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Boat Forum / Building / May 2007bolt metal type in aluminum mast
I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in there right now) and put a machine screw in there. I have some options in terms of metal types and there have been a number of discussions about corrosion and so on with SS in contact with aluminum and how really useless the isolating compounds are in a saltwater environment etc etc So what's the metal of choice? I'm seeing brass, SS, silicon bronze, zinc plated steel.. what's optimal for the aluminum (non-)interaction? These other discussions don't have a consistent conclusion. (And no rivets, this is a small job, I really do need the machine screw). This is a freshwater boat and has been for a long time. However I wouldn't rule it out of taking it down the east coast one day. Thanks for any comments, -CB Use stainless, preferably 316 and a teflon anti-seize goo. > I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in > there right now) and put a machine screw in there. [quoted text clipped - 15 lines] > > -CB The stainless is more noble than aluminum, hence the stainless will corrode the aluminum. But, since the bolt is so small compared to the mast the damage done by the corrosion will be very, very, very slight and the damage will not be localized at the hole, but over the entire volume of the aluminum. At least, that is what I've been told and how I've done similar work on my mast. Use the goo though. This does not sit well with me. I thought that localisation was why stainlless screws ended up in powdered aluminium and came loose. The way I learnt it, the actual corrosion was across the boundary of the two. >The stainless is more noble than aluminum, hence the stainless will >corrode the aluminum. But, since the bolt is so small compared to the >mast the damage done by the corrosion will be very, very, very slight >and the damage will not be localized at the hole, but over the entire >volume of the aluminum. At least, that is what I've been told and how >I've done similar work on my mast. Use the goo though. I have a serious concern about a machine thread in the relatrively thin wall of an almmunium mast. Al does not takew threads well, and then not fine threads, adn then most thin metals a happier with coarser threads. Use a reivet if at all possible. Ironically, even self-tappers of reltatively coarse thread, with large flutes, are better in thin sections. Apart from that, use 316 and some sort of insulator. It's worked on many boats. > I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in >there right now) and put a machine screw in there. [quoted text clipped - 15 lines] > >-CB >I have a serious concern about a machine thread in the relatrively >thin wall of an almmunium mast. Al does not takew threads well, and >then not fine threads, adn then most thin metals a happier with >coarser threads. Aluminum riv-nuts are easily obtained, easily fitted, and reasonably replaceable when the time comes. Screw a SS screw into a riv-nut Brian Whatcott Altus OK > Aluminum riv-nuts are easily obtained, easily fitted, and reasonably > replaceable when the time comes. Screw a SS screw into a riv-nut Brian, do you have a link to a source for these riv-nuts? I can't seem to find them. :> Aluminum riv-nuts are easily obtained, easily fitted, and reasonably :> replaceable when the time comes. Screw a SS screw into a riv-nut :Brian, do you have a link to a source for these riv-nuts? I can't seem :to find them. Mcmaster-Carr sell them. www.mcmaster.com, look for "rivet nut" in the fastener section. > :> Aluminum riv-nuts are easily obtained, easily fitted, and reasonably > :> replaceable when the time comes. Screw a SS screw into a riv-nut [quoted text clipped - 4 lines] > Mcmaster-Carr sell them. www.mcmaster.com, look for "rivet nut" in > the fastener section. THANK YOU! >> Aluminum riv-nuts are easily obtained, easily fitted, and reasonably >> replaceable when the time comes. Screw a SS screw into a riv-nut > >Brian, do you have a link to a source for these riv-nuts? I can't seem >to find them. I googled "riv-nut" for 30,000 hits. I googled "riv-nut supplier" for 3000 hits. Here is one of them. It has a cross-section diagram to give you the idea http://www.cardinalcomponents.com/fasteners.htm Brian Whatcott Altus OK Here is a related question. My Imron-painted aluminum mast and boom has the usual assortment of fittings and attachment methods typically found on a twenty-year-old boat now on it's fourth or fifth owner. For the most part the paint is in good condition, (and stainless bolts protected with TefGel or similar) but here and there next to older stainless fittings, or around a weld there are the usual smaller-than-a-pea pustules of corrosion. Sooner or later I plan to have the entire mast repainted professionally but in the meantime what is the best etch/prime/paint touchup method to minimize continuing damage? I assume there is a more or less standard sequence and bill of materials used by aircraft or aluminum boat maintenance shops. Any suggestions re online or local vendors would be much appreciated. Scott S/V Itchen >Here is a related question. My Imron-painted aluminum mast and boom has the >usual assortment of fittings and attachment methods typically found on a [quoted text clipped - 12 lines] > > A moisture cured urethane - and best would be an aluminum filled moisture cured urethane. paul oman progressive epoxy polymers >>> Aluminum riv-nuts are easily obtained, easily fitted, and reasonably >>> replaceable when the time comes. Screw a SS screw into a riv-nut [quoted text clipped - 9 lines] > > Brian Whatcott Altus OK I see that now. I must have had an blond moment. Thanks for the link, Dan ... >I see that now. I must have had an blond moment. Thanks for the link, >Dan I'm so envious: wish *I* could have a blond moment, or brown, black or gray. I specialize in pink moments. so to speak..... Brian Whatcott Altus OK > I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads > in there right now) and put a machine screw in there. [quoted text clipped - 7 lines] > bronze, zinc plated steel.. what's optimal for the aluminum > (non-)interaction? Don't even consider brass if you want any strength. It has none and is even worse in a salt water environment as it will lose the zinc that is compounded with copper to make brass.  SignaturedadiOH ____________________________ dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico CB, It is correct to consider the dissimilar metal as an issue, but in this application, it isn't the major one. This is so because the fastener will not be subjected to immersion, only wetting. So, in application, the problem is not so different than on a motocycle case or an aircraft. Stainless in an anti-seize grease is fine. The bigger issue is the thin wall of the mast. Rivets are much better and if removing the fastener periodically is an issue, you should use an aluminum riv-nut riveted in place and then use a 316 screw. Steve > I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in > there right now) and put a machine screw in there. [quoted text clipped - 15 lines] > > -CB On Fri, 25 May 2007 18:40:03 +0200, "Steve Lusardi" <stevenospam@lusardi.de> wrote stuff and I replied: I disagree with ther first bit. If the boat is to be used _exclusively_ in FW environments, then you may get away with it. But the OP did mention possible marine trips. Also, a boat by its spidery nature (don't want to climb the mast) and less industry pressure compared to even automotive, will tend to get less inspection in whatever setup. Also, actual constant immersion can sometime be better than moist environments....less oxygen and other acids etc. >CB, >It is correct to consider the dissimilar metal as an issue, but in this >application, it isn't the major one. This is so because the fastener will >not be subjected to immersion, only wetting. So, in application, the problem >is not so different than on a motocycle case or an aircraft. > Stainless in an >anti-seize grease is fine. The bigger issue is the thin wall of the mast. [quoted text clipped - 22 lines] >> >> -CB Your geting replys Cb. Hows it goin'g > I need to tap out a 1/4" hole in my mast to 5/16" 18 (no threads in >there right now) and put a machine screw in there. [quoted text clipped - 15 lines] > >-CB >Your geting replys Cb. Hows it goin'g > [quoted text clipped - 17 lines] >> >>-CB In theory you want to use either aluminum or something very close to aluminum on the galvanic scale. So, in a perfect world you would purchase an aluminum bolt and use that. Unfortunately this is not a perfect world and an aluminum bolt is not very strong, wear resistant, or available (although they are made), so you go for the next solution and pick a metal close to aluminum on the galvanic scale. Say, Monel, for example. Locate a monel bolt and use that. Again, a bit uncommon but available. Or, you can do what nearly every maker of aluminum spars in the world does. Use stainless and insulate it as best you can. If it were my boat I would drill a hole and either tap it or screw in a sheetmetal screw, depending on the load to be placed on the "bolt", and then install the screw/bolt using something like 3M 5200 for a sealant/insulation. I have stainless bolts installed in both the boom and mast on a 40 ft. sloop, afloat in salt water, in the tropics, that have been there for ten years or more with no signs of corrosion to date. Bruce in Bangkok (brucepaigeatgmaildotcom) SignaturePosted via a free Usenet account from http://www.teranews.com Ok so what I've done is tapped it out (yeah it's a fairly thin wall but this particular application doesn't have any kind of pulling load on it (it's a stopper in the boom/yoke downhaul slider)) and then put in an SS screw (probably < 316 because my local chandlery is such a crap place), and I've insulated the connection point not with anti- seize but with a coating of 3M 4000 that I happened to have lying around. Time will tell.. I'm sure it's fine for the application. Thanks for everyone's comments, especially those regarding the aluminum riv-nuts. I will look into those for next time. Cheers, -CB > >Your geting replys Cb. Hows it goin'g > [quoted text clipped - 44 lines] > -- > Posted via a free Usenet account fromhttp://www.teranews.com > Time will tell.. I'm sure it's fine for the application. Thanks for >everyone's comments, especially those regarding the aluminum riv-nuts. [quoted text clipped - 3 lines] > >-CB Riv-nuts cost peanuts and go on with a pop rivet gun. But if you were paranoid about electrolytic corrosion, there is a species of rubber expanding nut with an internal metal threaded insert. That might suit some low strength applications too. Brian Whatcott Altus OK Brian Whatcott brought forth on stone tablets: >> Time will tell.. I'm sure it's fine for the application. Thanks for >>everyone's comments, especially those regarding the aluminum riv-nuts. [quoted text clipped - 9 lines] > species of rubber expanding nut with an internal metal threaded > insert. That might suit some low strength applications too. Indeed. These are commonly used (or were...) for mounting air conditioner condensers in cars, where galvanic corrosion is also a problem. Unfortunately, however, the only ones I have seen have the threaded insert made of brass. Not a good choice in a marine environment. bob s/v Eolian Seattle > Ok so what I've done is tapped it out (yeah it's a fairly thin wall >but this particular application doesn't have any kind of pulling load [quoted text clipped - 11 lines] > >-CB Basically that is exactly the way my mast was fitted out nearly 40 years ago when it was built, except that I have two SS screws holding in a very short section of the mast track. No problems to date :-) >> >Your geting replys Cb. Hows it goin'g >> [quoted text clipped - 44 lines] >> -- >> Posted via a free Usenet account fromhttp://www.teranews.com Bruce in Bangkok (brucepaigeatgmaildotcom) SignaturePosted via a free Usenet account from http://www.teranews.com so are you actually listening to what you have asked about? > Ok so what I've done is tapped it out (yeah it's a fairly thin wall >but this particular application doesn't have any kind of pulling load [quoted text clipped - 60 lines] >> -- >> Posted via a free Usenet account fromhttp://www.teranews.com Human bevaviour: Bestiality with a brain > so are you actually listening to what you have asked about? > [quoted text clipped - 64 lines] > > Human bevaviour: Bestiality with a brain are you actually trying to troll me here? might want to work on your style a bit. -CB |
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