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Pilot Hole Screw Size for Fiberglass #14393252 06/14/22 02:41 AM
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learnin to fish Offline OP
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I’m installing the T-Top I am building for my boat tomorrow and am using #14 screws. Should I drill the pilot hole 1 or 2 sizes smaller than the screw size? I don’t want to crack the fiberglass but don’t want it too lose as this is going into the floor and must be a strong connection. I always countersink the pilot holes in the fiberglass to prevent cracking.

Last edited by learnin to fish; 06/14/22 02:41 AM.
Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14393858 06/14/22 05:53 PM
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bronco71 Online Content
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not sure of pilot size but if you tape the area with painters tape and use a step drill it is less likely to crack than using a regular drill bit....


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Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14393990 06/14/22 07:47 PM
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hopalong Online Confused
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size the bit so when held up to the screw you see the threads and barely any of the main shaft of the screw, don't remember what size I use for #14 but that is how I size bits for pilots.


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Re: Pilot Hole Screw Size for Fiberglass [Re: hopalong] #14394341 06/15/22 03:27 AM
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Duke 22 Online Content
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Originally Posted by hopalong
size the bit so when held up to the screw you see the threads and barely any of the main shaft of the screw, don't remember what size I use for #14 but that is how I size bits for pilots.

x2, or as I call it " The Eyeball Test".

Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14395079 06/15/22 10:21 PM
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hopalong Online Confused
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run the bit in reverse for a few turns first, it will keep it from chipping so bad.


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Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14449329 08/15/22 02:36 PM
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I struggle with this as well, and found this online - from a sailing site I think, but have found it to be accurate. As others have mentioned, I start with a smaller bit, and also drill in reverse and/or counter sync with a Philips bit depending on what I am mounting.

Formatting is bad due to forum, but the 3rd column is the FG pilot size


size clearance FG Wood
#4 9/64 3/32 5/64
6 11/64 7/64 1/16
8 13/64 1/8 7/64
10 15/64 9/64 1/8
12 1/4 5/32 9/64
14 9/32 3/16 5/32


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Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14449331 08/15/22 02:38 PM
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I struggle with this as well, and found this online - from a sailing site I think, but have found it to be accurate. As others have mentioned, I start with a smaller bit, and also drill in reverse and/or counter sync with a Philips bit depending on what I am mounting.


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Re: Pilot Hole Screw Size for Fiberglass [Re: learnin to fish] #14450186 08/16/22 01:59 PM
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Good info. flehan


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Re: Pilot Hole Screw Size for Fiberglass [Re: Duke 22] #14452028 08/18/22 01:39 AM
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Originally Posted by Duke 22
Originally Posted by hopalong
size the bit so when held up to the screw you see the threads and barely any of the main shaft of the screw, don't remember what size I use for #14 but that is how I size bits for pilots.

x2, or as I call it " The Eyeball Test".

That is howi do it.


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