting at marine stores or hardware stores I'd just as soon get that.

I'm sure there must be something commonplace but I can't find it due to not knowing what terms to search for.

[Reefmagnet]

Originally Posted by Tessellate

Anyone have other suggestions for material to use as a spacer pad? Like the original poster I'll be attaching fittings to mast/boom soon too (for a vang). But not sure where to get a rubber, nylon, or plastic sheet that I can fit as a corrosion barrier.

I saw the suggestion of a rollup cutting board at a dollar store, but I don't really know any dollar stores and could see myself wasting a lot of time driving around town to all of them, finding that none of them have the right kind of cutting board. The pad needs to be thin (<1/8") so it doesn't change the fit too much.

I've heard of people cutting up plastic milk jugs for this, but am not sure that's ideal - I need a pretty big piece (4"x8") and milk jugs have corners that will be hard to form well to the mast and boom. So if there's some thin rubber, vinyl, or plastic pad sheeting at marine stores or hardware stores I'd just as soon get that.

I'm sure there must be something commonplace but I can't find it due to not knowing what terms to search for.

I use the PET plastic from juice bottles. never actually done a large enough piece that would require it, but I'd be very surprised if a cut out section with corners couldn't be flattened with a heat gun.

[Uncle Bob]

Originally Posted by Tessellate

Anyone have other suggestions for material to use as a spacer pad? Like the original poster I'll be attaching fittings to mast/boom soon too (for a vang). But not sure where to get a rubber, nylon, or plastic sheet that I can fit as a corrosion barrier.

I saw the suggestion of a rollup cutting board at a dollar store, but I don't really know any dollar stores and could see myself wasting a lot of time driving around town to all of them, finding that none of them have the right kind of cutting board. The pad needs to be thin (<1/8") so it doesn't change the fit too much.

I've heard of people cutting up plastic milk jugs for this, but am not sure that's ideal - I need a pretty big piece (4"x8") and milk jugs have corners that will be hard to form well to the mast and boom. So if there's some thin rubber, vinyl, or plastic pad sheeting at marine stores or hardware stores I'd just as soon get that.

I'm sure there must be something commonplace but I can't find it due to not knowing what terms to search for.

Perhaps a piece of old inner tube from a tire shop ?

[Dave22q]

Gasket material from an auto supply store will work as well as the very expensive bedding compounds from a marine supply store. Cut to size and glue to the fitting using any adhesive or even vasoline. Drill, tap and screw.

[Rorzech]

I don't mess with dis-simalar metals. I would get some of those pop rivets that are threaded. It's a kit with the riveter having a threaded puller thats removable and you replace it with the machine threaded screws that come with it. Mine came with about 5 sizes. Greatest thing for making removable covers and things. I love it.

[TrentePieds]

Any auto supply store will sell you garden variety gasket material in 12 x 12 in sheets or even bigger in the roll.

It is not insulating the mounting pad of the fitting from the mast that is the trick. That's dead simple. It's the fastenings that are difficult - in fact often impossible - to insulate effectively!

If you can get them, use fasteners made of metal that is as close as you can get in the galvanic table to the kind of metal the base is made of and choose fasteners of a metal that is "less noble" (see galvanic table) than the metal of which the base is made so that the fastener, rather than the base, will corrode.

Unfortunately, all metals useful for fasteners is "more noble" than the metal in an aluminum mast, so almost invariably, if there is corrosion, it will be of the mast rather than of the fastener. But fortunately is takes years for the corrosion to become so serious that the fitting comes off the mast. When that becomes imminent, remove the fasteners one at a time, drill out the fastener holes for the next size fastener up, refasten with that size of fastener, and you'll be good again for several more years. The mast won't be seriously weakened by "opening up" the holes for the fasteners.

Generally speaking the layer of oxide that protects aluminum against corrosion regenerates as soon as atmospheric air hits a newly cut surface.

I've had good results in the past drilling the mast for a tap hole for an appropriate diameter stainless steel machine screw, hand-tapping the hole carefully, and setting the screw in heavy duty contact cement - delicately known around here as "monkey snot".

You can afford to take this approach because the mast and fittings are not continuously immersed in seawater. Most of the water that gets on them, at least around here, is rainwater, so it has negligible effect as an electrolyte.

TrentePieds

[Donald Duque]

As was already mentioned, a layer of electrical tape or two would work just fine if you use good tape such as 3M. It is easy to apply and assemble. You don't need a third hand to hold the extra piece. this works well especially when you are mounting some hardware such as spreader lights or antennae mounts aloft.
I've also used "nutserts" as in the reply above, but unless you are careful and get them installed perfectly, there is the potential for the screw to stick in the insert and have the whole thing twist in the mast or whatever the substrate it is mounted in. This is especially true when removing the screw after it has been in place for some time. Then you are really "screwed".