Stellina
Well-Known Member
I'm putting 5mm screws into GRP and was wondering what the pilot hole diameter should be? 4.5 is for wood but would 4.8 be better to avoid cracking?
Pilot hole size for screws in GRP
- Thread starter Stellina
- Start date
alt
Well-Known Member
Re: Pilot hole size for GRP
I was using 4mm screws on the weekend so used 3.5mm pilot hole, and if it was a little tight I just used an up-down motion with the drill a few times to make the hole ever so slightly bigger. Remember it's better to be too small at first, because it's easier to make the hole bigger. Not so easy to do it the other way round!
I was using 4mm screws on the weekend so used 3.5mm pilot hole, and if it was a little tight I just used an up-down motion with the drill a few times to make the hole ever so slightly bigger. Remember it's better to be too small at first, because it's easier to make the hole bigger. Not so easy to do it the other way round!
VicS
Well-Known Member
3/16" is a bit smaller than 4.8mm. A no. 14 would be good choice.
Alternatively what's wrong with 4.6mm or 4.7mm.
Alternatively what's wrong with 4.6mm or 4.7mm.
earlybird
Well-Known Member
I'm assuming that you're talking self-tappers. If so, be aware that, like woodscrews, they are sized, in the UK at least by gauge number, and what is sometimes sold as 5mm. is really gauge 10, or about 4.8mm . In any event, I would guess even 4.5 mm pilot is rather too large, giving only a hair's breadth of thread depth, with little strength. I would guess that less than 4mm. is needed. Follow the suggestion of starting small and opening out in 0.1 mm steps. Working the screw back and forth to cut it's thread is good, I find a dab of grease allows a smaller pilot hole, and better thread depth. If you must, cracking can be reduced by carefully counterboring with, say, a 5mm drill to the depth of the gel coat. Do this first, preferably with a hand drill, otherwise you may end up with a 5mm. hole!
VicS
Well-Known Member
Earlybird is correct about the gauge sizes in the UK but he failed to notice that you are in Norway.
He is also right about counterboring through the gelcoat.
I usually put the screws in with a dab of epoxy adhesive. They unscrew again OK if you do not clean them.
He is also right about counterboring through the gelcoat.
I usually put the screws in with a dab of epoxy adhesive. They unscrew again OK if you do not clean them.
Stellina
Well-Known Member
Thanks, and apologies; the screws are 4.8mm. I've just been into the garage to check. I was confused because I also bought M5 bolts for the places I can bolt rather than screw. My last experience of screws into GRP was not good but I understand now that perhaps it was just the gelcoat cracking. I have a 4mm and 4.2mm drill as well as 4.5mm so I will start small...
oldsaltoz
Well-Known Member
The hole size should be based on the size of the shank, than add or take a bit off pending on the tread type, a thick close tread will need more room, a thin sharp thread (timber screw will do better in a smaller hole.)
Also note: if the screw is a vey tight fit it will crack the glass or at least the gelcoat.
Heating the screws can make the job simple, but do not fully tighten a hot screw, let it cool then finish tightening.
Never use self tappers on core filled laminates, balsa or foam cores will not handle moiture very well at all.
Avagoodweekend......
Also note: if the screw is a vey tight fit it will crack the glass or at least the gelcoat.
Heating the screws can make the job simple, but do not fully tighten a hot screw, let it cool then finish tightening.
Never use self tappers on core filled laminates, balsa or foam cores will not handle moiture very well at all.
Avagoodweekend......
Lee_Shaw
Well-Known Member
[ QUOTE ]
I'm putting 5mm screws into GRP and was wondering what the pilot hole diameter should be? 4.5 is for wood but would 4.8 be better to avoid cracking?
[/ QUOTE ]
4.5mm better IMO
I'm putting 5mm screws into GRP and was wondering what the pilot hole diameter should be? 4.5 is for wood but would 4.8 be better to avoid cracking?
[/ QUOTE ]
4.5mm better IMO
pappaecho
Well-Known Member
I thought that was why they sell a 4.2mm drill - to provide a pilot hole for 5mm screws?
Lee_Shaw
Well-Known Member
[ QUOTE ]
I thought that was why they sell a 4.2mm drill - to provide a pilot hole for 5mm screws?
[/ QUOTE ]
4.2mm is one of the pilot hole sizes for a 10's self tapper, it all depends what you're going into.
4.2mm is most widely used as the drill out for an M5 tap.
I thought that was why they sell a 4.2mm drill - to provide a pilot hole for 5mm screws?
[/ QUOTE ]
4.2mm is one of the pilot hole sizes for a 10's self tapper, it all depends what you're going into.
4.2mm is most widely used as the drill out for an M5 tap.
Adaero
Well-Known Member
[ QUOTE ]
Never use self tappers on core filled laminates, balsa or foam cores will not handle moiture very well at all.
[/ QUOTE ]
I have a load of gear that needs attaching to my boat shortly such as horseshoe buoy, storage pouches, winch handle holders, Para-Anchor attachment plates, etc.
I read this http://www.yachtsurvey.com/hardware_attachments.htm which is a very good guide for the major attachments but what about minor bits and pieces where it’s easiest to just use a self tapping screw?
What can you recommend as the best fixing method for balsa cored hulls?
Thanks
Never use self tappers on core filled laminates, balsa or foam cores will not handle moiture very well at all.
[/ QUOTE ]
I have a load of gear that needs attaching to my boat shortly such as horseshoe buoy, storage pouches, winch handle holders, Para-Anchor attachment plates, etc.
I read this http://www.yachtsurvey.com/hardware_attachments.htm which is a very good guide for the major attachments but what about minor bits and pieces where it’s easiest to just use a self tapping screw?
What can you recommend as the best fixing method for balsa cored hulls?
Thanks
