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Tapping threads in mast

Dec 1, 2020
86
CAL 27 Illahee / Brownsville WA
I am planning on drilling and tapping 1/4 - 20 thread for SS machine screws to hold an additional cleat and a small pad-eye to the side of my Cal 27 aluminum mast. I purchased the drill/tap kit and figure this should be a pretty easy process, the only minor concern is to avoid wires for lights and vhf that are "someplace" inside the mast. All halyards are on the outside.

I plan on drilling very slowly to make sure I "just get through" the wall.

Suggestions on this project are desired. Are there some tricks or techniques I can use to make this a trouble-free project.

Thanks,
 
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Likes: ebsail

RoyS

.
Jun 3, 2012
1,167
Hunter 33 Steamboat Wharf, Hull, MA
Wires are usually in a plastic conduit. You can find that by looking for the rivets that secure the internal pipe straps.
 
Oct 22, 2014
15,863
CAL 35 Cruiser moored EVERETT WA
Is the mast up or is it down on stands in the yard?

Some masts have pvc conduit tubes attached to inside the mast which captures the wires. Keeps them from banging inside the mast as you sail. Most often the conduit is attached to the front of the mast using aluminum rivets near top, perhaps middle, and near the end of the conduit run (which could be a couple of feet above the mast base. Not all smaller boats have this conduit.

Installing the cleat on the side of the mast is often a safe action. Doing the work with the mast on stands in the yard is a lot easier as you can look inside and know what your dealing with.
 

dLj

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Mar 23, 2017
1,777
Belliure 41 Snug Harbor, Lake Champlain
Good advice above - I'll only add in when you go to tap the hole, kerosene is a good cutting fluid to use with aluminum. Dip your tap into kerosene just before starting to tap the hole. You probably don't need much more than that as the wall thickness is quite thin. But if your tap seems to be getting stuck add more - back it out, clean the tap, dip and go back to cutting the threads. Make sure if you do that, you don't cross thread. Soft stuff that aluminum to a tap....

dj

p.s. Why are you threading the mast holes? What kind of fastener are you putting in? I'd recommend monel, but that's hard to find...

 
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Likes: Bob S
Jan 11, 2014
7,746
Sabre 362 113 Fair Haven, NY
Conduits are usually located at the front of the mast.

If you don't use Monel fasteners use a good anti-corrosion compound like Tef-Gel it is a bit on the pricey side, but tub will last you a lifetime.
 
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Likes: AaronD
Dec 1, 2020
86
CAL 27 Illahee / Brownsville WA
Good advice above - I'll only add in when you go to tap the hole, kerosene is a good cutting fluid to use with aluminum. Dip your tap into kerosene just before starting to tap the hole. You probably don't need much more than that as the wall thickness is quite thin. But if your tap seems to be getting stuck add more - back it out, clean the tap, dip and go back to cutting the threads. Make sure if you do that, you don't cross thread. Soft stuff that aluminum to a tap....

dj

p.s. Why are you threading the mast holes? What kind of fastener are you putting in? I'd recommend monel, but that's hard to find...

The mast is still standing on the boat so seeing inside is not possible. Good to learn that wires may be in conduit and/or toward the forward end of the tube making the side mounting a safer location.

I was thinking of using basic SS machine flat top machine screws to fit the fittings flush. Would Corrosion-X work for mounting the SS to the mast threads - I have that. I would rather not have yet another lifetime supply of something new. Another person suggested using 3M4200 since it can be busted free with some heat if needed. Corrosion-X thoughts?

Thanks for the tip on kerosene as a cutting fluid.
 

dLj

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Mar 23, 2017
1,777
Belliure 41 Snug Harbor, Lake Champlain
I don't know what exactly you are mounting, but I'd prefer something like:

I've always found SS threaded fasteners to be a problem years down the road. But that's up to you.

dj
 
Dec 1, 2020
86
CAL 27 Illahee / Brownsville WA
I'm mounting a 3rd cleat to the side using a two-hole high-strength nylon type cleat which needs a 1/4" machine screw which is what I was thinking of using the tap for. I also have a small 2-hole ss pad-eye for a turning block to be mounted near the base of the mast on the side also. Again, 1/4" hole in the pad-eye.

Your link points to rivets, are you thinking those are a better choice?
 

dLj

.
Mar 23, 2017
1,777
Belliure 41 Snug Harbor, Lake Champlain
I'm mounting a 3rd cleat to the side using a two-hole high-strength nylon type cleat which needs a 1/4" machine screw which is what I was thinking of using the tap for. I also have a small 2-hole ss pad-eye for a turning block to be mounted near the base of the mast on the side also. Again, 1/4" hole in the pad-eye.

Your link points to rivets, are you thinking those are a better choice?
For the pad eye, yes. The cleat, I don't know, would have to see details. You might need threaded fasteners for that one.

You had said you needed a flush mount top, don't see monel threaded fasteners for that, but could look elsewhere and see if you find them. I'm quite leary of using stainless steel threaded fasteners in an aluminum mast for anything requiring long term structural strength. For long term structural strength in that application I prefer rivets, particularly monel rivets. But you are at your boat and know what constraints you specifically have and may have reasons that I can't see from here.

dj
 
Jan 19, 2010
9,954
Hunter 26 Charleston
@dLj recommended kerocene for a cutting fluid. That would work well. I have also had good results using WD40. I would try to find aluminum threaded bolts instead of SS. As far as technique goes... Pilot the hole with a drill bit that is at least one size down from the size bolt you plan to use. When turning the tap...turn a half turn with the tap and then back it up a quarter turn, then another half turn ... repeat. By continually backing it up, you break off any burrs that form and you minimize the risk of damaging the threads. Keep squirting the cutting fluid in there to wash out any burrs.

1620315890566.png
 

Joe

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Jun 1, 2004
7,416
Catalina 27 Mission Bay, San Diego
This is only my opinion, but I quit using tapped fasteners in the mast because the wall is too thin to cut an adequate number of threads. I used threaded fasteners to install rigid vang brackets some 20 years ago, but ever since I've used self tapping screws. They get a very secure bite. You need to use the proper corrosion prevention, of course. Tef-gel on the threads and plastic tape on any other surfaces that touch.
Also, I don't use rivets on this boat. SS rivets are hard to remove... self tapping screws... not so much.
 
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Likes: Bob S
May 27, 2004
1,696
Hunter 30_74-83 Ponce Inlet FL
On a slightly different point...
When you set up your drill bit, measure 3/16" from the tip of the bit and put 3 or 4 wraps
of painter's or duct tape on the bit at that point. That will stop the bit going in too far.
If it's not all the way through, re-wrap another 1/16 of an inch further down and try again.
 
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Likes: kappykaplan
Jan 7, 2011
2,833
Oday 322 East Chicago, IN
I tapped holes for cheek blocks for my lazy jacks, and holes in the boom for the strap eyes and a SS bracket for the Mack Pack ...all 1/4 20 and SS screws. I did use Lanocoat on the screws, so far (6 years later), no issues. I have to remove 4 of the screws for the bracket each winter, and no corrosion or issues (I am I. Fresh water though).

Greg
 
Mar 26, 2011
2,916
Corsair F-24 MK I Deale, MD
This is only my opinion, but I quit using tapped fasteners in the mast because the wall is too thin to cut an adequate number of threads. I used threaded fasteners to install rigid vang brackets some 20 years ago, but ever since I've used self tapping screws. They get a very secure bite. You need to use the proper corrosion prevention, of course. Tef-gel on the threads and plastic tape on any other surfaces that touch.
Also, I don't use rivets on this boat. SS rivets are hard to remove... self tapping screws... not so much.
Boom, maybe. But you never use self-tappers in the mast because the sharp point will catch on rope and cut wires. This is one of those rare "never, evers." So far, you've been lucky. Wait until one snares a halyard or chafes through a wire.

I've always found lubed bolts and rivets (drill them out--no problem--sometimes it helps to grab the head with small Vice Grips if it spins) easy to remove.

Rivets are the strongest connection on thin spars (dinghies). I did a bunch of shear and pull-out test for PS Magazine. The OPs mast probably falls in the middle.
 
Last edited:

Joe

.
Jun 1, 2004
7,416
Catalina 27 Mission Bay, San Diego
Insert the screw, then remove the screw and clip off and dull the tip, replace the screw... No sharp protruding screw.
 
Jul 1, 2010
861
Seaward 25, Catalina 350 Erie, Pa
FWiW, I'd use monel rivets instead of stainless bolts. You won't get much in the way of threads tapped in the thin wall of a mast. If you do go with 1/4" screws, I would use fine thread 1/4 -28. You'd get a little more threads to hold with a fine thread screw.
 
Mar 26, 2011
2,916
Corsair F-24 MK I Deale, MD
FWiW, I'd use monel rivets instead of stainless bolts. You won't get much in the way of threads tapped in the thin wall of a mast. If you do go with 1/4" screws, I would use fine thread 1/4 -28. You'd get a little more threads to hold with a fine thread screw.
Course threads in aluminum, even if thin. Check any engineering manual on the topic. Fine threads in soft materials are a common misconception.
 
Mar 26, 2011
2,916
Corsair F-24 MK I Deale, MD
Insert the screw, then remove the screw and clip off and dull the tip, replace the screw... No sharp protruding screw.
I have tested this. A coarse threaded bolt will give more hold unless the material is less than 1/2 bolt diameter, a problem normally solved by using smaller bolts. Or rivets. You won't see a pro or OEM use self tappers in a spar, not even on a dinghy. The exception is a self tapping bolt, which is something different (uses standard coarse threads..

This is textbook mechanical engineering, refined over the years, not my opinion.