Size of pilot holes for 1/4" screws in transom?
 

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.

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).

Also a good idea to use a Chamfer bit and round off the holes to prevent gelcoat cracking from the screws.

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.

I think you will be fine, you just want to round off the hole which sounds like exactly what you did.

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.

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.