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Aluminum Mast has stripped screw holes for halyard sheave box


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I've searched all over this forum and can't believe this hasn't been addressed yet.  Or if it has, I can't locate it.  So here goes,  Doing maintenance on my aluminum mast, I've found that 3 of the 6 holes that secure the spinnaker halyard sheave box to the leading edge of the mast are stripped. 

1. Tapping and installing larger screws is not possible.

2. because of the contour of the mast, installing a RivNut or Helicoil is not possible because there is not enough clearance inside the mast interior before it comes in contact with the box. 

3. I thought about doing a braising (soldering) to the holes and retapping, but learned that heating up the mast area to accomplish this only weakens the aluminum.

4. filling hole with JB weld (for metal) or similiar Loctite product and retapping seems to be a weak solution.

5. backing plates are not possible

6. don't want a jury rig situation of using copper wire or aluminum foil to stuff in the hole before fastening the screw.

This has to be a common problem.   What do the pros do?   Thanks in advance. 

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Steve, got any pictures you can share to get a sense of what you're dealing with.  I have some ideas, but hesitant to make suggestions with out more info.

There is always TIG the holes back up, drill and tap.

--Matt

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1 hour ago, Steve K said:

I've searched all over this forum and can't believe this hasn't been addressed yet.  Or if it has, I can't locate it.  So here goes,  Doing maintenance on my aluminum mast, I've found that 3 of the 6 holes that secure the spinnaker halyard sheave box to the leading edge of the mast are stripped. 

1. Tapping and installing larger screws is not possible.

2. because of the contour of the mast, installing a RivNut or Helicoil is not possible because there is not enough clearance inside the mast interior before it comes in contact with the box. 

3. I thought about doing a braising (soldering) to the holes and retapping, but learned that heating up the mast area to accomplish this only weakens the aluminum.

4. filling hole with JB weld (for metal) or similiar Loctite product and retapping seems to be a weak solution.

5. backing plates are not possible

6. don't want a jury rig situation of using copper wire or aluminum foil to stuff in the hole before fastening the screw.

This has to be a common problem.   What do the pros do?   Thanks in advance. 

SS Rivets?

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2 hours ago, Steve K said:

What do the pros do?

A Pro would take it to a spar shop to have it repaired. Very likely by welding in new alloy or filling as @egon wrote.

Really no room for a backing plate of 2-3 mm? Really?

Relocate the holes nearby  (3 diameters). Pop rivets tend to be a better choice than threads for small stuff (You didn't say how big of a boat).

Zip ties? Hose clamps? 

 

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More info. Boat size. Mast wall thickness current fasteners. Did they strip because of corrosion or thin wall thickness? 
 

structural plates or even spreaders are welded on aluminium masts. Often there are plates welded at the foot for deck stepped. Generally no typical aluminium parts are ever heat treated after welding. They do not grind out aluminium welds because of the local loss of heat treatment. The extra material from the weld and plate makeup for any loss of strength. 
 

Personally I would hand mast guy or a decent aluminium welder  a box of beer(Aussie currency) to fix it. 
 

They will simply fill it in with rod. The backside of the old hole will have a lump that can be tapped.
 

They weld entire boats. 

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4 hours ago, Steve K said:

4. filling hole with JB weld (for metal) or similiar Loctite product and retapping seems to be a weak solution.

Don't dismiss this concept.

There are specialized epoxies made for this very purpose. Loctite has (or had) one called Form-A-Thread.

I'd investigate it further.

Also, can the entire box be epoxied down on its flanges and fastened?

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1 hour ago, CaptainAhab said:

They will simply fill it in with rod. The backside of the old hole will have a lump that can be tapped.
 

They weld entire boats. 

Depends on what alloy the mast was and what the expectations of strength for the fasteners are. Welded 6061 will be less than half the strength of the original metal and extra thickness will do little good without thru bolting. 

Yes, they weld entire boats, but not usually from a heat treatable aluminum alloy - the loss of strength makes that a fool's errand. If the mast is a strain hardening alloy, like 5083, then it can probably be successfully welded. 

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I'm not 100% buying the idea that a helicoil wouldn't fit. My mast is full of them. You can get helicoil inserts of various lengths, including those sized for thin material. The trick is getting them to stay in place. This is solved by coating the outside thread in super strength loctite. Remove the sheave to punch out the helicoil tangs.

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They weld 6061. You are correct you lose 1/3- 1/2 the strength. If you double the thickness of the original material it is still stronger. Have you never seen a welded mast?

The top of the mast in our old 32’ IOR was tapered. They took a circular saw and cut a slot. Clamped it tight and welded it. The rig loads were over 5ksi. 

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On 8/11/2020 at 10:48 PM, CaptainAhab said:

They weld 6061. You are correct you lose 1/3- 1/2 the strength. If you double the thickness of the original material it is still stronger. Have you never seen a welded mast?

The top of the mast in our old 32’ IOR was tapered. They took a circular saw and cut a slot. Clamped it tight and welded it. The rig loads were over 5ksi. 

If you are only seeing 5 ksi then plastic or wood would work. Twice as thick and half the strength will not hold fasteners the same, as most of the stress is resolved in the first few threads. May be enough, maybe not - depends on the loads. Yes you can weld 6061, I do it all the time - but you have to be aware of the result, which is 6061-T0 mixed with the 4xxx or 5xxx filler rod you are using. 

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