Pre-drilled hole size for #8 screw in transom.

[tyler_bee]

Lame question but I don't want to over/under drill this.

I'm replacing an old garboard drain plug and bought 1in #8 screws for it as I cannot access behind the drain for through-bolts.

What size should I drill the pilot holes?

Too small I fear cracking/chipping excess glass.

Edit: Added an older pic of the transom, the old holes are now filled and sanded.

 

[blifsey]

Re: Pre-drilled hole size for #8 screw in transom.

Not sure of exact size needed but you are correct that too small will cause crack when tightening. Unlike wood, fiberglass can absorb/expand as screw is put in. Also, I've had luck drilling a very tiny pilot hole first, countersink that with a countersink bit larger than desired final hole size and then drilling final size. The countersink helps prevent chipping around edge of hole. Go very slow especially when cutting through gel coat. Once you've done one, the fear diminishes

I squirt 4200 or 5200 into hole before putting screw in. Have read to always use 5200 below waterline.

EDIT: originally had "chamfer" and, of course, meant "countersink" realized mistake after reading reply below mine.

 

[seabob4]

Re: Pre-drilled hole size for #8 screw in transom.

Tyler,
Yes, I am here as well. 1/8" bit, "wallow out" a bit because it is thick glass, c/sink the gel to keep from cracking/chipping out...

And plenty of 5200!

 

[Fireman431]

Re: Pre-drilled hole size for #8 screw in transom.

#8 screw get a 1/8" bit (for hardwoods). Use a 1/4" bit to campher.

 

[seabob4]

Re: Pre-drilled hole size for #8 screw in transom.

#8 screw get a 1/8" bit (for hardwoods). Use a 1/4" bit to campher.

Believe it or not, the end of a #2 Apex screw bit works real well...trick of the trade, you know....

 

[rwidman]

Re: Pre-drilled hole size for #8 screw in transom.

Lame question but I don't want to over/under drill this.

I'm replacing an old garboard drain plug and bought 1in #8 screws for it as I cannot access behind the drain for through-bolts.

Was the original through bolted?

Does the boat have an electrical system? If so, the original was probably bonded using one of the bolts and the replacement should be also.

As for the pilot hole for screws (not bolts), it needs to be a little larger than it would be for wood. As others have said, if the hole is too small, the gelcoat will crack.

 

[tyler_bee]

Re: Pre-drilled hole size for #8 screw in transom.

1/8th worked great. Not asmuch "Meat" as I would've liked, the hole edge in the flange is damn near the edge of the larger hole for the drain tube. - Edit, just looked now, the holes shown in the above photo have about the same distance as the new ones. It just feels close when you are drilling it.


Waiting on some Epoxy Resin to dry before I actualy 5200 it in. I just Dry fit and things looked acceptable.

The Epoxy says I can sand after 24 hours and full cure in 72 hours. It's all white now, no pink, and feels quite hard. Do you suppose I should still wait for the 72 hour mark before I have 5200 come in contact with it?

I filled the old bolt holes with epoxy and drilled new ones for the new garboard drain.

Thanks for ya'll input. I was thinking about ya while I was dry fitting thinking, "Man, it's nice to have all the help from here". (=