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Long drill bits let you bore through a thick post, beam or stacked framing without drilling from both sides. The right choice depends on the hole diameter, how deep you need to drill and whether you need a clean, accurate hole or just a passage for a cable or bolt. A longer bit is not automatically better: extra length makes a bit easier to bend, wander and bind.
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Choose the bit type for the hole
For general holes in framing lumber, a long auger bit is usually the most useful choice. Its screw tip pulls the bit into the wood, while the flutes carry chips out of the hole. A 12- to 18-inch auger is a sensible starting range for many timber jobs. Check the bit’s usable cutting length, not just its overall length; the shank and tip take up part of the listed dimension.
For larger, flatter-bottomed holes, use a Forstner bit with an extension if the manufacturer permits it. It can make a neater opening than an auger, but a wide Forstner bit demands more torque and is a poor match for a small cordless drill. Spade bits are inexpensive and fast for rough holes, but long versions can wander and leave ragged edges. They work best for utility holes where appearance does not matter.
For clean holes through thick stock, consider long wood auger bits. If the job calls for a larger opening, compare them with long spade bits, which cost less but usually give up control and finish quality.
Long-bit options at a glance
| Bit type | Best for | Trade-off | Typical buying check |
|---|---|---|---|
| Auger | Deep holes in framing timber | Can grab hard; needs chip clearing | Length, diameter and shank fit |
| Spade | Fast, rough utility holes | More wandering and tear-out | Hex shank and usable length |
| Forstner | Cleaner, flat-bottomed holes | High torque demand; extensions add runout | Drill capacity and extension rating |
| Ship auger | Deep, demanding framing work | Often costs more and may be overkill | Cutting length and compatible drive |
Match length and diameter to the job
Measure the timber and mark the required depth before shopping. A 12-inch bit may not cut a 12-inch-deep hole once you account for its tip, chuck clearance and the space needed to keep the drill square. For a hole through a 10-inch post, leave enough extra length to start straight and clear chips. If you need substantially more reach, a purpose-made long bit is generally more reliable than stacking extensions.
Diameter matters as much as length. A 1/2-inch hole is much easier on a drill than a 1-inch hole at the same depth. Large holes remove more wood, create more friction and raise the risk of a sudden bind. Buy the smallest diameter that safely fits the bolt, pipe or cable. For an oversized hole, work up in stages if your bit and drill setup allow it; do not force a large cutter through at full speed.
Check the shank before buying. A round shank fits a standard three-jaw chuck, while a hex shank can reduce slipping and may suit a quick-change holder. The holder must still be rated for the bit and torque involved. Long bits amplify small amounts of chuck runout, so a bit that appears straight when spinning slowly is worth checking before drilling a deep, visible hole.
Use a drill with enough control
A variable-speed cordless drill with a proper side handle is preferable for large, deep holes. A compact drill may handle a small auger in soft pine, but it can stall or twist out of your grip in a knot or dense hardwood. A hammer drill should be set to rotary-only; hammer action is for masonry, not timber. An impact driver is not the default choice for drilling with long augers: its pulsing action can make the bit harder to control, and many large bits are not intended for impact use.
If your drill has a mechanical gearbox, use its low-speed range for large bits and high torque. Start slowly, keep both hands on the tool and brace the work securely. Do not hold a loose post between your knees. If the drill has an auxiliary handle, fit it; a long bit can produce a sudden reaction when its cutting edge catches.
When the bit binds, release the trigger rather than leaning harder. Back the bit out gently to clear the flutes, then check for a knot, embedded fastener or packed chips. For a deep hole, withdraw the bit periodically to clear chips, especially in green, resinous or wet wood. Packed chips increase heat and make the bit more likely to seize. Keep hands away from the rotating bit and wear eye protection; long shavings can be sharp.
A practical buying checklist
Before ordering, confirm four things: the cutting depth, the hole diameter, the drill’s chuck capacity and the bit’s shank type. Look for a sharp, well-formed screw point and clean flutes. A dull bit makes the drill work harder and is more likely to burn the wood or bind. For occasional work in softwood, an inexpensive spade bit may be adequate. For repeated deep holes, hardwood or accurate boring, a quality auger is easier to control and typically clears chips better.
Buy the shortest bit that reaches the work. If you need a very long bit only once, an extension can be cheaper, but each added connection introduces more wobble and another place to loosen. For regular work through thick timber, a single-piece bit of the required length is the more dependable setup. Whichever you choose, test it on scrap of similar wood before drilling the finished post or beam.