Tapping a thread in fibreglass?

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emnick

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I will be removing the stern tube on our boat soon. Last time I had it out the threads (see pic) where the stern tube bolts to the small skeg were not so good. I think when I take it out again they will need re tapping. Its a solid block of fibreglass. What do you think the best way of re threading, The stern tube holes won't allow a bigger bolt. Is it feasible to fill the hole with epoxy or something similar/ better ??? drill it and re tapp or is there some sort of "helicoil" that could be used.
Thanks for any help
PS sorry about poor pic quality
 

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emnick said:
I will be removing the stern tube on our boat soon. Last time I had it out the threads (see pic) where the stern tube bolts to the small skeg were not so good. I think when I take it out again they will need re tapping. Its a solid block of fibreglass. What do you think the best way of re threading, The stern tube holes won't allow a bigger bolt. Is it feasible to fill the hole with epoxy or something similar/ better ??? drill it and re tapp or is there some sort of "helicoil" that could be used.
Thanks for any help
PS sorry about poor pic quality
Click to expand...
Two good answers above but after filling with epoxy and tapping I would epoxy bond a length of silicon or aluminium bronze studding into the tapped holes so in future you only have to remove nuts and not disturb the threads in the skeg again.

www.solocoastalsailing.co.uk
 
Can you drill the holes deeper and tap good glass at the bottom of the hole? And maybe use some blue Loctite?

The problem with epoxy is that the repair is barely the thickness of the threads and is generarlly not very strong at all. You would need to over drill first to get enough bond area. And add fibers; they add a lot of toughness.

I like the stud idea best; fiberglass threads can be strong, but they are not for repeated use.
 
Plum said:
Two good answers above but after filling with epoxy and tapping I would epoxy bond a length of silicon or aluminium bronze studding into the tapped holes so in future you only have to remove nuts and not disturb the threads in the skeg again.

www.solocoastalsailing.co.uk
Click to expand...
The problem with that is that you wouldn't be able to unscrew the outboard bearing from the stern tube because the studs will prevent its being rotated.

The only way then of removing the outboard bearing then would to be remove the inboard bearing from the sterntube, and withdraw the sterntube and stern bearing in one piece.
 
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Poignard said:
The problem with that is that you wouldn't be able to unscrew the outboard bearing from the stern tube because the studs will prevent its being rotated.

The only way then of removing the outboard bearing then would to be remove the inboard bearing from the sterntube, and withdraw the sterntube and stern bearing in one piece.
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The stern tube is one piece it is fixed to the stern bracket..............dont ask me how I know. But the studs are a good idea for future. I could possibly dove tail the holes a bit and would never pull out?
 
Poignard said:
The problem with that is that you wouldn't be able to unscrew the outboard bearing from the stern tube because the studs will prevent its being rotated.

The only way then of removing the outboard bearing then would to be remove the inboard bearing from the sterntube, and withdraw the sterntube and stern bearing in one piece.
Click to expand...
Very good point. The last time I took mine out it came out with the sterntube still attached so no unscrewing was necessary.

Www.solocoastalsailing.co.uk
 
thinwater said:
One problem is that filling deep holes in a vertical surface, without bubbles, is not so simple. Then there is the problem of adding fibers.
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You don't need fibres. As the WEST manual states, the enlarge hole is first wetted out with unthickened epoxy. The epoxy which is to bear the load is thickened with 404 or 406 filler.

A plastic washer can be used to prevent its oozing out
 
thinwater said:
Can you drill the holes deeper and tap good glass at the bottom of the hole? And maybe use some blue Loctite?

The problem with epoxy is that the repair is barely the thickness of the threads and is generarlly not very strong at all. You would need to over drill first to get enough bond area. And add fibers; they add a lot of toughness.

I like the stud idea best; fiberglass threads can be strong, but they are not for repeated use.
Click to expand...
I don't think I can drill deeper as it could break into the boat. Thank you
 
Poignard said:
View attachment 130655

From Bonding Fasteners & Hardware
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I've used this method successfully on a number of occasions. I use WD40 on the fastener to ensure it can be undone, although I might consider wax as an alternative. I use 404 filler. Disposable plastic syringes can be bought cheaply in large numbers on Amazon or eBay to aid application - I find the 20ml ones good.

Leave 24 hours or so to set. I find that occasionally (1/10 or 2/10 of the time) it pulls out when you test it, but otherwise it's a very solid thread - the bolt can be removed or retightened and you can put a decent amount of weight on it.
 
Poignard said:
You don't need fibres. As the WEST manual states, the enlarge hole is first wetted out with unthickened epoxy. The epoxy which is to bear the load is thickened with 404 or 406 filler.

A plastic washer can be used to prevent its oozing out
Click to expand...

A. The need for fibers depends on how long the threaded area is. In this case, probably not.
B. Thickeners. They (including 404 and 406) generally reduce the thread holding ability because of microbubles entrained during the mixing process. I have tested this, and the reduction is considerable. But without bubbles or fiber, the epoxy is quite brittle and can break. They would strengthen the epoxy if they could be mixed without any air entrainment, but that would have to be done under vacuum... which is done for some products.
C. Yes, a washer will hold it in. But how do you know you got the bubbles out of the back? The best you can do is inject from back to front and hope. Pretty effective.

No not impossible, but not simple on a vertical surface. The best method we have.
 
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