Threads on mast hardware

[seawolf]

I will be installing a couple of mast steps from ABI a few feet below the masthead.
Would it be safe to assume since the thickness of the mast wall is not great, it would be better to use flat head machine screws with fine threads rather than coarse, to provide more of a bite?
Advise appreciated.
Seawolf

[Brian Duff]

What size boat do you have ?

[Ian McColgin]

I've always used a proper tap and put in a suitable fine thread SS bolt with some loctite. That's much better contact than any sheetmetal screw which works better, as designed, on sheet metal.

G'luck
Ian

[seawolf]

Brian,
My boat is a 1962 Columbia 29.

Ian,
Thats what I have done on other hardware on the mast and boom.
Just getting some confirmation from the experts.
I did notice that my mast shape doesn't allow for a flush fit for the base of the mast step, it rocks a bit so I guess some sort of pad will need to be fabricated out of west system or teak for a flush and secure fit.

I made a pad to mount a main sail halyard winch on the mast out of the west system epoxy and filler and it worked pretty well. I took the aluminum base off the jib halyard winch and used it as a plug in some plaster of paris, and poured a mixture of west system and high density filler to create another base.
Unfortunately my resist wasn't very effective and I had to break and sand the plaster of paris off the new base. It turned out pretty good though, and the winch bolted on nicely.

[Ian McColgin]

A pad is ertainly needed. WEST will work if you thicken it enough as will many other epoxies but a pad (it need not be teak) is nicer as it allows for change and choise.

Many do install with sheetmetal screws and things like fairleads are often low enough load that the distinctly inferior grip of a sheet metal screw will not be a problem. And it's certainly faster to drill one hole and force the screw in. For my own use and for any work I've done for customers, however, I always drill, tap and use a bolt on aluminum masts and booms.

G'luck

[Ian McColgin]

Reread and realize I very misinterpreted the post by not reading "machine screws (aka bolts) and instead thinking of sheet metal screws. My apologies.

I take it that you're planning on those fold-out steps. I'd be inclined to shape the inside surface of the step a little rather than adding a pad. Should be easy to just lay a bit of say 80# across the mast and rub till the step takes the shape. This makes a subtle error that will vanish in whatever you use for bedding.

G'luck

[seawolf]

Ian,
The sanding to form the shape of the mast onto the inside surface of the aluminum folding mast step is a clever idea. Now I know why I get on this forum occasionally.
Thanks for the input.

[Jim Fulton]

Originally Posted by Ian McColgin

Many do install with sheetmetal screws and things like fairleads are often low enough load that the distinctly inferior grip of a sheet metal screw will not be a problem. And it's certainly faster to drill one hole and force the screw in. For my own use and for any work I've done for customers, however, I always drill, tap and use a bolt on aluminum masts and booms.

I agree with what Ian says here. I also use appropriately-sized machine screws for mounting hardware on aluminum masts and booms. However, many people (including some of my friends) do not agree. In a separate thread, I asked about objective engineering data on the relative holding power of machine screws vs, sheet metal (self-tapping) screws in aluminum. I have seen no replies. Does anyone know of any such data? Or is it all just opinion?

Jim Fulton

[Brion Toss]

Hello,
I once conducted a come-along test with a padeye in a section of scrap spar, once with sheet metal screws, and once with machine screws. We tore the former out fairly easily, and couldn't budge the thing with the latter, even when we got down to just two screws.
Sheet metal screws have such big, deep threads because the threads are meant to act as backing -- there's enough space between threads so that they can sandwich the sheet metal. When dealing with thicker stuff, like, oh, a mast, one must drill a bigger hole, just to get the screw in. So rather than having the thread projecting beyond the hole, as with sheet metal, they merely scrape against the sides of the hole, with very little "bite". That's why they can pull out.
Now of course given enough screws and a light enough load, you can have hardware stay put. But with how much reserve strength? Clearly, the right tool for the job, for anything heavier than a winch handle pocket, is a tapped screw, with a good rivet a distant second.
Fair leads,
Brion Toss

[seawolf]

Funny how the topic changed from machine screw threads to sheet metal screws, but it is all good information.
Brions come along test on machine screws vs. sheet metal screws seemed to give a good answer on what is appropriate for the best holding under a load.
Great device, that come along. It may be a good device to have on board for helping a jury rig.
Thanks to all who replied to my post. And fair winds to all.
Seawolf aka Bob