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Old 11-11-2010, 10:42   #1
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Fine or Coarse Threads

I am mounting some steps (1 1/2" x 1/8" flat stock) on my mast and want to use stainless machine screws.

Which thread holds better 10-24 or 10-32 when tapped into the fairly thin (1/8th?) aluminium?

I was going to avoid the problems with dissimilar metals but the 3/16"dia aluminium rivits just do not inspire confidence. I guess some sort of goop like Tefgel will be required if I use the screws...........m
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Old 11-11-2010, 11:21   #2
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If I wasn't concerned about corrosion, I'd go with an 8-36. Your kinda pushing the limit on length/diameter with a #10 fastener, especially where there is a safety concern. If you think corrosion may be an issue, as courser thread size would be better, but a 10-24 will only give you 3 engaged threads (pushing the lower limit IMHO).
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Old 11-11-2010, 11:28   #3
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I would use fine tread and tuff gel the dissimilar metals is not an issue in this case might be submerged but works fine on a mast.
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Old 11-11-2010, 11:39   #4
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I'd suggest dipping the screw threads in Tef-Gel or some kind of dissimilar metal anti-seize goop before screwing. Also, it's important to remember that stainless is harder than aluminum so care is required when screwing them in.

I've got rivets on my mast steps (4 screws/step) and haven't had a problem. I'm 6'/200lbs if that helps. The PO installed the steps and he appears to be a bit shorter in the legs than I am.
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Old 11-11-2010, 11:48   #5
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Into 1/8 aluminum, coarse thread would be okay. You cannot give it a full load like you might with thicker material. The threads will probably pull out before the machine screw breaks under a tensile load. Put in as many machine screws as what makes sense to share the load.

Using Tef-Gel is mandatory, if you ever want to remove it and you do not want electrolysis to corrode away the aluminum.
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Old 11-11-2010, 11:53   #6
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I used 4 each 1/4 inch structural rivets on our steps that were rated for 1300 pounds of shear strength each. That equals 5200 pounds of shear strength per step. I've gained some weight, but not that much...lol. It was much faster popping rivets than having to drill and tap fifty holes.
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Old 11-11-2010, 12:06   #7
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I used 4 each 1/4 inch structural rivets on our steps that were rated for 1300 pounds of shear strength each. That equals 5200 pounds of shear strength per step. I've gained some weight, but not that much...lol. It was much faster popping rivets than having to drill and tap fifty holes.
SS or Al rivets? For metal compatibility I like Al rivets but are they strong enough?
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Old 11-11-2010, 12:22   #8
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I would use fine tread and tuff gel the dissimilar metals is not an issue in this case might be submerged but works fine on a mast.
Id also use UNF (fine) threads.

Apparently 10-32 (UNF) bolts are stronger, and can be installed at higher torques, than 10-24 (UNC).

Shear strength is only about 60% of tensile strength. The tighter the screw, the more friction there is to resist loosening, so more clamping force and friction helps reduce shearing loads.

Find the Right Fastener for your Needs with PSB's Material Strengths Chart
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Old 11-11-2010, 12:29   #9
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Threads are my job

Contrary to popular belief, the contact area and load carrying potential of a fine thread are greater than the coarser thread.
Even though the tap drill size for a 32 is larger than the one for 24 tpi
the greater turns of thread more than makes up for that.

Since an 8-32 and 10-32 are the same pitch go with the biggest one that will fit in the space allowed.

Tapping into thin metal is also easyer with the fine thread, less chance of chewing up the hole and getting no threads.

The fine threads will also generate higher holding tension for a given torque.

For one of my customers I had to calculate the contact area of a 10 pitch vs 40 pitch thread and was a bit surprised.
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Old 11-11-2010, 13:53   #10
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I tend to use helicoil inserts when tapping into an aluminium mast. And I use plenty of apropriate barrier media such as Duralac, or similar
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Old 11-11-2010, 17:39   #11
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As stated previously, fine threads or ultra fine threads are better. As a general rule, you need at least 5 fully engaged threads in similar metals to get full joint strength. Since you are going into aluminum, you would need more than this. This is something that is pretty easy to calculate. For anyone who does a lot of modifications to their boat, I would suggest getting a copy of Machinery's Handbook which has an unbelievable amount of info and covers this in more detail.

Another thing to keep in mind is that you want to actually put torque on the bolt. With sufficient thread engagement, you want to torque to 70-80% of the yield strength of the bolt. In shear this helps you because it creates a friction joint which carries the load rather than just putting the bolt in shear. It also keeps the bolt and threads from fatiguing which is very important.
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Old 11-11-2010, 18:08   #12
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I've always understood to use coarse threads in aluminum. It's a soft metal and fine threads rip out easier and can cross thread without really trying. Never had a problem with coarse threads I've tapped. The factory gear on my mast is course thread and fine after more than 40 years.
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Old 11-11-2010, 18:12   #13
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We've got mast steps, installed with 8-32 screws. None have stripped out, nor has there been a corossion problem.
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Old 11-11-2010, 19:30   #14
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You could also use nut rivets to install in the holes, then thread the machine screws into these.

This gives your machine screw more material to thread into without having to put reinforcing pads in or on the mast.
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Old 12-11-2010, 07:00   #15
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Nothing personal to anyone but I think folks here tend to over think things a bit too much sometimes. Keep it simple drill and tap and go sailing lol. I have seen hundreds of steps put on with just #8 or #10 MS and they work fine the other ideas are great and will work fine too but why do it the hard way when the simple way works just fine? Just had to say it lol. Not trying to put anyone down.
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