Screwing into GRP - advice please

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cpedw

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I want to attach a fitting to GRP using self-tapping screws, I think 5mm diameter. Can anyone advise the right size of hole to drill to get a good grip but prevent the gelcoat from cracking please?
Is there a simple proportion between pilot drill size and screw size?
Thanks,
Derek
 
HI,
the hole should be the same as "the core" of the screw;
this will probably give the correct drill dimension of 4,5-4,75 mm letting just the windings trhu -- try 4,5 first!
 
Theres an easy way to get a good grip from a screw in grp..

DONT USE ONE!

Use a bolt instead. The screw will not bite well in grp, and end up cracking the gelcoat. Bolts hold better, and you can use a bit of stickerflex on the thread and under the head to make it completely watertight.
 
If you must screw do as ZED_NIVEL says however to allow the screw to go in easier use tallow as you would in wood and it a waterproof sealer or if you don't have any rub the screw threads with ordinary toilet soap it also acts as a lubricant.
 
Drill to the root size as said,then use a countersink to widen the gelcoat part of the hole to the diam of the thread,then add a drop of sikaflex and tighten the screw....but if you ever think you will be putting your hand anywhere behind the fibreglass,do your skin a favour and file off the sharp tip of each screw using a metal file and a pair of pliers/vicegrips/vice
 
Personally ? Having had many threads pulled in GRP over years from others attempts ... I'm of the blind bolt brigade. There are plenty of drill hole, pass bolt and assembly through ... till assembly head sits against work piece ... tightening bolt expands and locks whole thing in place never to come out again.

I have 3 places where timber rubbing strake has lifted .... the above will pull them up very nicely thank you. No need to get behind, sealant and stopper to finish off ...
 
I like the idea of blind bolts but don't you have to drill a bigger hole than the bolt to allow the toggle to pass or is there a type available that will allow the toggle to pass and open up using the same diameter hole as the bolt itself? I can't get to the inside of the hull to washer and nut as normal and the fitting I am using is quite small so don't want gaping holes. Any ideas? Hope I've made myself clear. Happy New Year!
 
One point to remember is that most self tappers are A2 and then you get ***p type as well. have sheared off many a screw and ended up using bolts or set bolts, my advice would be if its easy to get behind use bolts as they can be got out easier.
Happy new everybody /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
 
You know that putting a pulling strain on GRP will result in eventual failure. Using a bolt instead of a screw is indeed best.

Another solution is to drill a full size hole and place a piece of backing timber, medium density, behind the hole, using "no more nails" or similar to secure it. Drill a pilot hole in the wood and turn in the screw: when tight, the load will be spread to the backing timber with no localised strain on the GRP. Worth considering.

If a screw into the GRP is the only solution, use the largest diameter possible, to better engage the GRP.
PWG
 
Passage hole ...

Yes you do have to drill a larger hole than the bolt ... but the hole is filled with the tube like compressor.
Or other way is to drill and insert a Rawlplug .... then use a conventional screw. Much better than relying on a GRP thread .. which is brittle and fails easily.

The post about the backing piece of wood is excellent and true - IF you can get in behind.
 
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