John
In my opinion, it is always best to rivet into alloy masts and spars if you can.
If you use stainless steel self-tappers, the amount of "bite" for screws is limited to the wall thickness of the spar, so even self-tappers may work loose over time - a possibility made worse if they are out of sight and/or out of reach; and if, over time, dis-similar metal corrosion welds them nicely together, they won't come loose but one day it may prove difficult to remove the screw without damage to either the screw or the wall.
In addition, in the case of a mast, self-tappers will leave their sharp ends protruding into the cavity, with the potential to fray halyards and possibly electrical cables against them.
If you are riveting plastic to alloy, or alloy to alloy, then alloy rivets will be fine. If you are riveting stainless steel to alloy, then you MUST use Monel rivets, and you should isolate the stainless fitting from the alloy wall [again, to avoid dis-similar metal corrosion].
Like screws, rivets come in different diameters and lengths, and so need to be selected to take into account both the fitting hole diameter, and the combined depth of fitting and spar wall. A matching diameter washer is often desirable, sometimes necessary. For occasional jobs, a hand riveting tool and a strong wrist is all you need.
Rivets can always be drilled out, and knocked through, leaving a clean hole that can be re-used; and even a hand drill will normally do the job.
Whatever you decide to use, check for electrical cable ducting before applying drill to mast!
Geoff Sheddick
Parker 27/146 "Stroller'
Geoff Sheddick
Parker 27/146 "Stroller'