Tapping thread in alloy mast

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dunedin

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Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
Should anything be used on threads to reduce corrosion between screws and mast?
 
dunedin said:
Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
Should anything be used on threads to reduce corrosion between screws and mast?
Click to expand...

For something close fitting like a screw thread Tef-gel is a lot less messy to use than the common Duralac ( barium chromate) paste. There is also another newer "Duralac Green" which is chromate free.

Consider an insulating gasket between the stainless steel fitting and the mast

Also consider a finer thread than the usual coarse M8 x 1.25mm if the wall thicknes is a bit thin for a 1.25 mm thread (there are 1.0 and 0.75 mm alternatives)

Also consider Rivnuts or similar rather than tapping a thread in the mast wall
 
Be extremely careful if using the original yellow Duralac. It stains clothes very effectively. It's also pricey so you don't want any of it to go other than where it's needed. On the other hand, IME one tube lasts a lifetime.

Derek
 
dunedin said:
Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
Should anything be used on threads to reduce corrosion between screws and mast?
Click to expand...

Another approach that I have used is to rivet a stainless steel plate onto the mast using lots of smaller rivets then tapping the stainless plate to mount the fitting.

The stainless plate would need a plastic gasket between the mast and the plate.

Generally I would use a larger pitch thread in softer material like aluminium.

BTW I have fixed mast steps onto mu mast with 4 M6 stainless crews tapped directly into the my mast which I consider is better than rivets.
 
Thanks all. Tef-Gel or Duralec sounds like what I was thinking of.

Was thinking to mount on thin layer of Sikaflex to insulate. This partly because also very slight curvature of mast behind flat plate.
Interestingly none of the original fittings attached by the spar maker seem to have any gasket underneath. And like Roger Shaw the handful of mast steps are fitted with machine screws.

Rivnuts are interesting. But would I need Monel ones? Where could one source a small quantity (only 4 needed) at the right metal spec? Also it was suggested that if mast wall thicker these might be too shallow, as designed for thin metal?

Thanks again
 
You can buy small quantities of Ali rivnuts on ebay.
They come in varied lengths for metal thickness.
What is the fitting?
If it has M8 fixings for good reason, you need some serious backing plate in the mast.
If it's under not great load, you could bush the fixings down to M5 and just tap the mast wall.
 
i need to add a new main sail guide gate to my zspar mast to stop the runners/cars falling out when raising or lowering the main. there are just burred threads where one used to be. the bolts are steel though but i would have to re-thread the holes.

are rivnuts put into a hole then you screw into those? thanks
 
Nothing to do with tapping the thread, but a gentle reminder - think about the run of cables and halyards down the inside of the mast and don't drill into any of them!
 
UK-WOOZY said:
are rivnuts put into a hole then you screw into those? thanks
Click to expand...

They're put into a hole, drawn (like a rivet) with a special tool to secure them in place, resulting in a strong thread with good depth.

Regarding dunedin's question in post #8, stainless rivnuts are all that's required**. Best inserted with Duralac/Tufgel.

** I'd much prefer steel rivnuts to aluminium, so that any corrosion potential is between the rivnut and the mast rather than the rivnut and the fastener. This is one reason the better masts have stainless thread inserts for major fasteners.
 
macd said:
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** I'd much prefer steel rivnuts to aluminium, so that any corrosion potential is between the rivnut and the mast rather than the rivnut and the fastener. This is one reason the better masts have stainless thread inserts for major fasteners.
Click to expand...
Bugger that, it's a recipe for a big hole corroded in your mast.
 
The OP hasn't told us what the fitting is, so we have no idea what loading the fastening may be subjected to, or how permanent it is to be. At one extreme we have glue or double sided sticky tape, but at the other, for a proper job, it will be rivets or rivnuts. Personally, I wouldn't rely on tapped holes in thin walled mast for anything that mattered.
 
I would and have threaded directly based on advice from Allspar. for the simple reason that if you do have a corrosion problem with fastener you can then drill out the hole and add a rivnut later.
This was for a halyard jammer.
 
September 20th
UK-WOOZY said:
I need to add a new main sail guide gate to my zspar mast to stop the runners/cars falling out when raising or lowering the main.
Click to expand...

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?
 
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Thanks Rszemeti, that's helpful.

But the long-bolt method sounds clever and I've had rivet tools work erratically and break before, so perhaps the purchase would be less smart than learning the technique. But unless it's fairly difficult, I'm surprised there's any market for the tool.

I've always used monel rivets on my alloy mast, and been wary of aluminium. I guess for a low-load component like the mast-gate, I could use aluminium and replace them when they go powdery. Isn't that safer than stainless, which will corrode the surrounding alloy?

EDIT:

I've since watched on Youtube and seen in earlier forum threads, that rivnuts are definitely easy to set without cleverdick tools, (vid below) and that the smallness of a stainless (cathode) rivnut relative to the size of an alloy (anode) mast, means there's unlikely to be much corrosion. So that's all good. :encouragement:

 
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