Self tappers spinning in fibreglass hollow sections

[thinwater]

If inserts don't make sense to you, an easy and very strong way is to remove the core (bent nail or other) and fill not with an epoxy-wetted spunge (good) but with some chopped fiberglass cloth wetted with epoxy. Just fill the hole 2/3 with epoxy, snip up some bits of finish glass and stuff them in with a skewer, and then fill with epoxy. Very strong.

Remember that that whenever reinstalling a self tapper in any material--plastic, fiberglass or metal--ALWAYS feel for the existing threads (turning backwards and feeling for the "click" is one way). You will very soon strip the threads if you cut new ones each time. In general, using a power driver when reinstalling self tapers is a mistake. At the very least, hand start them first, in the existing threads, and turn the torque way down.

 

[johnphilip]

Remove the brass inserts and fit stainless steel female Big Heads.

Use machine screws to secure the floor panels. (with anti-seize grease on the threads, or use brass/bronze machine screws).

View attachment 203524

Thanks for the idea but I do not see how it works in this instance?

 

[johnphilip]

Could you fibre glass some small nuts into the present holes in the frames and use small bolts with counter sunk heads, so that instead of screwing the sole boards down, you would bolt them in position? You may have to enlarge the screw holes in the frames slightly but once the nuts were captive in set fibre glass there would be no weak spots along the frames.

Afraid I cannot see how the nuts would be held in place very well. The wall thickness of the frames is probably only 3-4 mm

 

[johnphilip]

A simple solution is to soak a piece of sponge in the epoxy, then poke it into the hole in the GRP, so you have most sticking through the hole but a bit still on top. The foam expands and holds itself in place, let it set. Then trim flush and drill a pilot hole for your self tapping screw size. Obviously clean the hole in the GRP first if it is dirty.

Cheep as chips, and very easy.

I like this wheeze, thanks

 

[johnphilip]

I've used something like this in the past They aren't stainless, but they last fine with a squirt of ACF50

https://www.screwfix.com/p/suki-drill-in-threaded-sockets-m6-x-10-5mm-4-pack/181gx

This may worth a go thanks, not as messy as I tend to get with epoxy.

 

[johnphilip]

You can get countersunk rivet nuts for exactly that problem
View attachment 203510

Or you can get “reduced head rivet nuts” which only require a tiny countersink (in metal they pull in almost flush anyway).
View attachment 203511

Do you resin these in or what stops them pulling out or rotating , the ribbed side looks pretty fine for gripping glass fibre

 

[Plum]

Do you resin these in or what stops them pulling out or rotating , the ribbed side looks pretty fine for gripping glass fibre

See my post #13. I have used the ones with both countersunk and reduced heads, M3 and M4 internal threads, once set with the pop rivet tool the tube of the riv-nut swells up inside/behind the grp so it does not pull out and as long as you squeeze the pop rivet tool enough they do not turn.

 

[Poignard]

Thanks for the idea but I do not see how it works in this instance?

In the absence of more detail, I have assumed the hollow frames are only accessible from above, that the floor panels need to be removed from time to time,

and that the Big Heads would fitted with the threaded bosses downwards and the flanges epoxied in place.

That is what I would do, but what I would do is not necessarily what you, or anyone else, would choose to do.

I try not to advise, but merely to suggest a possibility.

 

[Mister E]

Rivernuts will grip steel vehicle panels about 1 mm thick so shoulddo 3 mm. Also if the hole is not completely circular then when they squash in place the generally deform non circular thus less likely to turn.
Ebay do kits of rivnuts that have a hand tool similar to pop rivet pliers for setting them. Mine was less than a tenner and came with a selection of alloy rivnuts.

 

[oldbloke]

For floorboards I would use simple rawlplug type plastic cavity wall fixings

 

[Ian_Edwards]

I've used SS rivetnuts for cavity fixing on the boat. The principle problem I had was compressing the rivetnuts with hand tools.
The SS work hardens and I found it hard to get enough compression to prevent the rivetnut from spinning, which makes it very difficult, if not impossible to tighten or release the bolt.
I had some success covering the outer surface with thickened epoxy, letting cure before compressing the rivetnut.

 

[Refueler]

For floorboards I would use simple rawlplug type plastic cavity wall fixings

Cut off the split tang ends and just use the top tube part.

 

[mikegunn]

I've used something like this in the past They aren't stainless, but they last fine with a squirt of ACF50

https://www.screwfix.com/p/suki-drill-in-threaded-sockets-m6-x-10-5mm-4-pack/181gx

There are several sources for stainless steel threaded inserts (Google’s your friend).. I’ve used them on several occasions, particularly the blind hole versions. In my opinion it’s best to coat the O.D of threads with a very thin film of epoxy adhesive before installation.
Mike