Can I tap that? (Fiberglass)

Quickstep192

Anarchist
841
160
Chesapeake
I’m installing a new plexi cover over my engine panel. The screws go into solid fiberglass a little over 1/4” thick. 
 

Can I thread the glass and use 6/32 machine screws instead of #6 sheet metal screws?

A test on scrap seems to have produced a decent thread, but I’m wondering if I’m overlooking something.  
 

Can I get a decent thread in fiberglass? 
 

 

Overbored

Anarchist
711
59
So. Cal
it is not about strength to hold on a plastic cover but make sure you counter sink the gelcoat to help prevent the gelcoat from cracking. tapping is better because it cuts the treading, removes the material and the sheet metal screw cracks and displaces the material which almost always cracks the gelcoat.

 

SloopJonB

Super Anarchist
68,782
12,402
Great Wet North
It works fine as long as there won't be a lot of stress on the fastener.

I have tapped glass on deck so I could install lifeline stanchions single handed - tap the holes, install the fasteners then go below and install backing plates and nuts.

Just the fasteners in the tapped glass was surprisingly strong.

 

IStream

Super Anarchist
10,865
3,046
I've done this a number of times and it works well. As noted above, it's important to countersink the area to prevent gelcoat damage. Drill a small pilot hole and then countersink to a diameter larger than the final hole size will be. I would suggest you use the largest screw you can get away with (e.g. 8-32 rather than 6-32) and at least 32tpi. Use a tap drill suited for a 75% thread, lubricate the tap with water or rubbing alcohol, and clear the flutes of swarf frequently as you tap. Do your best to keep the tap from leaning over as you tap.  

 

Quickstep192

Anarchist
841
160
Chesapeake
Unfortunately, #6 is as big as I can go. It might be ideal to go 10-24 for the deeper, coarser threads, but the fasteners are right at the edge of the cover as it is. 
 

I plan to use butyl tape to seal the plexi, so the screws have to have enough bite to be able to compress the butyl some. 
 

I’d use caulk instead of butyl tape, but I want to be able to get it apart later without a wrestling match or a lot of cleanup. Is there a “non-adhesive” caulk?

 

weightless

Super Anarchist
5,607
583
Yes. You can also cast threads with thickened epoxy. That's my go-to on molded bits with gel-coat. Take a tad of release (wax, grease, whatever) some machine screws a pinch of thickened epoxy a drill twist. Mix well. Flavor to taste.

The plexi might be happier if it can float a bit -- oversized holes and some kind of flexible schmoo, perhaps.

 

toddster

Super Anarchist
4,265
999
The Gorge
My boat was built with things attached to fiberglass with sheet metal screws.  Virtually every single one of them has pulled out over the years.  I've been replacing them with machine screws threaded into the plastic.  (Or with C-clips, depending on application.) All of them seem fine, ten years on.  They'll even self-tap into an appropriately sized hole.  

 

AaronD

New member
35
17
Newberg, OR
I tap fiberglass all the time (including today, as it turns out). As @Overbored said, sheet metal screws often crack the gelcoat. @thinwater posted specs at https://sail-delmarva.blogspot.com/2017/02/stainless-steel-bolt-specifications.html (down at the bottom), saying that threads in FRP should hold full screw / bolt strength at ~2x screw size (I.e., threads tapped into 1/2" solid FRP should hold full strength for 1/4" bolts; ~3/8" for #10s, etc.) Or 1x thickness for G10.

 

Marty Gingras

Mid-range Anarchist
I tap fiberglass all the time (including today, as it turns out). As @Overbored said, sheet metal screws often crack the gelcoat. @thinwater posted specs at https://sail-delmarva.blogspot.com/2017/02/stainless-steel-bolt-specifications.html (down at the bottom), saying that threads in FRP should hold full screw / bolt strength at ~2x screw size (I.e., threads tapped into 1/2" solid FRP should hold full strength for 1/4" bolts; ~3/8" for #10s, etc.) Or 1x thickness for G10.
Ooooh.  That's good stuff.  Thanks both.

 

Gabe_nyc

Member
270
22
Bayside
How do you guys feel about RivNuts?

If you’re not familiar with them, it’s like a pop-rivet except that it leaves a threaded hole. 

Easy, cheap etc. It’s great if you are looking to make tapped holes in a flange, but it cannot be installed in a blind hole.

I like them a lot but have only installed them on a boat once and then sold it shortly after so I don’t know if there are any long-term issues.

More here:

https://www.mgexp.com/forum/mgb-and-gt-forum.1/aluminum-bonnet-gas-strut-mount.3933045/

1F166511-2280-48F6-9D26-951742774407.jpeg

 

slug zitski

Super Anarchist
6,292
1,246
worldwide
How do you guys feel about RivNuts?

If you’re not familiar with them, it’s like a pop-rivet except that it leaves a threaded hole. 

Easy, cheap etc. It’s great if you are looking to make tapped holes in a flange, but it cannot be installed in a blind hole.

I like them a lot but have only installed them on a boat once and then sold it shortly after so I don’t know if there are any long-term issues.

More here:

https://www.mgexp.com/forum/mgb-and-gt-forum.1/aluminum-bonnet-gas-strut-mount.3933045/

View attachment 502095
That flange at the top limits mounting options 

a standard helicoil is more versatile 

 

Gabe_nyc

Member
270
22
Bayside
My boat was a Dufour that had some cabin panels riveted in place into FG approx 3mm thick.

RivNuts worked but the FG was not thick enough for helicoils.

On the other hand, coils would have worked great on a blind hole and other places.

 

toddster

Super Anarchist
4,265
999
The Gorge
How do you guys feel about RivNuts?

If you’re not familiar with them, it’s like a pop-rivet except that it leaves a threaded hole. 

Easy, cheap etc. It’s great if you are looking to make tapped holes in a flange, but it cannot be installed in a blind hole.

I like them a lot but have only installed them on a boat once and then sold it shortly after so I don’t know if there are any long-term issues.

More here:

https://www.mgexp.com/forum/mgb-and-gt-forum.1/aluminum-bonnet-gas-strut-mount.3933045/

View attachment 502095
I've only encountered old riv-nuts that were either installed wrong or had eroded their way loose, so they just spin uselessly.  Or corroded into lumps. Maybe if you have access to the back you could add a dab of epoxy, but then why not just throw on a washer & nut?  Similarly have had some success, but not great, with pop-toggles. At least they don't corrode.  

shopping


 

Kenny Dumas

Super Anarchist
1,224
465
PDX
Not saying it’s a good idea but I saw some big ass primaries on a 50’ that worked fine for a decade that were tapped and screwed to maybe 2 cm thick coamings. Owner freaked when he discovered it but they were dry and looked ok. Installed backing plates of course, but interesting data point anyway. 

 

Gabe_nyc

Member
270
22
Bayside
I've only encountered old riv-nuts that were either installed wrong or had eroded their way loose, so they just spin uselessly.  Or corroded into lumps. 
Thank you.

This is the kind of information I was looking for.

Unfortunately I do not have good pictures of this. The first picture is not my boat. The wood panel that holds the back cushions ran about 6-7 ft long. I had a leak and wanted to take it off. It was pop-riveted in place to a shaped fiberglass liner. I circled here where 3 of the pop-rivets are on this boat (there were 12-15 in all).

The second picture is (was) my boat. I circled where 2 of the pop-rivets used to be. This particular location did have access to the back of the panel by removing the electrical panel. However, this was a pain, and also, most of the other locations did not afford such access.

Because the RivNuts were completely covered over by the wood panel, if I had been concerned about them spinning I could have sanded away a bit of the gel coat at the front and added a dab of epoxy.

How we’re the ones you had seen installed wrong? If you follow the instructions it’s pretty straight but there ARE lots of silly people out there who maybe do not get the correct grip length or whatever. Earlier in this thread somebody mentioned about chamfering the edge of the hole. I had not heard about that and had not done it, but this is more of a cosmetic issue, no?

If an installed RivNut works loose, can it be re-tightened with the installation tool or do you recommend drilling out and replacing?

C25BBE7B-BEB6-4C1B-AF0D-EDF81A7D2506.jpeg

2C5766E8-0252-4088-979D-89DF50CF2E30.jpeg

 
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