S/Steel BLIND hole fixings required

[squidge]

Hi Has any one come across stainless steel blind hole fixings?
I need some so that i do not have to cut holes in my wood work to tighten the bolts which will hold my new stanchion bases on.
Ive been doing a search but the ones ive found so far are to flimsy or require to large a hole to get the fixing under the deck.
M8 / M10 X 100mm would be just the ticket.

Thanks

 

[Freebee]

what you may need are called rivnuts but what are they fastened to? cos rivnuts are set with special pliers and clinch onto the surrounding substrate if its fibreglass then they will probalby not hold fast.

 

[Stemar]

I don't think I'd want to use blind fixings for stanchions - I just don't think they'll be strong enough for something that your life could depend on.

I'd want bolts through a backing plate under the deck if possible, or at least bolted through substantial penny washers.

 

[tugboat]

I agree with Stemar - imagine yourself halfway over the side with only the top of a stanchion between you and the briney. Would you have wanted to take any shortcut? I think you are imagining something like those bolts where the nut has folding wings that flip out on the other side. Even if they exist, in this case they wouldn't be strong enough in the hinge, also you'd need a biggish hole in the deck to fit it through - potential for all sorts of leaks. Sorry -not the answer you wanted but best to give an honest opinion.

 

[fireball]

There are some bolts with captive rawplugs available for concrete fixings, the company I work for use these to fasten shelving units to the floor - the maint engineer wouldn't use them if they were dodgy ... but they were 1/4" diameter!!

I don't know if you can get similar bolts in smaller diameters - I'll ask tomorrow.

 

[jfm]

I agree with Stemar

Boat builders attach plenty of things into blind holes but they invariably do it by glassing tapping grade aluminium plate into the GRP laminate, then they drill and tap the hols for the fitting.

This is a pretty expensive process and if there were good blind fixings you can be sure they would use them. But they dont, because there aren't. Sorry, not the answer you wanted.

 

[William_H]

All right agreed it's a pretty critical application however. How would it go if you got a bolt 8mm and about 70mm long. Cut the head off and put a bend in the bolt about half way along that is a rightangle or tighter. Thread the bent part of the bolt into the hole turning it so that the bent part sits under the deck and the threaded part extends out through the deck so the stanchion base can fit over it. Tighten up a nut on top and cut off excess thread. The bent bolt should give some hold in the deck especially if the base is thick denying the bolt from bending out again. Now this is just a thought and not necessarily strong enough to hold the inside corner of a stanchion base but might be suitable for less critical fastenings. (here comes the flack) regards will am I telling you how to do a bodgy job?

 

[squidge]

Thanks for all the input.
I do not think that if the job is done correctly with a substantial blind hole fixing which will be a lot stronger than the M8 nut and bolt it is replacing there will not be a safety issue.
The kind of thing i am looking for is not found off the shelf at B&Q but used to build bridges .If i could get some in the correct grade s/steel i would be more than happy with the strength of the finnished job. (Although i will have to go up in bolt size)
It may be that the wood below the deck has gone soft or i am unable to find or make the fixings in which case i will have no option other than to remove the bright work to get at it.

Thanks again
Squidge

 

[jfm]

Will that would work initially but it's unsuitable for a critical job. The pressure on the boat on the underside would be immense because all the tension in the bolt would be applied to a small area in contact with the curved side of the bent part of the bolt. That would mean possible splitting of the boat material, and/or working loose soon, etc. In contrast, a plate or a big penny washer allows high bolt tension because the force is spread over a wider area, so less pressure on the GRP. Tension in bolts is needed to stop the fitting from wobbling, and in critical applications tension in the bolt reduces stress fatigue in the bolt's metal.

Not picking a fight - I think you knew this, it's a bodge as you say! Also there might be an issue in stopping the stud from spinning round as you tighten the nut. Lastly, it would look awful on the top side!

 

[jfm]

"blind hole fixing which will be a lot stronger than the M8 nut and bolt it is replacing"

The issue isn't stength of the bolt, it's stength of the boat. Backing plate/penny washer is needed due to weakness in the boat.

What are these blind bridge-building fixings that you mention like? How do they work mechanically? There are plenty that work well in steel and concrete, but I'd be intrigued to know if there are some that work with GRP.

 

[snowleopard]

agreed that rawlbolt type fastenings which may work fine in concrete would simply split most boatbuilding materials when cranked up tight enough to grip.

i don't believe there are and methods of fixing that can be done without getting to the back of the deck, unless there is enough wood to take some really long screws.

you could solve the problem for the future by using 'Bigheads' which are nuts or studs fastened to a large perforated plate that can be held in place with epoxy so you effectively have a tapped hole to bolt into. unfortunately they are a bit hard to come by as the manufacturers have a minimum order of 100 of a type, at around £1 each. if you're lucky you may find a chandler who stocks them.

 

[squidge]

Thanks Snowleopard ill take a look at them.

Jfm, I am sorry to say that the reason i am having to replace or refit all of my stantion bases is in part due to the M8 bolts which have sheared off at some time. I only found this out when i drilled out the 2 horizontal bolts and found that the remaining 3 rd bolt had long since demised. This has lead me to start removing the bases which look fine from the out side but have suffered from bolt failure.
As i say it is very possible that i shall have to take the hard road if the supports are soft or rotten.I will find out on Saturday.
If you are interested i will try to get a photo to you once i find the right thing.
Thanks again for everyone's input.

Best regards
Squidge