Features September 2018 Issue

How to Tap and Rivet Into A Sailboat Spar

Tapping holes is a skill that requires some practice. Start with a piece of scrap metal, because mistakes in a spar are impossible to erase.

Single taps cost about $15, though you will need a handle. A small set (#8 through ¼-inch) sells for about $60. Carbon steel taps are fine for aluminum. High speed steel is required for stainless steel. Chose “starting” taps for typical through-threading work. Taper and bottoming taps are used sequentially for blind holes and are harder to start.

1. Use cutting fluid (3-in-1 Oil also works for small jobs), for both drilling and tapping. This improves the accuracy of the threads.

tapping sailboat spar
Drew Frye

The riveter and rivet need to be properly aligned in order to ensure maximum clench strength.

2. Drill the hole square and the correct size. It is best to carefully match the tap to the fastener but drilling to the nearest 1/32-inch is generally sufficient in aluminum. A #10 coarse thread screw (24 threads per inch) calls for a #25 tap drill. A #10 fine thread screw (32 threads per inch calls for a #25 tap drill. (Most screw suppliers have a table to determine the correct tap and drill size.)

3. Start the tap with firm downward pressure, turning and resetting your hand for the next turn without disturbing the tap. Keep the tap very square. You should be able to feel the tap bite within ½ turn, and the threads should be stable enough for full turns after about two full turns.

4. In thin materials (less than ½-inch diameter), continue turning until the tap is well through and is turning easily. In thicker materials you will need to clear the metal swarfs (chips). These should start to thread out through the flutes in the tap where they can be brushed away. Forcing the tap back out over larger swarfs can damage the threads in soft materials.

tapping sailboat spar
Drew Frye

The blind (inside) end of the rivet can protrude approximately one quarter-inch before being clenched and upset.

5. Back the tap out gently. Blow the remaining chips out with a straw or small brush.

Machine screws threaded into soft aluminum spars need to be carefully tightened.

If you feel any decrease in resistance, stop. For testing, we used these torque values: 3/16-inch coarse-thread screw in 12 gauge aluminum (.08 inch thick), 10 inch-pounds torque; a 1/4-inch coarse thread screw in 10 gauge aluminum (.1089 inch), 20 inch-pounds torque


Rivets are more forgiving of operator error or inexperience. So long as the hole is drilled the correct size, the rivet is the correct draw length, and the materials are pressed together when the rivet is fully upset, all that is needed is a hard squeeze. If the rivet is too long, it may fail to clench properly and will be weak. Often, repeated squeezing will correct for minor over-length situations.

riveting sailboat spar
Drew Frye

Flutes in the tap allow metal swarfs (chips) to peel out of hole that is being tapped.

If the rivet is too short, the blind end will not be able to fully upset on the backside. The fitting must be pressed firmly to the spar; if the there is a gap, this increases the draw up length and may cause failure. Although rivets are very good at drawing the hardware tightly to the spar, fit is essential. The range of close-fit for each rivet length is small.

Blind rivet tool for rivets up to 3/16-inch costs $40-$60.

Removing Fasteners

Once corrosion has settled in, removing small fasteners from aluminum is a challenge. Machine screws should be installed with anti-seize (see PS July 2018). If stuck, a few good raps with a hammer followed by an all-day soaking in PB Blaster is often helpful. Blind rivets must be drilled out, but that is usually quite easy, since the rivet guides the drill and only the head flange must be drilled off. If the rivet begins to spin, grab the head with vise grips. Failing that, either remove the head with a sharp chisel (easy if the drill made any progress) or delicately grind it off. Pry up the remaining bits of the head and punch it through. Generally the hole is completely undamaged and is reusable.

Comments (8)

I'm surprised no one has mentioned using a "drill-tap". It is a combination drill and tap that does the work in one motion, i.e...drills the hole and taps it in one passing. I've used these on my mast a number of times and it makes the job easy particularly when your up the mast in a bosun's chair. I get them at Tacoma Screw, a professional fastener store, and they are more expensive but worth it in my opinion as you are usually only doing one size screw at a time.

~ ~ _/) ~ ~ MJH

Posted by: MJH | September 5, 2018 11:53 AM    Report this comment

Periodically as part of my job, I need to drill and tap a few hundred holes or set a few hundred pop style, blind rivets.

If you want to drill and tap a quality threaded hole you need to use quality tools.
You want high speed steel taps and drills. A decent tap will cost you $5 to $10 in a small size. Likewise a proper drill will cost a few dollars each. If you bought them at Home Depot or Menards, etc you likely bought an inferior product. Forget about carbon steel taps and drills. Most are junk. If you are doing stainless you want to use cobalt alloy spiral point taps and cobalt alloy drills.
Look for the places where professionals buy their tools.
You want a spiral tip tap. That kind of tap pushes the chips forward of the tap into the hole, so the tap will tend not to jam.
To drill and tap aluminum, Tap Magic Aluminum is the best product I have found. Much better than WD40, or 3 in 1 oil, or any other light oil.
The most consistent way to tap a hole in thinner metal like an aluminum spar is to use a good cordless drill. Set the torque clutch to a low level, and engage the drilled hole and pull the trigger. The clutch will ratchet when the torque has been reached. Reverse to break the chip, put it back into forward and continue. Before you start drilling your spar, get a piece of 6061-T6 aluminum in about the same thickness as your spar and PRACTICE! Do 10+ drill and tap routines and then once you are comfortable with it and then go for it. Its very possible, and more than likely that if you use a spiral point tap that you will run the tap fully into the hole without stopping or stalling the drill clutch. The spiral point tap is key to being able to do that. By using a cordless drill you can make much more consistent tapped holes once you get the hang of it. Using a hand tap holder is fraught with problems. You will tend to bend the tap to the side and wallow out the hole. Using a cordless drill with a spiral tip tap makes drilling and tapping a hundred holes a realistic job. Oh.. and always mark your holes with a center punch!

Regarding rivets, or more appropriately Pop style blind rivets, don't overlook the use of a Hydraulic Riveter. They are run on compressed air and allow you to set steel, stainless steel and monel rivets up to 1/4" without problems. Once you have used one to set a number of rivets you will consider it a necessary tool. They start at about $80 from a big discount nationwide tool store. But a good hand riveter that can set a 3/16" stainless rivet is not cheap either. Blind rivets are easily removed with a drill. Drill off the head and punch the back end of the rivet into the hole.

Posted by: Dave9111 | August 26, 2018 9:26 PM    Report this comment

"high speed steel" NO NO NO, Use cobalt tipped bits for drilling stainless, using a cutting oil or lubricant, and do it a SLOW speed. You should be able to see the flutes in the drill bit; if they're a blur the bit is moving too fast. Keep adding lub as you drill to keep the bit cool.

Posted by: PeacheyKeen | August 22, 2018 5:56 PM    Report this comment

Lots of good comments on tapping. It is certainly a skill that requires practice, and there are many good machine work sites on-line. I first learned the craft 40 years ago.

Rivets were only recommended for light spars (less than 0.08" thick), typically found on dinghies and smaller trailerable boats. They are very well proven in this application and outperform screws. They are easily removed by drilling with a 3/16-inch drill, often much easier than removing seized screws. Finally, rivets require less practice and are often more appropriate to beginning boaters for this reason. So long as the holes are drilled true, the rivet is the correct length, and the parts are held firmly together while upsetting, there is little that can be done wrong.

Machine screws are recommended for thicker spars. Backing plates and nuts are even better.

Tefgel and other grease-type anti-seize products can make it easier for screws to back out if already loose, but if tightened to the correct torque, it isn't a problem.

Posted by: Drew Frye | August 22, 2018 2:30 PM    Report this comment

If using tapped threads, is there a concern for the screw backing out of the spar? I know that a SS screw needs to be coated with something to minimize galvanic corrosion while in an aluminum spar, but is a thread locking compound a better or safer alternative?

Thanks for such awesome details on rivets vs threaded fasteners in the comments!

Posted by: 99expy | August 22, 2018 1:47 PM    Report this comment

Hoping that that is a typo by Piberman? $100,000 to remove and refurbish spars on a 60 foot boat?

Posted by: lesliegb | August 22, 2018 8:03 AM    Report this comment

Rivets are nice. For small boats. Serious cruisers use taps, not rivets. Why ? So the fastenings can be readily removed when required. Larger sailing boat/vessels to stay in "certification class" typically have the rigs removed every 5 years. And then taken completely apart. Why ? To check for corrosion. That usually means taking every thing down to bare metal. It's not a matter of finding corrosion. But where it is and repairing it with suitable metalwork welding. Rivets can't be undone. Taps can. Taking the rig on a 60' crusing Sailboat completely apart, down to bare metal, repainting and repairing as needed can easily cost $100k. Experienced blue water sailors know keeping the rig up is job one. And that means periodically taking it completely apart every so many years.

A comment on taps. But ones same quality as machine shops use. Not cheap. An experienced hand will do better with a high grade drill than with a hand or ratchet tap. Best of all is using a drill press turned by hand with the fixture is secured so the tap goes in 90 degree truth from the cardinal points of the compass. Tapping looks easy. Doing it correctly is not. If done correctly the machine screw will fit snugly.

Taps ought be done sparingly. Using machine bolts with back up washers/nuts often requires inspection plates. Good grade stainless bolts rarely fail if properly sized to their intended purpose. On the whole avoid rivets. Not nearly as strong as taps when properly done. And can't be easily undone.

Again the golden rule for masts is that everything ought be able to be removed when needed for inspection. Loosing a mast makes for a real bad day. Been there done that.

Posted by: Piberman | August 21, 2018 9:14 PM    Report this comment

A couple of comments:
1. First about tapping; Taps are extremely strong but brittle. It is very important to secure you work a rigidly as possible. Use aids to insure both the hole you drill and the tapping process are square to the work. If you can put your work in a vise on a drill press do it. You can then drill the hole square and gently start the tap into the hole at the lowest speed possible gently tightening the tap in the chuck so it will slip when some resistance is felt but the tap gets a start. Finish the hole with a two handed tap handle exerting equal pressure on each side of the tap driver. In thicker materials (and especially SS) after you have achieved a turn or two reverse the tap a half turn to break the cut chips and clear the cutting side for a new cut. I find using a paste cutting medium to be cleaner and more effective. Some larger marine stores carry this. I always go down a few numbers (up in size) on the drill index when drilling SS. If you do a lot of drilling and fastening with sheet metal screws or some drilling and tapping I have found an investment in a "numbered" drill index is money well spent. A 10-32 tap calls for a #21 bit. A 10-24 tap calls for a #25. Be ever so carefull and use a new tap for a part that is very important to you. Broken taps can be very difficult to remove.

Posted by: Leonidas06 | August 21, 2018 5:08 PM    Report this comment

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