Pre-drilling screw holes in fiberglass?

The Overfishin Condition · 03-27-2006, 07:48 PM

How much smaller diameter of a drill bit should you use than your screws when pre-drilling fiberglass? Thanks, TOC.

diamondfish · 03-27-2006, 07:53 PM

When I have drilled into fiberglass or plastic in the past I use a caliper to measure the width of the screw between the flights of the screw. I then match it up with a bit of similar size, try to avoid going smaller than the shank of the screw as it may cause spidering in the glass. Measure the diameter of the screw shank halfway up the screw.

Hope this helps.

D-tangle · 03-27-2006, 07:59 PM

I attempt to use a bit the sixe of the screw shank (Which is smaller than the outside diameter of the threads). If the shank contacts the fiberglass, it could jamb, split the fiberglass, or more likely snap. Too large of a drill bit and the threads won't grab and hold. I suggest using a scrap piece of Maple or Oak to try the screw and make sure you get the secure hold you need without getting too tight before the screw bottoms out.
For instance, lets say the outside diameter of the screw is 1/4" and the shank is 1/8". I would try a bit 1/8" or slightly larger in a hard piece of wood. If it drives too hard, go the next sixe up. I find having bits in the 64ths of an inch is handy for stuff like this. Too large, and you are going to have to plug the hole with wood and resin and either move slightly to the side and drill again, or drill into the plug and try again. Good luck.

Broknt · 03-27-2006, 08:17 PM

buy a alum boat. then you dont have to worry about it. haha just jokin erik what these nice fellers said up above is what i would do.

thad

Fshklr · 03-28-2006, 04:33 AM

what are you trying to mount Erik?...fish finder, cleat, rod holder,pedestool seat ????

something that requires strength may need a backer.

MacEFL · 03-28-2006, 06:03 AM

I agree with Dan. A finder should be find but a 'rigger or a cleat will NEED a backing of some kind!

KChookem · 03-28-2006, 07:02 AM

You might consider drilling in reverse until you get through the gel coat or top layer. This may reduce the cracking and splintering in the top coat.

Chetco River · 03-28-2006, 07:22 AM

Quote:

You might consider drilling in reverse until you get through the gel coat or top layer. This may reduce the cracking and splintering in the top coat.

If you use a sheet metal s/s screw it's not tapered so then you measure inside the body of the screw .Also if you use masking tapeto drill through it helps tp keep the chiping and tracking <walking> down.

Bandy76 · 03-28-2006, 07:32 AM

put a piece of tape down to keep from cracking the gel-coat.
their is a chart for clearence sizes of drills. If you are using sheet metal screws have clearence for the solid shank.
if using wood screws have clearence also for the solid part of the shank on top.
A little soap(house hand)on the threads helps in librucating when putting in.
for standard screws use the size of bolt for the clearence drill then add a small washer on the back for support. if possible I add a piece of wood or light gage alumin also for strength.