Tapping thread in alloy mast

[DinghyMan]

September 20th


I have exactly the same problem. I'm making a mast-gate from light aluminium so I'll use both Duralac and patches of cycle inner-tube as gaskets between the metals, but the metal of the rivnuts themselves hadn't crossed my mind. Is it even possible to insulate the inside end of a rivet, from the wall it butts-up against? Does anybody manufacture alloy-friendly rivnuts?

The question I wanted to ask when I searched for 'rivnut', is can I set rivnuts with my existing lazy-tongue rivet tool, or will I need to make a substantial purchase for this small job?

No, you can set rivnuts with either a dedicated tool or with a nut and bolt of the rivnut size; put nut on bolt, insert fully into rivnut, put rivnut in place, two spanners one on bolt head the other on the nut and tighten the nut against the rivnut drawing the bolt outwards and compressing the rivnut

See https://www.youtube.com/watch?v=I6H6wr7fJYo

 

[vyv_cox]

Bugger that, it's a recipe for a big hole corroded in your mast.

Not normally, as the anode is far bigger than the cathode. Localised surface corrosion is sometimes seen but very rare for the whole section to corrode away.

 

[rogerthebodger]

Not normally, as the anode is far bigger than the cathode. Localised surface corrosion is sometimes seen but very rare for the whole section to corrode away.

We have had lots of question about a stainless steel screw/bolt fitted in an aluminium that cannot be unscrewed.

This is of cause due to corrosion but has not been so extensive to loosed the screw or create a big hole. The only I have seen extensive corrosion at he interface between a largest stainless plate screwed to a mast without and isolation barrier.

 

[DownWest]

Have a look here: Make Ends Meet Ltd I use lots of their ally rivnuts. Cheap as chips.
Biggest monal rivets I have done up a mast, were 6mm. Tough bu++ers. Set them with a long tong riviter, not fun up there.
Why not use them with a washer? Got to be stronger than a thread cut in a thin mast?

 

[Greenheart]

Thanks for replies. I ordered stainless 5mm rivnuts and Allen-bolts, all 316. The tiny amount of free space available for the mast gate may make drilling and attaching a challenge, especially holding the lower nut close to the mast. I can see that a properly attached rivnut is likely a better fitting than a permanent rivet - though it's worth considering that the initial hole must be significantly bigger too.

 

[pagoda]

I fitted 3 mast steps in A4 stainless to get easy access to our mainsail bag.
The mast is a Selden product and close to 4mm thick beltween the boom and deck. I tapped for M6 x 13mm machine screws and used Duralec on the threads. The step bases are also isolated from the mast by thin nylon spacers. I've had the steps off once after a year to check for any signs of problems. Looks fine, the screws were not difficult to remove. I would not be that comfortable going to M8 in a 4mm aluminium wall thickness. Rivnuts are good with powerful fitting tools but may be a nightmare if you don't get them good and tight first time round.