are Well Nuts the way to go

[Dignity]

I can access my underfloor fuel tank via 12 screws in the floor, they go through the floor panel into 3 -4 mm thick fibreglass, it's an old Allison Vision. Over the years and some rough pounding the screws no longer hold so I need to somehow fix them in place. I have played around with Rivnuts/Nutserts in ally but am reserved on their use as I have an offcut of F/G the right thickness and of the few test samples I've done I've found that the rivnuts will start to turn after not too much pressure from the screw and this is not acceptable as I have had extreme trouble getting one out in a blind situation previously.

I have thought of using M6 x 14mm Well/Rawl Nuts as an alternative my only hesitation is that the nut part is brass (I haven't found any that aren't although I did find some that had SS bolts supplied) and I will be using SS machine threads, also the floor needs to be pulled down quite solidly. The floor size is 1800mm x 600mm.

There are 3 issues I need to address:

1. Whichever product I use I need to have confidence that it won't widen the hole and hence turn and make it impossible to remove the screw.

2. To pull the floor down tight will brass and SS bind, I will cover the nut and bolt in lanolin grease.

3. Is it likely that the tread on either the bras or SS will strip when I torque them down.

Alternately is there another product I can use (I have seen Rivnuts with anti rotation lugs but can't seem to source any locally)

Thanks in advance.

[robothefisho]

Use bigger screws?

Cut some stainless plate and sikaflex in place under the fibreglass and drill and tap for a metal thread.

[scottar]

If you can get to the back of the glass I would be making up backing plates out of some flat bar (alloy or stainless) and sikaflexing them in. I have done tinnie floors this way when the original holes in the frames have flogged out. Personally I wouldn't use brass.

[juggernaut]

Similar to Scottar - can you epoxy marine ply backing plates and instal stanless t-nuts into the ply backing plate - then use bolts or machine screws into the t-nuts?

[Dignity]

Use bigger screws?

Cut some stainless plate and sikaflex in place under the fibreglass and drill and tap for a metal thread.

Can't get under the fibreglass, it is the other side of the fuel tank well and under a sealed floor.

Scottar, juggernaut, similar advice but it is a blind fitting that I need.

I figure that well nuts being rubber might grab a bit better on the fibreglass.

[Noelm]

How about simple filling the holes and starting again?

[Dignity]

How about simple filling the holes and starting again?

Noelm, did that about 7 years ago, have to drill new screw holes further away as you can't use the same holes with only 3mm thickness which means bogging and re gel coating the existing holes.

[BigE]

google "blind screw in thread inserts" they may do the trick.

BigE

[Fed]

You could make up 12 little SS plates with nuts welded on them & a fixing hole at each end or maybe SS hollow wall anchors. Do they have to be perfectly flush or do you have a few mm to play with, is the top cover solid or thinner with folded sides Dignity? Got any pics of the cover & where it's getting screwed to?
Maybe SS hollow wall anchors?
https://alexnld.com/product/10pcs-st...l-fixing-tool/

[Dignity]

You could make up 12 little SS plates with nuts welded on them & a fixing hole at each end or maybe SS hollow wall anchors. Do they have to be perfectly flush or do you have a few mm to play with, is the top cover solid or thinner with folded sides Dignity? Got any pics of the cover & where it's getting screwed to?
Maybe SS hollow wall anchors?
https://alexnld.com/product/10pcs-st...l-fixing-tool/

Fed, only got around 1 to 1.5 mm to play with sO needs tObe relately flush. If I had to I could take out the 200 litre fuel tank, cut into the side walls, glass in some hardwood very tight area and would be a big PIA to do then reglass the cutout where I have a lot more space to work with. By accident I found a company in NSW that apparently has Well Nuts with SS nuts plus also in Nitrile, there should be no fuel around these fittings but one never knows.
I think I'll go down the Well Nut path as the rubber does have anti vibration properties and shouldn't enlarge the holes. I have some of the brass ones here and I'll play around with them as I have found the torque settings needed and see how it goes, i just need something to do some vibration tests over a period of time.
If I get a chance I'll post some photos,just trying to do it from the phone is a big PIA, it would be nice if Ausfish automatically resized uploads.

[Dignity]

google "blind screw in thread inserts" they may do the trick.

BigE

Did that after I realized rivnuts would still be a problem, I still have one in my truck drawers I can't free up, probably the one I forgot to put some lanolin on when I installed it. There are some varieties of rivnut that would do the trick but only in steel, there are a variety of toggle style fittings I've looked at that are avail in SS but I've discarded them as once again if the glass hole wears they will be difficult to remove. I think I'm stuck on Well Nuts, got to ring this company today to see if I can get hold of the SS ones. Nothing in boating is simple.

[Dignity]

OK, here is the the conundrum. Everything still needs to be cleaned up but pic of the floor, the problem area with fixings and what the underside of the floor looks like (in reverse order of course)

[DATCOL]

Hi Dignity Can you set some stainless nuts in the deck with epoxy glue then screw counter sunk from the top

[Mupster]

Fill the old screw holes with Techni Glue and then rescrew it again. It will be like new. That stuff sets hard and can be screwed into. An hours job!

[Dignity]

Datacol, only 3 mm to work with, the holes in middle pic, I'd probably drop more down the holes than I'd managed to set.

Mupster, sound interesting, I'll give it a go on some scrap first, do you think it would be worth drilling larger holes so that there's more body for the screws to purchase.