# Self tapping screws in the mast.



## paul77 (Jul 5, 2007)

Okay, so this may seem a little weird. In looking into proper mounting for some mast steps to put near the mast top, the clerk at the local marine supply store (seemingly knowledgeable fellow) said to use self tapping screws to affix to the mast. 

This seems a little odd... I expected to have to drill and tap holes to install these. Anyone used/heard of using self tapping screws?

The mast is aluminum.


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## sailingdog (Mar 19, 2006)

It really depends on your mast. If the mast is fairly thin wall construction, then self-tapping screws may be a better choice than machine screws, which really need a minimum thickness for them to be secure. If you are going to use machine screws, make the holes about 75% of the size of the screw, so that you've got enough material for the screws to bite into. For instance, if you're using 1/4" screws, the holes should ideally be 3/16" or so. 

A better option, IMHO, is using stainless steel pop rivets. I would not recommend aluminum pop rivets because they're considerably weaker. 

Whatever you decide to use for fasteners, you really should use TefGel, Lanocote or some other anti-galvanic corrosion gel on the fasteners to help prevent the mast and fasteners from reacting to each other.


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## paul77 (Jul 5, 2007)

Okay, his line of reasoning was similar regarding the mast thickness. Thanks for the rule of thumb, I'll check the thickness tomorrow and see if it's right for tapping. 

There is another selden rig at the yard (similar size to mine), which has the steps already installed. He does have screws, rather than rivets, holding them into place. 

Already bought the tefgel 

Cool,

paul


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## Gramp34 (Oct 5, 2006)

Your store guy probably was thinking sheet metal screws. There are true self tapping screws available with a notch in the end to cut threads, but they're going to be very tough to source in stainless.

Stainless sheet metal screws are strong enough to form threads in sheet metal, but in a mast extrusion I think you'll find you'll have a bear of a time turning them in and will likely torque the heads off screws.

Machine screws are much better. You'll need a proper size tap drill for the screw thread you're using. Use a tap drill size table, e.g., like at http://www.efunda.com/designstandards/screws/tapdrill.cfm Hardware stores often have the tap and proper tap drill in the same package. (75% of major diameter is too small for a tap drill.) Put lanocote or other lubricant on the tap as you're cutting threads to get cleaner threads and make the tap last longer.

Coarse threads (e.g., 10-24 vs 10-32, etc.) give more strength in softer metals like aluminum.

I like button head allen screws. You will probably have to get them from an industrial fastener supplier, check your Yellow Pages. The heads are nicely rounded and the allen wrench doesn't want to cam-out when you tighten or loosen them. The fastener supplier will also sell taps and tap drills.










Cheers,

Tim


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## jackytdunaway (Sep 11, 2006)

wouldn't self tappers leave a sharp point inside the mast that the halyards could rub against and get damaged?


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## sailingdog (Mar 19, 2006)

Tim-

If the mast wall is too thin, you can't trust a tapped hole to have any serious strength. The mast on my boat is such that the spar manufacturer said that nothing should be attached using screws, since they won't have a prayer of holding under any load. The winch bases and cleats are either through-bolted to a backing plate inside the mast or screwed to a base that is riveted to the mast. 

Jacky-

If you're pre-drilling the holes, you can always file the points off...but yes, that would be a possible issue with self-tapping screws.


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## sailaway21 (Sep 4, 2006)

You can get the self-tappers in aluminum.


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## Gramp34 (Oct 5, 2006)

sailingdog said:


> Tim-
> 
> If the mast wall is too thin, you can't trust a tapped hole to have any serious strength. The mast on my boat is such that the spar manufacturer said that nothing should be attached using screws, since they won't have a prayer of holding under any load. The winch bases and cleats are either through-bolted to a backing plate inside the mast or screwed to a base that is riveted to the mast.


I'm surprised the spar maker would say that. What size of mast is that thin?

Here's part of the article "Installing Hardware on Aluminum Spars" from DIY Boat Owner magazine (1998 issue #1 page 27):

_Machine screws are used exclusively by custom mast builders to fasten most hardware. More time consuming to install than rivets, they require drilling an undersize hole, then using a tap drill to make a threaded hole for the screw. Screws are identified by the size and number of threads. A No. 10-24 screw, for example, denotes a screw with a maximum diameter of 10 (.190) and 24 threads per inch. Look for National Fine (NF) screws; the finer the thread, the better the holding power.​_In this article they interviewed Danny Klacko of Klacko Spars. His company has built spars for a number of production boatbuilders, including for all the C&C boats before Tartan bought the name. C&C were high performance boats, so I would expect their masts to have been as thin as possible.

My previous statement about using coarse thread screws in aluminum stands corrected, although I'm puzzled by their example because 10-24 is coarse and 10-32 is the fine thread in that size.

I've got Klacko spars on a 1976 34' sloop (and a 1973 34' ketch, but that's another story). The boom was assembled with rivets, and much of the mast hardware with machine screws. When I got this boat most of the rivets on the boom were loose. All of the machine screws are tight with none of them showing any signs of working. The gooseneck was worn out, so this boat did do some sailing in that time. The mast is probably 1/8" thick, which is typical. There are no backing plates.

SD - the OP asked about putting steps up a mast. If a mast is so thin that machine screws would pull out, wouldn't you be concerned about the mast dimpling around each step if an adult climbed the mast? Those dimples would really weaken an already fragile mast.

Cheers,

Tim


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## Quickstep192 (Jan 6, 2001)

A local rigger taught me a clever trick tht lets you thru-bolt to a mast or boom. It's a lot of trouble, but if you want something to be really secure, it may be the way to go. If I were going to climb the mast, I would want to make sure the step wouldn't give way. 

Make the mounting hole slightly oversize so a screw with a string attached can pass through. Dental floss may be a good candidate for the string since it's thin, but still fairly strong. Use a screw that's a good bit longer than needed for the job. Here's the process:

Tie or tape a string to the end of a machine screw and drop it through the mast. Use a piece of wire with a hook on it to fish the string through the mounting hole. Once retreived, pull the end of the screw through the hole in the mast and throught the mounting hole of whatever you're planning to attach to the mast. The head of the machine screw is now on the inside of the mast and you're holding the threaded end. Start the nut onto the machine screw, holding the end and getting the nut as tight as you can by hand. Next, grab the threaded end of the machine screw with a pair of vise grip pliers and finish tighteneing the nut. Once satisfied, cut off the excess length from the machine screw and file it smooth. After this much work, I'd also be inclined to use some threadlock or a nut with a nylon insert to make sure it didn't get loose and fall down the mast.


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## sailingdog (Mar 19, 2006)

The finer thread pitch is necessary if you think about it. In a the fairly thin material of a spar wall, you want as many complete threads as possible, and the finer the thread pitch—the more complete threads you'll have in tapped in the wall. 

The real problem with using finer pitch thread screws is that they are far more susceptible to galvanic corrosion issues. 

As for machine screws pulling out... the real problem is that many people won't do a good job of tapping the holes. If the holes aren't tapped perfecty, then the strength will be severely compromised. I don't think the steps would dimple the mast under load, since their design probably spread the load out over a fairly wide area. The screws are more of a point load.


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## j34035 (Nov 10, 2006)

A good source for fasteners is McMaster-Carr on the internet. They are an industrial supply house with several locations and a huge inventory. They sell small quantities and ship really quick. If you can dream it up, they have it. WWW.mcmaster.com is their irl.
DD


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## Omatako (Sep 14, 2003)

Monel pop rivets. Strong as heck and won't let go.

And if the pop rivet dimples then any form of screw will do the same.

I used monel 5mm rivets for mast steps and had years of use without any issues.


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## RXBOT (Sep 7, 2007)

*Coarse vs Fine thread*

If the material you are tapping is thin then yes you should use fine thread. However it will strip out easier than a coarse thread. Generally fine thread is better suited for applications where a fine adjustment is needed,mostly for positoning or tensioning purposes.


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## Valiente (Jun 16, 2006)

What are the galvanic issues between the mast step material, the Monel rivets and the alu mast? I have a Selden mast 6 x 9 that seems reasonably thick-walled, but I would far prefer the ease of rivets to tapping a hundred screw holes as long as it didn't represent a compromise in strength (I am large and at the mast top would only accentuate a pendulum motion...)


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## christyleigh (Dec 17, 2001)

I also went with fine thread machine screws on my c320 after a long debate on the Catalina lists a few years ago. I've had my new set of mast steps in storage on the NC all last summer waiting for the urge and surge of energy to hit - it never did. Oh... well... maybe next year  One thing not mentioned here was was the need for some form of 'stopper' on your drill bit, even if its just tape, so that you only drill the 1/4" or whatever you need. Obviouis as it may be ......drilling through the halyard that's holding you up could spoil your whole day


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## TrueBlue (Oct 11, 2004)

Paul77,
You've probably noticed that Selden uses monel pop-rivets in most of the components attached to the spars. The rivets are certainly strong enough to bear the weight of the deck stepped mast base, booms and winches on both of my masts.


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## Valiente (Jun 16, 2006)

So would monel pop rivets trump machine screws? It would be significantly easier just to drill rivet holes than to tap 60-90 holes (three or four per step).


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## sailingdog (Mar 19, 2006)

I would go with either Monel or Stainless Steel pop rivets, coated well with either TefGel or Lanocote to minimize galvanic corrosion issues. Aluminum pop rivets aren't strong enough to use IMHO, but I've used SS ones with little problem. 

The one major point I'd make is that stuff attached using pop rivets should be stuff that doesn't have to be removed to be serviced. The line clutches and winches I've had to mount were attached using machine screws to a mounting plate that was riveted to the mast...this gives you the ability to remove the winch or clutch for maintenance, while giving the strength and simplicity of pop rivets for the installation.


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## rperret (Apr 11, 2003)

don't forget you'll need a big rivet gun - either a big daddy riveter, or a pneumatic rivet gun. something most people don't have in their tool shed...

Rick


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## sailingdog (Mar 19, 2006)

Sears sells a hand-held mechanical rivet tool that will handle 3/16" stainless steel rivets. IIRC, it's about $20 for it.


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## Omatako (Sep 14, 2003)

Valiente - I weigh in at a modest 114kgs and had my maststeps on with 3 off 5mm monel rivets per step. Never had one even come loose let alone pull out and in one event had a jib sheet wrap around the step and pull really or tight with no measurable effect. Also we had that boat for 5 years with the steps and no noticeable corrosion.

Rick - I have a "concertina" type handriveter that effortlessly pulls a 5mm monel rivet real tight. The only issues I had was the first riveter had a mandrel size a little too big and slipped when pulling. Got another and no more problems


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## Omatako (Sep 14, 2003)

*Concertina pop riveter*

I guess I need to explain a "concertina" pop riveter. Have a look at this:

http://www.fascor.co.za/ProductsRivetingTools/fh44.htm

I don't know if these are available where you are so don't know if this is of any help. I just know that these tools take no nonsense from rivets! The one in the link is the little one,for some reason the page for the larger version wouldn't open for me


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## Valiente (Jun 16, 2006)

Thanks for the tips. If I can have the confidence that monel rivets will hold me, the steps, and my size 13 feet (I may have to make my own steps out of 1/4" alu bar stock...), then a Lanacote or Tufgel-coated monel rivet would suit me far better than drill, tap, screw...no pun intended.


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## 66luders (Jan 22, 2007)

I have been planning to install mast steps too. I already bought ABI folding aluminum mast steps which have four 1/4 counter sunk holes drilled in them. I have some counter sunk 1/4 20 stainless machine screws... but I was worried about corrosion and the installation. So... I found 1/4 countersunk aluminum rivits. I even bought a cheap harbor frieght rivit tool. I haven't installed the steps yet because I am now concerened about the way the contour fits the mast... the steps seem to be designed for a slightly flatter or larger mast. I tried welding on some additional material to the back of one step so that it make better contact with the mast but it is very time consuming. I really want to get it done but I want it done right. I guess welding and grinding in a rounder contour for 20 steps isn't so bad.

Oh, but wait... The rivit tool can only fit the top two holes without interferance. I think I might be able to grind away some of the rivit tool and get it to do the other two holes too. 

What a PITA. I wish I bought different steps.


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## sailingdog (Mar 19, 2006)

66luders-

You'd be far better off not welding on additional material, since heating aluminum will reduce its strength considerably. IMHO, you'd be much better off making bases for the steps and then riveting the step and base to the mast or screwing the step to the base and then riveting the bases to the mast. It might even be faster and simpler than welding and grinding the 20 steps. It will also look a hell of a lot better than welded and ground steps probably would—especially in terms of uniformity. 

Finally, I wouldn't trust aluminum rivets. The tensile strength of aluminum rivets is far lower than that of stainless steel or monel rivets of the same size.


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## TSOJOURNER (Dec 16, 1999)

I cast my vote for the S/S rivets. 
Or get a monkey and train it to go up the mast for you...


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## soulesailor (Nov 18, 2007)

*mast steps*

In my opinion I wouldn't use self tapping screws or rivets. Hand tapped in machine screws are your answer. When you stand on the step the force is straight down and the strength of the fastener is, then, its shear strength (diameter not including threads). The threads are there to keep the machine screw in place and keep everything snug. This is where self tapping screws and rivets fail because they can work loose and get sloppy. Self tapping screws have too far a distance between threads to bite into the mast wall enough to not move when stood on repeatedly. Rivets are meant to hold things together, not shear loads or hoping the peened over heads are adequately strong to never budge. The pre-drilled holes in your steps will determine the size machine screw to use and this will lead to the correct size tap and drill bit to buy for those screws. Drilling holes and tapping in threads for them is extremely easy. Use high speed steel (HSS) for your drill bit and get your gear at a hardware store.


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## lucidsalt (Nov 20, 2007)

*Manukea*

Sounds like you are doing a lot of great work on Manukea. Do you have any recent pics of the boat online? A friend of mine owned Manukea from the late nineties to about 2005. I crewed on the boat once from the Bay Area to Southern California and enjoyed a fine passage.


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## lucidsalt (Nov 20, 2007)

*Manukea History*

Bret,

Private Message your email or some alternate contact than SailNet to me as I can't Private Message you back (too new a user on this forum).

I'll send you some an email about Manukea.

J.



66luders said:


> I have been planning to install mast steps too. I already bought ABI folding aluminum mast steps which have four 1/4 counter sunk holes drilled in them. I have some counter sunk 1/4 20 stainless machine screws... but I was worried about corrosion and the installation. So... I found 1/4 countersunk aluminum rivits. I even bought a cheap harbor frieght rivit tool. I haven't installed the steps yet because I am now concerened about the way the contour fits the mast... the steps seem to be designed for a slightly flatter or larger mast. I tried welding on some additional material to the back of one step so that it make better contact with the mast but it is very time consuming. I really want to get it done but I want it done right. I guess welding and grinding in a rounder contour for 20 steps isn't so bad.
> 
> Oh, but wait... The rivit tool can only fit the top two holes without interferance. I think I might be able to grind away some of the rivit tool and get it to do the other two holes too.
> 
> What a PITA. I wish I bought different steps.


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## sailingdog (Mar 19, 2006)

Rick-

The blue loctite probably functions quite well as a galvanic separator film, as well as a thread locking agent.


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