Pilot Hole Screw Size for Fiberglass

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.

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....

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".

run the bit in reverse for a few turns first, it will keep it from chipping so bad.

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

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.

Good info.

That is howi do it.