[rtate]

What size drill bit should I use to pre-drill for the screws that hold the drivers
down in my xls-encore's ??

[jparkhur]

Just a little one will do.  Then the screw will not crack the wood and still bite hard.  Four or fives sizes down from what your screw is.

[Peter J]

I use Vix bits so the hole is centered. I just did a similar project and I think Danny sends you two different screw sizes...one for woofer and another for tweeter.
The bit size in the Vix is 5/64 on smaller and 7/64 on larger. Theoretically the pilot should be same diameter as screw shank, but in the real world, MDF is very forgiving as long as the pilot isn't too big. You'll get a little volcano on the surface, which can be sanded off or touched with a countersink. That's part of why I like to prefit before finishing.

[django11]

I would go with the root size of the screw thread which is the inside diameter (R in the drawing).  You can just eyeball the drill bit by holding it in front of the screw.  Drill bit= inside thread of screw.  Don't over tighten.




FYI S is the shank diameter and not the size for a pilot hole.

[srb]

Theoretically the pilot should be same diameter as screw shank, but in the real world, MDF is very forgiving as long as the pilot isn't too big.

I use the exact screw shank root size for hardwood.  For softwood and MDF I usually use one drill bit size smaller (- 1/64") as softwood will compress more and MDF has a greater screw pull out potential than softwood, hardwood or plywood.
 
I also lubricate the screw threads with bar soap, beeswax or parrafin for smooth hole thread cutting.
 
Steve

[Peter J]

I would go with the root size of the screw thread which is the inside diameter (R in the drawing).  You can just eyeball the drill bit by holding it in front of the screw.  Drill bit= inside thread of screw.  Don't over tighten.




FYI S is the shank diameter and not the size for a pilot hole.

Django is right, shank is wrong terminology. Root is proper for wood screws. Perhaps I misled without intending. The illustration is a wood screw, the screws sent with kit are actually tapping screws, which have no shank.

SRB describes the theoretical ideal. I have to admit I don't adhere to it in every case, and do much by eyeball and seat-of-pants experience.

[rtate]

Thanks for the great info guys...