Screw/bolt size question

DougM · May 10, 2019

Don’t be overly concerned about the clearance hole size in the Scanstrut mount. use a screw that fits through the mount holes and drill and tap the mast for that thread size. By the way, the holes in the mounting bracket may be metric for whats its worth.

Ken Cross · May 13, 2019

I imagine 12 of them are more than adequate. How many #10 bolts would it take to hold you up?

Ken

thinwater · May 13, 2019

Chris & Lenore said:
I plan to use bolts to mount my 18 inch Raymarine radar to the mast.
The Scanstrut mount came with rivets that require a 3/16 hole. Number 10 screws seem to be the closest in size (I plan on using 10-32 and tap the holes).
Question - although there will be 12 screws, they seem small in size. Do I need to upsize to, say, 1/4 inch?

I can find many posts about using screws vs rivets but nobody mentions a size.

Thanks.

You can look up the strength and WLL with Google. Screws and rivets are not that different, assuming the mast is >0.1-inch thick, which I'm sure it is (I've lab tested both on masts).

This report was based on that size range, in spars, and included both sheer and tensile testing.
https://www.practical-sailor.com/issues/44_9/features/A-Riveting-Report-Spar-Fasteners_12480-1.html

There is nothing wrong with rivets, if installed properly. They are just as strong as machine screws. What ever you chose, do it neatly, and use Tefgel or equivalent.

FWIW, the sheer strength on these fasteners is about 1000 pounds, tensile, about 1/2 that. Also, 10-24 will be considerably (25-40%) stronger in tension in a soft metal, like aluminum, at typical spar thicknesses. Shear will be about equal. Just sayin', based on testing. A fine machine screw is much weaker in pull-out strnegth, in thin metal, than a proper rivet. Counter intuative, perhaps, but proven.

LeslieTroyer · May 13, 2019

You might look at using rivnuts rather than drilling and tapping. It will be stronger and easier.

Les

thinwater · May 13, 2019

Chris & Lenore said:
Interesting.
My mast specs at .125" thickness. So 10-24 would be stronger (in pullout) than 10-32?
I'm not a Practical Sailor subscriber so I can't read the article but thinking I should be.

Yes, fine threads are stronger in steel, but coarse threads are stronger in thinner aluminum. Below about 0.125, rivets are better than either. Think of what Hobies endure, using nothing but rivets.

FWIW, self-tapping screws are never the right answer in a mast, if for no other reason than because the sharp points will snag lines and cut wires.