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Old    John (johndk)      Join Date: Aug 2008       05-18-2013, 8:29 PM Reply   
I'm installing a trim tab, and have one pilot hole drilled so far (which I probably need to redo since it's offset, as I wrote in another thread).

I'm using 1/4" pan head sheet metal screws, and what I've read is that I should use 3/16" pilot holes. (The MB trim tab kit itself had no instructions.)

But when I tried to screw the screw in, it stopped with about 1/8" left to go, even with my corded drill. It was so tight, the drill couldn't back it out, and I had to grasp a screwdriver with a pair of pliers to do it. The pilot hole was longer than where the screw got stuck.

So, is 3/16" really the right diameter pilot hole for 1/4" screws, and I'm just doing something wrong? Or should I drill thicker holes, such as 13/64" or 7/32" or 15/64"? I will also be using 3M 5200 adhesive sealant.

In this video, the guy is just using a screwdriver... there's no way that would be possible in my case: http://www.youtube.com/watch?v=cV2BKm1s0iQ

Thanks.

Last edited by johndk; 05-18-2013 at 8:31 PM.
Old    John (johndk)      Join Date: Aug 2008       05-19-2013, 8:20 PM Reply   
13/64" worked. I actually got gel coat cracking around the 3/16 hole I tried (I hadn't done the countersink yet, but was just trying to briefly test the screw... now I know to never put in a screw even temporarily until you've countersunk the surrounding gelcoat off first).
Old    Travis (fman)      Join Date: Nov 2008       05-19-2013, 8:40 PM Reply   
Also a good idea to use a Chamfer bit and round off the holes to prevent gelcoat cracking from the screws.
Old    John (johndk)      Join Date: Aug 2008       05-19-2013, 8:43 PM Reply   
I just used a much larger bit (run in reverse) to countersink it and grind away the gelcoat about 1/16" around the hole. I saw that tip somewhere - hopefully it's as good as using a specific bit like you mentioned.
Old    Travis (fman)      Join Date: Nov 2008       05-19-2013, 8:57 PM Reply   
I think you will be fine, you just want to round off the hole which sounds like exactly what you did.
Old    Art (rallyart)      Join Date: Nov 2006       05-20-2013, 12:06 AM Reply   
Fibreglass is much tougher than wood, or sheet metal, and you may need a bigger hole than normal for a screw. If it's too tight it will bust the screw and that is a real pain. The threads will hold very well in glass compared to wood or sheet metal, even if the hole is a bit bigger.
Old    John (johndk)      Join Date: Aug 2008       05-20-2013, 12:13 AM Reply   
Yeah, I just don't understand why the various sources I watched/read said to use 3/16", even though it was clearly too small.

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