Latest News: Forums Technical mast fittings – self tappers

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  • #3861

    I was trying to fix a burgee bracket and a pulley for a Secomar self-inflating bouyancy aid to the top of my mast.

    I thought I would use self tapping screws, but I only succeeded in shearing three screws in the holes I had drilled.

    I’ve fixed cleats to the boom like this, for reefing lines.

    I don’t understand why the aluminium mast should be so difficult.

    Is there something I need to know?

    Colin Parkstone


    Your find that the mast at the very top is much thicker and also has the taper weld meeting at the top.

    Do try and drill out the screws if they went into the mast itself as your have the points of the screws next to the halyard and will rip it to shreads.

    Try a larger drill bit and after each drilling try the screw until it fits.

    C P


    Why not use pop rivets ?

    Mine stays up there all right.


    Strangely enough I was recently thinking of asking on the forum for any advice as to when to use rivets and when to use self tappers – so perhaps I can post that question here. Any advice would be appreciated. I have some clamcleats to fix onto the boom for the slab reefing lines which the head of my rivet gun wont fit inside so they will have to be attached with self tappers – but if you could use either, does it actually matter which form of fixing is used?




    Thanks for your advice.

    I did try bigger and bigger drill bits, going up in steps of .5 mm, until I found the one that was .5mm narrower than the one that gave the screw complete clearance (is that clear? The test holes were not drilled in the mast itself!).

    I thought that .5mm undersize was just about right, but clearly not.

    Is there a limit to the thickness into which one can screw a self-tapper?

    I’ve since bought a pop rivetting set, but I’ve never used one of these before, so there is the opportunty for more errors …

    It’s not obvious to me how you gauge the size of pop rivet to use. You need to know how thick the mast material is where you are going to put the rivet, but it is not obvious how to measure that. Or doesn’t it have to be very accurate?


    when to use rivets and when to use self tappers

    I heard that pop rivets hold stronger than screws… Also, some say that you should use ‘monel’, not plain SS rivets to reduce corrosion (but still, dip in Duralac before installing)… I think length of the rivet is not problematic as long as you don’t undersize severely…

    Hopefully someone more experienced will comment.

    Good luck.



    pop rivets

    1) Diameter tends to be limited limited by size of hole in the thing you are fitting
    2) Drill the hole to that size and you can gauge depth,which gives you the length of rivet needed ( slightly longer than depth drilled)
    3) Main advantage is there is no screw point to foul/damage ropes inside the mast
    4) Not sure about stronger than screws, but spreaders fixed to mast with rivets, and certainly with a burgee holder,there is very little strength required.
    5) get the rivets from a boat shop that knows what they are talking about,and you should get suitable ones for alluminium
    6) Stainless steel rivets take a lot of force to ”pop” them, so I would advise against them unless you have an expensive pop rivet gun.

    hope this helps



    Stainless steel rivets still corrode due to galvanic corrosion, similar to the corrosion you see with stainless steel screws. Monel. on the other hand, does not seem to have galvanic problems with aluminium. Monel is more then adequate for our purposes and we do not need the extra strength stainless steel rivets bring. SS rivets are expensive and hard to pop.

    For things that are not loaded heavily, such as the burgee holder, I would use aluminium pop rivets. They pop easy, are cheap and don have a galvanic problem with the (Aluminium) mast.

    As for drilling; I always measure the core diameter of a screw and I never drill smaller than that diameter. The outer points of a caliper are usually sharpened and allow the measurement of the core diameter between the thread. Measure the thickest part, near the screw’s head.


    Thank you all – very helpful as usual.

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