ATTACHING CLEAT ON MAST

[maine50]

I would like to add 2 cleats on my aluminum keel stepped mast. Is tapping threads the best way? The mast section is 9.4 x 6.1 with minimum wall thickness .155. The bolts would likely be 5/16. What's better- course or fine threads if tapping? The attachment point would be about 8' from base.

[a64pilot]

It’s the way I added cleats and rope clutches, and even my whisker pole track.
I’d say fine thread definitely and use lots of tefgel, and even then I feel that the SS screws may eventually become rivets.

[billknny]

Originally Posted by maine50

I would like to add 2 cleats on my aluminum keel stepped mast. Is tapping threads the best way? The mast section is 9.4 x 6.1 with minimum wall thickness .155. The bolts would likely be 5/16. What's better- course or fine threads if tapping? The attachment point would be about 8' from base.

The general "Rule of Thumb" for thread engagement is 3 threads. This is a bit simplified, but for most of the work we do, it is satisfactory. If you are using 5/16x18 bolts, and your mast is 0.155 you're ALMOST there. The downside of fine threads is they are shallower, so grab less metal. A 5/16x24 really won't get you that much extra strength.

That's a long and round about way of saying: Either works just fine.

Double up on using TefGel. I have pulled screws out of masts that I installed 10 years before with TefGel. Nothing other than a screwdriver was needed. This stuff works.

For cleats, rivets really aren't a good idea. Rivets big enough are hard to install with hand tools. For really high loads, a well installed rivnut is an option.

[roverhi]

Aluminum is a soft metal. Fine threads are much more susceptible to damage from cross threading and corrosion. Stick with coarse threads. Current boat is having it's 50th birthday and haven't had a problem with the coarse fasteners other than trying to remove them. Screws were probably installed without an insulating medium as they were a bitch to remove when I stripped the hardware off the mast. Still after much travail with impact screw driver, heat, and penetrating oil, got every thing off and the coarse threads in the mast are all still good.

[maine50]

Thanks for the input guys...

[jamhass]

I have had excellent luck with drill and tap and using Tefgel. In addition, I use black PVC gasline tape on the underside of whatever I am attaching to the mast as a "corrosion buffer" This tape is sold by plumbing stores (Home Depot, Ace Hardware, etc) for wrapping gas lines where they exit the ground. It is 2 inches wide and quite thick. Stick it to the bottom of the part and trim with a sharp knife.

[oleman]

Has anyone considered pin-set aluminum rivets for this blind hole application. These rivets are used on transit buses to attach aluminum to aluminum in blind through holes where the material is too thin to tap and pop rivets will not seal properly and leakage will occur. These rivets are permanent after setting by driving the pin home. Removal is by drilling the head and driving the base on through. The pin does not penetrate the base, just expands so the fixture if properly installed is weather tight and would be aluminum to aluminum. The are available at Mcmastercarr in 3/16" and 1/4" with a specific bit requirement for the hole. They work well in dry land settings but we have not used on the vessel except in dry areas. Looking for someone else to verify corrosion resistance.

[MJH]

Originally Posted by maine50

I would like to add 2 cleats on my aluminum keel stepped mast. Is tapping threads the best way? The mast section is 9.4 x 6.1 with minimum wall thickness .155. The bolts would likely be 5/16. What's better- course or fine threads if tapping? The attachment point would be about 8' from base.

Practical Sailor recommends course threads for spars.

I recommend the use of a "drill-tap" to do the job in one passing through the spar skin, particularly if you are in a boson's chair.

~ ~ _/) ~ ~ MJH

[Morgali]

Aluminum mast I would do RivNuts... But I am not an experienced sailor, nor am I an aluminum specialist, but at least with rivnuts you are spreading the working load of your attachment over a greater area, and giving far more area for thread to bite as well. Check youtube for home-made rivnut fasteners instead of buying the 200+ dollar tool.

Bonus to this method is that you can remove and replace bolts easily, and torque higher than you would be able to in the metal itself.

[a64pilot]

Originally Posted by oleman

Has anyone considered pin-set aluminum rivets for this blind hole application. These rivets are used on transit buses to attach aluminum to aluminum in blind through holes where the material is too thin to tap and pop rivets will not seal properly and leakage will occur. These rivets are permanent after setting by driving the pin home. Removal is by drilling the head and driving the base on through. The pin does not penetrate the base, just expands so the fixture if properly installed is weather tight and would be aluminum to aluminum. The are available at Mcmastercarr in 3/16" and 1/4" with a specific bit requirement for the hole. They work well in dry land settings but we have not used on the vessel except in dry areas. Looking for someone else to verify corrosion resistance.

Sound like what the aircraft industry call Cherry Max rivets, they are as far as I know the only pop rivet with the strength of a solid.

[a64pilot]

Originally Posted by a64pilot

Sound like what the aircraft industry call Cherry Max rivets, they are as far as I know the only pop rivet with the strength of a solid.

But for God’s sake no rivnuts or gripnuts.
The reason is very quickly you have a rivet, the reason is the SS screw seizes into the rivnut and then the rivnut just spins, so instead of drilling out an aluminum rivet, you now get to drill out a SS screw, and oh by they way did I mention that this screw your trying to drill out spins freely?
Rivnuts are in my opinion the spawn of the Devil, even though they are widely used in non structural applications on aircraft. Note the non structural part

[SVTatia]

Originally Posted by oleman

Has anyone considered pin-set aluminum rivets for this blind hole application. These rivets are used on transit buses to attach aluminum to aluminum in blind through holes where the material is too thin to tap and pop rivets will not seal properly and leakage will occur. These rivets are permanent after setting by driving the pin home. Removal is by drilling the head and driving the base on through. The pin does not penetrate the base, just expands so the fixture if properly installed is weather tight and would be aluminum to aluminum. The are available at Mcmastercarr in 3/16" and 1/4" with a specific bit requirement for the hole. They work well in dry land settings but we have not used on the vessel except in dry areas. Looking for someone else to verify corrosion resistance.

monel pop rivets - the recommended rivets for use with aluminum for strength and corrosion resistance.