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Old 01-12-2021, 14:35   #1
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Fiberglass rivets

I recently installed a tiller clutch on my Freedom 21 and needed to add cleats to each side of the cockpit to be able to adjust the control line. One side has an open locker so no problem accessing behind the fiberglass there, but the back of the other side of the cockpit is pretty much completely impossible to reach. Of course, a standard pop rivet is only usable in metal (and maybe G10) because those materials are hard enough to withstand the pressure of the rivet in the hole.

I eventually found a solution that seems to work perfectly for this and just wanted to share it in case anyone else encounters a similar challenge. The answer is these 'banana peel' rivets intended specifically for softer materials. Instead of expanding laterally to jam into the hole, they split open at the end and bend backwards to push against the back of the hole and spread the load over a wider area. They look like this:

So far it seems to be working great:


Now before you break out your pitchfork - I definitely don't claim these are anywhere near as strong as nuts and bolts with backing plates. But for a light-load situation like a tiller clutch (which automatically slips before a high load can be applied) they are a quick and easy solution that seems to work great. I'd also consider them for other non-critical applications like winch handle holder or a line coil hanging hook.

If you have any alternative solution that would also solve this problem, I'd definitely be interested in it.
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Old 02-12-2021, 05:00   #2
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Re: Fiberglass rivets

Toggle bolts?
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Old 02-12-2021, 06:16   #3
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Re: Fiberglass rivets

Another option are Jack Nuts or Blind Threaded Inserts, two variations of the same thing. Again, not as strong as a fender washer and nut in fiberglass, but probably stronger than aluminum pot rivets.
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Old 02-12-2021, 06:34   #4
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Re: Fiberglass rivets

There are bunches of designs for gripping in situations that are both blind and soft, probably due to the problem of gripping sheet rock. The granddaddy of them is Gord's toggle bolt, which does require a larger hole, but has great spread. They are hard to find in stainless steel. Here's a stainless steel backing plate/nut that can then take a 1/4 by 20 SS bolt:

https://www.boltdepot.com/Product-De...kaAqcREALw_wcB
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Old 02-12-2021, 07:13   #5
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Re: Fiberglass rivets

Yeah a toggle bolt would absolutely be stronger, but there's a couple problems due to their design for interior home applications:

1. They're pretty much impossible to find in 316 stainless. If you can find stainless: it's 18-8, the thread sizes are limited, and they are crazy expensive
2. You need a massive hole (McMaster says a 3/8" bit for a 1/8" bolt, which is huge)
3. I haven't actually tried them in fiberglass, but I imagine in drywall it's the softness of the material that lets the wings dig in and stop turning. Would fiberglass be too hard and slippery? Maybe the roughness of most hidden surfaces would counteract this.

I like @fourlyons suggestion better - I'd definitely be interested if anyone has done strength comparisons between rivnuts and banana peel rivets. You do need a special tool for those. McMaster carries rivnuts specifically for composites, but unfortunately not in aluminum or SS (https://www.mcmaster.com/rivet-nuts/...nd-composites/).
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Old 02-12-2021, 07:48   #6
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Re: Fiberglass rivets

Stainless steel rivet nuts are common - and came up in another thread recently. Here's an Amazon page with type 304:

https://www.amazon.com/s?k=stainless..._ts-doa-p_4_26
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Old 02-12-2021, 11:21   #7
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Re: Fiberglass rivets

I might be wrong, but it seems like those standard rivnuts would have exactly the same problem as a standard pop rivet in softer materials like composites.
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Old 19-12-2021, 01:26   #8
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Re: Fiberglass rivets

Fibreglass is a hard material to crush; its not sheetrock! I see absolutely no problem using pop rivets to hold a lightly loaded cleat; I've used pop rivets on fibreglass for other pieces of hardware. Depending on the thickness of the fibreglass, you could also use sheet metal screws.
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Old 19-12-2021, 04:02   #9
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Re: Fiberglass rivets

Quote:
Originally Posted by nuku34 View Post
Fibreglass is a hard material to crush; its not sheetrock! I see absolutely no problem using pop rivets to hold a lightly loaded cleat; I've used pop rivets on fibreglass for other pieces of hardware. Depending on the thickness of the fibreglass, you could also use sheet metal screws.

I used alloy pop rivets on racing motorcycles. The very thin and lightweight streamlined fairings were affixed using riveted brackets. Epoxy was used on the brackets between the GRP and the lightweight steel brackets.


Pop rivets were chosen as they have a lower profile in the airflow, countersunk ones.



In the fast bits of the IOM TT course, just sticking a finger out into the airstream caused a 200 RPM drop in engine speed at 150 mph plus.


Correctly fitted, no issues. Lots of cleats and guides fitted as OE on our mast with monel pop rivets.
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