Find out on this page what a pilot hole, clearance hole and countersink in timber or metal is, when and why you should use them and how to work out the drill size to use for correct pilot & clearance hole sizes.
Countersinking, drilling a wood screw pilot hole and drilling a clearance hole is handy during carpentry work for four reasons;
It is especially important when working with hardwoods, on thin strips of/narrow timber or near the edge of the timber because in these instances the timber is more prone to split.
To countersink a hole is to cut a V shaped groove into the surface that will accommodate the tapered bottom of the screw head. The screw head being the widest part of the screw can often be the part that causes the timber to split as you bury the screw head below the surface of the wood. Countersinking the hole first prevents this from happening and looks neat enough to leave, or can be filled over for decorating too.
If you don't want to countersink and fil over the screw, you can use screw cups like these instead.
Special countersink drill bits are available to cut the shape, although you may just be able to use a large HSS drill bit instead if you don't have one.
This hole is drilled in the timber slightly smaller than the threads of the screw. It allows the screw to pass easily through the timber, but because the hole is slightly smaller than the diameter of the threads, the threads still have plenty of timber to bit into and grip. Again this makes tightening the screw easier and less likely to split.When working with hardwood or particularly thin or brittle pieces of timber a pilot hole is essential to prevent the wood from splitting.
Special pilot hole bits are available that also have a countersink on them so you can do both jobs with one drill bit.
A clearance hole ensures the two materials are pulled up tight when they're screwed together. Without a clearance hole, as you wind the screw in a gap can develop in between that doesn't get closed up - no matter how deeply you bury the screw head! The piece of timber you are fixing (the first piece the screw enters) should not be gripped by the screws threads - the screw should pass straight through it until the head pulls it towards the wall/piece being fixed to.
Because the two holes for wood screws (pilot and clearance) do a completely different job to each other they are different sizes.
It is easy to ensure you have the right size by holding the bit in front of or behind the screw to compare whether it is larger/smaller than the threads.
Yes! I normally have two drills on the go to save time. One will have a clearance hole drill bit, the other a countersink and pilot hole drill bit. Alternatively you may use one for all these holes and keep a screwdriver bit on the other depending on the job and type of holes I am drilling.
To save time drilling all those holes If you can afford them Dewalt Flip Drivers have a special quick release holder that fits in the chuck of your battery drill and holds a screwdriver bit like a pozi-2. Flick the quick release mechanism that releases the bit holder and on the other end is the pilot and countersink bit - all in one. They save loads of time not having to keep undoing the chuck and swapping the bits over which adds up when you have a lot of timber to fix.
To hide large screw holes in finished work-pieces I use wooden pellets inserted the same direction as the grain and cut out of off-cuts of the timber I'm filling.
Have a question or comment about drilling pilot holes?