# How big to predrill for holes in fiberglass.



## jephotog (Feb 25, 2002)

When predrilling in wood I use the following rule.

Softwood: drill bit two sizes down from screw size.
Hardwood: one size down from screw size.

I have pulled some hardware from my West Wight Potter, and filled the holes with epoxy. I am unable to use bolts when mounting so need to use screws. What sized drill bit should I use to pre-drill before mounting the screws.


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## Matt (Aug 11, 2015)

jephotog said:


> When predrilling in wood I use the following rule.
> 
> Softwood: drill bit two sizes down from screw size.
> Hardwood: one size down from screw size.
> ...


id follow the hard wood rule however absolutely make sure to countersink the gelcoat else you get cracks. i always epoxy the screws before installing them aswell usually use a product called marine tex, but have been known to use gflex aswell.


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## SHNOOL (Jun 7, 2007)

If we are talking cored fiberglass, I thought you were supposed to OVERDRILL, the hole like 2 sizes up, backfill with resin or epoxy let dry, then drill to proper size (not up or down) for he bolt (not screw). Now I know what you mean, you cannot always use bolts (no access to the backside)... but they better be non load bearing. But yeah, I'd go 1 size down. I am not sure it's possible to do it without cracking the gelcoat around it with screws.

if you have issues with bolts because of no access, consider pop-rivets, although I have no idea how you'd water-proof it.


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## Stumble (Feb 2, 2012)

Shnool has it right.

To drill a hole in a cored boat you need to first protect the core. The best way to do this is to drill a pilot hole then take a forstner bit from the underside and ream out a plug from the core. tape over the core and backfill with thickened epoxy. This gives a solid surface and a completely protected core. If you can't get to the back size for some reason then you need to drill out a larger hole then you can tap the epoxy plug. 

Tapping into epoxy isn't ideal, and i wouldn't do it for something that is heavily loaded, but for small things it will work. And if it ever pulls out you can just ream the hole and fill it again.


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## travellerw (Mar 9, 2006)

For solid fiberglass I follow the hard wood rule.


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## TejasSailer (Mar 21, 2004)

Hardwood guidelines haven't worked for me in fiberglass. I have a piece of solid scrap fiberglass that is used as for testing starting with the appropriate hardwood guideline and going incrementally with larger bits until success.


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## kd3pc (Oct 19, 2006)

One of the sailing rags had a chart last month....

fastener size clearance hole Pilot hole (FG) Pilot hole (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 (1/4") 9/32 3/16 5/32


best of luck, and don't forget to counter sink/taper the top of the newly drilled hole to keep the gel/glass from cracking...it also provides a place for sealant.


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## GeorgeB (Dec 30, 2004)

kd3pc, can you restate what the column headings should be in your chart? They got a little scrambled in the posting. I understand that the first column is the fastener size, but what are the next three columns?


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## Erindipity (Nov 29, 2014)

You might consider Threaded Inserts. They can be screwed or pounded into blind holes with Epoxy for setting, and then you can use regular bolts, machine screws, or nuts just as you would in Metal.
I've used Brass and Stainless inserts in G10 Fiberglass, but not yet on a Boat.










Epoxied-In Archives - Yardley

¬Erindipity


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## SloopJonB (Jun 6, 2011)

"cutting" screws - wood, self tappers etc. should have the hole drilled to the size of the minor axis of the screw - that is the diameter at the bottom of the teeth.


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## jephotog (Feb 25, 2002)

Stumble said:


> Shnool has it right.
> 
> To drill a hole in a cored boat you need to first protect the core. The best way to do this is to drill a pilot hole then take a forstner bit from the underside and ream out a plug from the core. tape over the core and backfill with thickened epoxy. This gives a solid surface and a completely protected core. If you can't get to the back size for some reason then you need to drill out a larger hole then you can tap the epoxy


This was my plan I pulled all the hardware on my West Wight Potter with the intention of doing this. Turns out there is no core on the boat so a lot of effort to fix something not broken. 
I have drilled tapered and butyled and bolted all hardware where I could.
Near the bow there is a number if things too close to the sides of the boat to get nuts behind a bolt.

What is the order of drilling?
Do you start out with a small pilot hole then taper before drilling the correct sized hole or do you just mark the hole then taper first?


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## Stumble (Feb 2, 2012)

jephotog said:


> This was my plan I pulled all the hardware on my West Wight Potter with the intention of doing this. Turns out there is no core on the boat so a lot of effort to fix something not broken.
> I have drilled tapered and butyled and bolted all hardware where I could.
> Near the bow there is a number if things too close to the sides of the boat to get nuts behind a bolt.
> 
> ...


I generally don't bother with a pilot hole in fiberglass unless I am protecting the core, or going to be using the pilot to center a bigger hole. I find brad point bits of the correct size work fine on their own. But I also sharpen my bits regularly as well.


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## albrazzi (Oct 15, 2014)

Stumble said:


> I generally don't bother with a pilot hole in fiberglass unless I am protecting the core, or going to be using the pilot to center a bigger hole. I find brad point bits of the correct size work fine on their own. But I also sharpen my bits regularly as well.


Ditto on the Brad points, step drilling is not necessary on soft materials and I will drill first and countersink after. I have some countersinks left over from the old days basically a taper shape with a diagonal hole through at an angle that does the cutting instead of the flute style. I haven't looked but if I could find that style I would buy some.


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## SloopJonB (Jun 6, 2011)

I've never heard of using brad points on anything as hard as glass - don't the little tips on the flutes break off a lot?


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## Stumble (Feb 2, 2012)

SloopJonB said:


> I've never heard of using brad points on anything as hard as glass - don't the little tips on the flutes break off a lot?


Not brad nails, brad point drill bits. I have never had then break on glass, kevlar, or carbon fiber. They even chew through G10 pretty well. The only issue is they do have a good bit of tear through on the back side, and because fiberglass is so abrasive they need to be sharpened (quick pass with a fine metal file) from time to time.


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## SloopJonB (Jun 6, 2011)

I know that - ref. the "flutes" comment. I have always found those little scoring tips or "wings" on a brad point bit to be fairly fragile so I never use them on anything but wood.


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