# Screwing into aluminum



## Ed Kinney (Sep 26, 2016)

Sailheads!

I'm preparing to mount a Raymarine Rotavecta onto the top of my mast (pictured) and I need to know how this is done. The Windex that is there was secured with what looks like a wood screw (pictured) and Raymarine included two screws (also pictured) that to me look like just plain old wood screws. 

So should I just drill some pilot holes and screw the base into them? Is it really that easy? Doesn't the kind of metal that the screws are made of make a difference? Should I line the holes with some sort of sealant? Or do I need to get out the tap and die set and prepare more formal holes?

Any tips would be appreciated! 

Thanks!


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## midwesterner (Dec 14, 2015)

Those are panhead sheetmetal screws. Usually you drill a pilot hole a little smaller diameter than the screw. I would guess that it's a good idea to bed the screws with butyl tape.


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## JimsCAL (May 23, 2007)

Agree those are sheet metal screws. But the top of your mast is much thicker than typical sheet metal. You should be able to drive them into an undersized hole there, but it isn't going to be easy. A better installation would be to tap the holes and use machine screws.


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## colemj (Jul 10, 2003)

I would tap the holes and use machine screws. Also coat the screws with Tefgel, Duralac, or similar to prevent corrosion, and use a locking washer to prevent backout. Maybe use Locktite instead, which will provide some corrosion resistance and lock them in. I don't see any need to bed it with butyl tape, as it isn't a leak concern.

The problem I always have with sheet metal screws is that if the pilot hole isn't almost the same size as the threads, then driving them through that much aluminum tends to snap them before they sit fully. Then you have a real problem. And if you make the hole large enough to prevent this, then the holding is marginal. The cheap quality screws provided with most things make the problem worse.

Mark


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## Skipper Dan (Mar 31, 2017)

You could use self-tapping screws. If the mast head has any depth to it you need to drill and tap which is not easy in aluminum, go slow back and forth a lot.


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## krisscross (Feb 22, 2013)

Chances are you will not be removing that bracket any time soon. I would just use aluminum rivets. Much simpler and a better mount.


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## albrazzi (Oct 15, 2014)

Skipper Dan said:


> You could use self-tapping screws. If the mast head has any depth to it you need to drill and tap which is not easy in aluminum, go slow back and forth a lot.


Anything is better than nothing on a Tap, Kroil, any tapping compound, I used to use this pink stuff kinda thick I don't remember the name now and in a pinch lithium grease will help a bunch.
+1 on tefgel I have a small tube that will probably never run out, it was cheap too.
SS self tappers will work good I would pre-drill which kinda defeats the purpose but they hold better than a sheet metal screw IMO.


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## colemj (Jul 10, 2003)

krisscross said:


> Chances are you will not be removing that bracket any time soon. I would just use aluminum rivets. Much simpler and a better mount.


I have had way too many aluminum rivets corrode and fail, even when just connecting aluminum parts to aluminum. If you go with rivets, get monel rivets.

Mark


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## Skipper Dan (Mar 31, 2017)

I predrill self tappers. They are just better than sheet metal screws. Either will break when going into aluminum that is thicker if not drilled.


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## Rocky Mountain Breeze (Mar 30, 2015)

As has been stated, drilling and tapping for a machine screw is far better than using a panhead sheet metal screw in aluminum thicker than 1/8". I would use a 10-32 machine screw and apply removable threadlocker (blue) when installing it. A tapping fluid such as Tap Magic would be good to use for aluminum but you could also use plain old mineral spirits, just remember to keep the tap perpendicular to the axis of the hole and back it out a quarter turn or so for every half turn. Rivets, especially aluminum pop rivets come loose and there is no way to tighten them. I would use an Allen head screw although a 5/16 hex head would be fine also.


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## Captsteve53 (Sep 13, 2017)

colemj said:


> I would tap the holes and use machine screws. Also coat the screws with Tefgel, Duralac, or similar to prevent corrosion, and use a locking washer to prevent backout. Maybe use Locktite instead, which will provide some corrosion resistance and lock them in. I don't see any need to bed it with butyl tape, as it isn't a leak concern.
> 
> The problem I always have with sheet metal screws is that if the pilot hole isn't almost the same size as the threads, then driving them through that much aluminum tends to snap them before they sit fully. Then you have a real problem. And if you make the hole large enough to prevent this, then the holding is marginal. The cheap quality screws provided with most things make the problem worse.
> 
> Mark


Drilling and tapping is the professional way to go using SS bolts and use *DURALAC* as this will not only stop any galvanic corrosion issues but will also work as a thread locker, IMHO Duralac far exceeds Tefgel for dissimilar metal combinations as:https://www.fisheriessupply.com/saddington-consultants-plus-duralac-anti-corrosion-jointing-compound

Cheers Steve


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## colemj (Jul 10, 2003)

I'm pretty sure Duralac isn't a thread locker. I would use a mechanical lock washer if you use it.

Mark


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## Captsteve53 (Sep 13, 2017)

In my 40 odd years of using Duralac on threads i have never had one back out, the good side is that it will unscrew under normal wrench pressure,IMHO (YES IT'S NOT ADVERTISED AS A THREAD LOCKER)

Also saying that I'm NOT recommending Duralac for highly stressed vibration area's where the normal loctite or similar is the preferred chose , for simple normal attachments such as mast type fittings it will work well in these situations, and yes a lock washer if it makes you feel safer (Stainless type), using products like LOCTITE in these situations means you have no chance to ever remove with out heat or other difficult items to use at the mast head!


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## colemj (Jul 10, 2003)

Captsteve53 said:


> using products like LOCTITE in these situations means you have no chance to ever remove with out heat or other difficult items to use at the mast head!


Locktite, vibratite, and similar have different versions for different holding strengths. All have versions that can be removed easily by hand tools.

Mark


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## Captsteve53 (Sep 13, 2017)

MARK,

I can only assume you are speaking from a practical experience level and not a book, It's just that after trying to remove any type/grade of thread lockers used in Aluminum/Stainless combo's (especially mast heads) for me after 40 years of commercially doing this has always been a disaster with the SS bolt (usually small diameter snapping off way before it un do's) 

Just my experience talking! Nothing Personal-or from a book!

Cheers Steve


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## RegisteredUser (Aug 16, 2010)

Locktite 'blue' does not require heat.


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## colemj (Jul 10, 2003)

Captsteve53 said:


> MARK,
> 
> I can only assume you are speaking from a practical experience level and not a book, It's just that after trying to remove any type/grade of thread lockers used in Aluminum/Stainless combo's (especially mast heads) for me after 40 years of commercially doing this has always been a disaster with the SS bolt (usually small diameter snapping off way before it un do's)
> 
> ...


Just to be clear, I personally would use tefgel or duralac and lock washers in this application - not thread locker.

The thread locker discussion was a bit of an aside. But blue locktite and vibratite V3 has never failed to release with hand tools for me. I mostly have success without heat on red locktite if I carefully apply an impact driver. Yellow locktite - forget about it.

Mark


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## hellosailor (Apr 11, 2006)

And if Loctite Red has been around for a couple of years...it won't require heat either. The stuff all goes bad in the bottle, two-year toss date. FWIW.


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## Bleemus (Oct 13, 2015)

When I worked on Flyer II, the aluminium former Whitbread winner converted to luxury cruiser, we used Duralac for everything. While not advertised as a threadlocker I found it worked well for that purpose. If your local marine supply doesnt carry it find a business that serves commercial fisherman. They always seemed to have it.

Due to health and enviromental concerns it seems like Duralac has fallen out of favor. Just wear rubber gloves or go with TefGel though I dont think it works as a thread locker.

TefGel on Amazon Affiliate. http://amzn.to/2xS4jqF


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## FreeAgent (Apr 19, 2017)

Rocky Mountain Breeze said:


> As has been stated, drilling and tapping for a machine screw is far better than using a panhead sheet metal screw in aluminum thicker than 1/8". I would use a 10-32 machine screw and apply removable threadlocker (blue) when installing it.


When tapping into aluminum, it is best to use a *coarse* thread. 10-24 in this case. Otherwise the threads are too easy to strip. I would use Blue Loctite. May not NEED heat to remove, but it does help.

It is possible to use the self tapping screws, but you need to make sure the hole is large enough and you need to lubricate the screw. Then you turn it in, back it off, and repeat until it form the thread. Easy to break screw and to mess up head, so not the best way. Not to say I have not done that many times! May be an idea to practice on a piece of aluminum of about same thickness until you find the best hole size.


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## Rocky Mountain Breeze (Mar 30, 2015)

In my 2 year schooling to become a Tool and Die Maker it was repeatedly stressed that fine threads always have better holding strength than coarse threads. I can look up in my Machinists handbook to see if there is a difference between steel and aluminum holding strength but I would be interested in your source for thread strength information.


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## FreeAgent (Apr 19, 2017)

Rocky Mountain Breeze said:


> In my 2 year schooling to become a Tool and Die Maker it was repeatedly stressed that fine threads always have better holding strength than coarse threads. I can look up in my Machinists handbook to see if there is a difference between steel and aluminum holding strength but I would be interested in your source for thread strength information.


My source is 30+ years of experience working on masts & rigging. For confirmation, try a Google search for tapping aluminum sheet coarse fine.

https://www.google.ca/search?as_qdr....1.10.3343...0i22i30k1j33i160k1.0.ofcrm6uTvHw


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## colemj (Jul 10, 2003)

Rocky Mountain Breeze said:


> In my 2 year schooling to become a Tool and Die Maker it was repeatedly stressed that fine threads always have better holding strength than coarse threads. I can look up in my Machinists handbook to see if there is a difference between steel and aluminum holding strength but I would be interested in your source for thread strength information.


Both of you are correct. Fine threads have better overall holding strength, but in a corrosive, galvanic environment, they are more apt to strip out than course threads.

Mark


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## Maine Sail (Jan 6, 2003)

colemj said:


> *I would tap the holes and use machine screws.* Also coat the screws with Tefgel, Duralac, or similar to prevent corrosion, and use a locking washer to prevent backout. Maybe use Locktite instead, which will provide some corrosion resistance and lock them in. I don't see any need to bed it with butyl tape, as it isn't a leak concern.
> 
> *The problem I always have with sheet metal screws is that if the pilot hole isn't almost the same size as the threads, then driving them through that much aluminum tends to snap them before they sit fully.* Then you have a real problem. And if you make the hole large enough to prevent this, then the holding is marginal. The cheap quality screws provided with most things make the problem worse.
> 
> Mark


Bingo on drill & tap & snapping of sheet metal screws...!

I would only add that if your mast head is thin-wall aluminum that you may want to use a fine thread as opposed to course thread. For a windex, it really won't matter thought either would be ok.

Fine thread in thin walled aluminum is what many spar makers use, Harken recommends as well as the two professional riggers I use & sub out to for certain tasks. Of course the issue of fine/course is not set in stone because some mast builders, like Z-Spar/US Spar often use metric standard pitch too, when spar thickness exceeds 4mm.. I always used to call the spar maker and ask, based on the extrusion, but after most of them said to use fine thread I settled on that for applications where a machine screw is needed and the extrusion is thin...

Masts by nature leak like a sieve so there's no real need to seal it...


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## FreeAgent (Apr 19, 2017)

Ed Kinney said:


> So should I just drill some pilot holes and screw the base into them? Is it really that easy? Doesn't the kind of metal that the screws are made of make a difference? Should I line the holes with some sort of sealant? Or do I need to get out the tap and die set and prepare more formal holes?
> 
> Any tips would be appreciated!
> 
> Thanks!


Ed, you probably now have more advice than you need 
Those screws are sheet metal screws. They are intended for use on sheet metal! (in aluminum, say 1/8" or less) If you are screwing into the mast head that looks to be about 1/4" aluminum, then drill and tap. You are not looking for strength here, just something that will work. 
I would use a standard course thread like 10-24. Others say fine threads will be stronger. Perhaps in some applications, but only 10% stronger and not recommended for soft metals like aluminum.
If you don't want to strip the threads, use 10-24 stainless screws and put a little blue thread locker on to reduce corrosion, lube as you screw in and avoid screws loosening later from vibration. 
Drill and tap a piece of scrap aluminum first and try the screws in that. It will give you an idea of how loose or tight the screws will be. You do need to use the right size drill bit for the thread you are creating. 
It is that easy


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## colemj (Jul 10, 2003)

Ed, I decided to bring up a related topic before you mount your Rotavecta.

That wind unit isn't really appropriate for sailboats, and was designed for power boats. The difference is in the accuracy of the wind angle reading. It works OK when going faster and the apparent wind is forward of the beam. But for a relatively slow sailboat with wind from all angles, that unit is reported to twirl and hunt and be useless at times.

However, I don't have personal experience with it. The above was gained when researching wind instruments by reading many on-line reports from others who installed them on sailboats.

I bring this up in case you may have the opportunity and desire to return it for RM's sailboat version of a masthead wind transducer before you actually drill holes and mount it.

Mark


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## FreeAgent (Apr 19, 2017)

may need new wind instrument myself, so this is interesting:

Rotavecta vs Vane


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## Maine Sail (Jan 6, 2003)

colemj said:


> Ed, I decided to bring up a related topic before you mount your Rotavecta.
> 
> That wind unit isn't really appropriate for sailboats, and was designed for power boats.
> Mark


Very good catch Mark!!


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## Rocky Mountain Breeze (Mar 30, 2015)

If the mast is thin wall aluminum a Riv-nut would probably work much better than any direct threaded fastener. Drill a hole, insert the riv-nut, expand the base, and then mount your piece with stainless steel screws.


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## roverhi (Dec 19, 2013)

Aluminum is a very soft metal. Tapping for fine threads is asking for trouble from cross threading and offers very little advantage over coarse threads for strength. Have never found a fine thread on the few masts I've worked on, always coarse threads.

Self tapping screws in aluminum are asking for trouble. The threads on these screws are so coarse that only part of a thread is actually gripping the aluminum. Have seen them work and strip out all too often where it's almost never happened on a drilled and tapped coarse machine screw.


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## RegisteredUser (Aug 16, 2010)

Why not back the mount with the correct vhb tape and then fasten with screws...


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## hellosailor (Apr 11, 2006)

VHB tape? In case there are 135mph winds in the cabin, trying to rip the lids off? (G)

After it has had 48 hours to set, that stuff is almost as bad (good?) as epoxy. Sure as hell wouldn't need 50 screws with it though.


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## colemj (Jul 10, 2003)

Actually, VHB tape is difficult to install correctly. Yes, it will hold windows in skyscrapers, but installing it to that performance level takes far more prep and care than just peeling off the backings and sticking things together. Most don't know that there is a specific primer for it. I've given up many man-years of experience learning how to work with that stuff correctly.

I do love it now.

Screws won't be necessary if VHB is installed correctly.

You won't get VHB installed correctly.

Mark


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## john61ct (Jan 23, 2017)

Link to this primer?

Maybe rocket science is required to get full spec'd loading strength.

But just following some basic common sense

https://www.3m.com/3M/en_US/bonding...?storyid=b3996cbd-9954-455f-8e72-88e452ca38c0

will give pretty amazing results, assuming you pick the right tape of course.


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## colemj (Jul 10, 2003)

I work with a lot of VHB in different applications and have learned a lot by trial and error. Hanging a picture on a wall - no problem. Joining a plastic transducer to a painted aluminum masthead - possible, but I'll bet nobody here has the experience with this stuff to do it successfully.

I wouldn't try it.

Mark


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## FreeAgent (Apr 19, 2017)

Rocky Mountain Breeze said:


> If the mast is thin wall aluminum a Riv-nut would probably work much better than any direct threaded fastener. Drill a hole, insert the riv-nut, expand the base, and then mount your piece with stainless steel screws.


He said he was mounting the instrument on the TOP of the mast that he showed a picture of.

Anyway, this thread seems to have run it's course :2 boat:


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## Ed Kinney (Sep 26, 2016)

Thanks, all!


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## RegisteredUser (Aug 16, 2010)

A tad off subject....
VHB does isolate dissimilar metals...if that if of concern.


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## hellosailor (Apr 11, 2006)

VHB works best when it is flat-on-flat. It needs matching surfaces. So with the typical curved mast sides and little flat space being free on the mast truck...not a prime candidate for VHB. Unless you can find adequate flat-to-flat up there.


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