[Brian Duff]

Yes, flat head machine screw fasteners. Only the mast wall gets a countersunk and clearance hole, the splice plate receives the tapped hole . IF the head of the #10 is too thick, use # 8's (or a diffenet angle or undercut screw head, or any of so many options...)
I do not know what mast you have, but guessed at its wall based on the dimensions you gave. It is important to obtain the 'drawing in' of the splice plates to the mast wall. This is another reason that the splice pieces shouldn't be one piece. When splicing something like a 'pipe' mast where it is easy to obtain an extrusion that fits well inside, we split it lengthwise so the fasteners can draw the plates tight to the mast wall. I usually use three complete rows of fasteners each side of the splice, then two more rows with half as many fasteners.
Russ L is right in that the idea is to avoid stress risers. We also (sometimes, depending on the budget of the client) thin the plates toward the ends, as as antoher way to taper the material away. Most spar builder splices I have worked with have not done either type of tapering. some of the builders of spars use just three rows of fasteners each side, with a rather closer fastener spacing. Some have way more fastneners...

call your spar builder and have a chat with them about your plans, thats what we do.

heat works to loosen corroded stuck fasteners on all masts- anodized, powdercoated, awlgrip, or bare, you just gotta be careful ! Impact tools also are very effective at removing stuck fasteners. PB Blaster can help too. If a splice with mechanical fasters was corroded enough to "fall" apart, that should be clearly visable to even an untrained eye. Use some 'goop' to help slow the corrosion of fasteners in aluminium.


Self tapping screws are only to be used on wood. Do NOT use self tappers on aluminum or fiberglass, as it is WRONG. Self tappers are weak becasue they have both a smaller root diameter and must have less thread grip as well (in such hard materials) becasue the threads cannot compress the material away like in wood. It seems to me that self tapper screws in fiberglass are alway leaking, and the surely delaminate the fiberglass where they go through. thats for another thread though, if someone wants to start one....