Impact driver bits: the 1/4 in hex shank and which bits fit

01What shank does an impact driver take?

An impact driver takes a 1/4 in hexagonal shank, the standard driver-bit shank described in ISO 1173 and, in the US, in ASME B107.4. Instead of a jawed chuck, the tool has a quick-connect holder that the hex slides into and locks. The DeWalt DCF845, the Milwaukee 2953-20 and the Ryobi PBLID01 all list a 1/4 in hex holder, and Makita lists the same for the Makita XDT13Z.

The hex size is the same across brands and across the Atlantic. ISO names it in millimetres, in shank forms such as E 6.3, while US makers use the inch size, but it is one shank. A Wera bit made in Germany fits a Milwaukee, DeWalt, Makita or Ryobi driver, and a US-brand bit fits a European holder made to the same standard. Wera states that its insert bits fit holding tools made to DIN ISO 1173-D 6.3.

The table below lists the impact drivers in the AccessoryFit database with the holder and speed each maker publishes.

02What is the difference between insert bits and power bits?

Insert bits are short bits that sit in a bit holder, while power bits are longer and lock directly into the driver's quick-change holder. Both have the same 1/4 in hex; the difference is length and the retaining groove. The Makita E-01666 Impact XPS 35 pc. impact bit set shows both types in one set: its contents are listed as (25) 1 in insert bits, (6) 2 in power bits, (2) magnetic nutsetters, insert bit holder, socket adapter.

The Wikipedia summary of ISO 1173 describes the two shank forms. Form C 6.3 is the insert-bit shank, used in magnetic or friction-fit holders. Form E 6.3 is the power-bit shank, which carries a groove so the ball detent in a power-tool holder can lock it in place. Power bits are commonly 2 in long, for example Milwaukee's ECX power bits and Wera's 851/1 Z.

In practice, a power bit goes straight into an impact driver. An insert bit needs a holder first, either a magnetic holder or a screw-lock sleeve, which then goes into the driver. Most impact bit sets, such as the Milwaukee 48-32-4004 SHOCKWAVE Impact Duty driver bit set (32 pc.), include a magnetic holder for that reason.

03Do you need impact-rated bits in an impact driver?

Yes for screwdriving: impact-rated bits are made for the torque peaks an impact driver delivers, while standard bits share the same shank but can fail early. Impact-rated bits have a torsion zone, called TZ by Wera and SHOCKZONE by Milwaukee, which absorbs torque peaks so the tip does not break or wear early. The Wikipedia article on screwdriver bits adds that standard bits tend to fail sooner in impact drivers because their harder heat treatment is more brittle.

The reverse case is a common misconception. Impact-rated bits do not only work in impact drivers: Milwaukee and Wikipedia both note that they also work in drill/drivers and hand holders, though they bring little advantage there. One set of impact-rated bits can therefore serve a drill and an impact driver alike.

Makers mark impact-rated bits clearly: Milwaukee uses SHOCKWAVE Impact Duty, DeWalt uses IMPACT READY and FlexTorq, Bosch uses Impact Tough and Makita uses Impact XPS. Milwaukee's and Makita's sets list a 1/4 in hex shank; the Bosch and DeWalt set tables on file do not state the shank size, though both are sold for impact drivers. For set comparisons by published contents, see the shortlist of impact driver bit sets.

04Can you use drill bits in an impact driver?

Only drill bits with a 1/4 in hex shank fit an impact driver directly. Milwaukee's SHOCKWAVE RED HELIX impact-rated twist bits have that hex shank, and Milwaukee says they fit both impact drivers and drills. The DeWalt DWA1790IR IMPACT READY titanium nitride step drill bit set, 3 pc is listed with a 1/4 in hex shank, and so is the Milwaukee 49-56-0505 1/4 in DIAMOND MAX hole saw bit. Starrett makes flat wood (spade) bits with a hex shank for impact machines.

Many drill bit sets do not have hex shanks. The DeWalt DWA1240 Industrial Cobalt Alloy Steel PILOT POINT drill bit set, 14 pc and the Milwaukee 48-89-2332 RED HELIX Cobalt drill bit set, 29 pc, for example, have 3-flat shanks, made to grip in a jawed chuck. Three flats help a round shank resist slipping in jaws, but they do not make it a hex, and such a bit will not lock into an impact driver's holder.

Hole saws sit in between. Some arbors have larger hex shanks that need a jawed chuck rather than an impact holder: arbors with a 3/8 in hex need a drill chuck of that size or larger. The Bosch HBT15M 15 pc. general-purpose bi-metal hole saw set lists its arbor parts as 2 adapters, 2 HSS 1/4 in hex pilot bits. Whole-range bit guidance by material is gathered under drill bits by material.

05Is there a drill bit adapter for an impact driver?

Yes. A hex-shank chuck adapter is a small keyless chuck on a 1/4 in hex shank; it locks into the impact driver's holder and lets jawed bits fit. Industrial suppliers list these hex-shank mount chucks for impact drivers, and Milwaukee's M12 FUEL installation drill takes the same idea further with interchangeable heads, a 3/8 in metal chuck head and a 1/4 in hex head, according to Milwaukee's listing.

Makita sells one as a named part. Its 763198-1 adapter puts a keyless chuck on a 1/4 in hex shank, and Makita says it lets an impact driver hold round- or hex-shank bits up to 3/8 in, turning the driver into a driver drill. That capacity is smaller than a full-size drill chuck, so large reduced-shank bits and heavier hole saw arbors still need a drill.

An adapter solves fit, not suitability. The AccessoryFit fact base holds no manufacturer guidance on how ordinary twist bits behave when driven through a chuck adapter under impact, so that question is left open here. Drill chuck capacities, for anyone choosing a drill instead, are compared in the guide to which bit shanks fit your chuck.

Adapters also run the other way. Makita's B-59259, according to Makita, lets an SDS-plus rotary hammer drive 1/4 in hex bits, and its short shank helps limit damage if hammer mode is selected by mistake. Chuck and hex adapters on SDS-plus hammers are for rotation-only use. The SDS shank family itself is explained in the guide to rotary hammer bit shanks.

06Which other hex-shank accessories fit an impact driver?

Anything built on the same 1/4 in hex fits, which takes in nut drivers, hex key bits, bit holders and some screw extractors as well as screwdriver bits. Milwaukee's catalogue gives a useful map of the range.

• Nut drivers. Milwaukee's impact-rated magnetic nut driver set covers 1/4, 5/16, 3/8, 7/16 in hex heads, and its shorter insert set adds one larger size, per Milwaukee.
• Hex key bits. Milwaukee's driver bit sets include short hex (Allen) insert bits from 1/16 to 1/4 in on a hex shank, and some sets add metric hex sizes.
• Magnetic bit holders. Milwaukee sells them with a hex shank in three lengths, listed as compact, 3, 6 in, and any of them turns a short insert bit into something the driver's holder can lock.
• Right-angle attachments. DeWalt says its FLEXTORQ right-angle attachment is impact rated and has a magnet to hold fasteners.
• Screw extractors. Milwaukee rates only the smaller sizes in its double-ended extractor set for an impact driver, run at low speed in reverse.

07Which bit tip matches which screw?

Match the bit's marking to the screw recess: PH for Phillips, PZ for Pozidriv, T or TX for Torx and SQ or R for Robertson square drive. The shank is the same for all of them, so tip choice is a question of the screw, not the driver.

Phillips and Pozidriv cause the most mismatches. Pozidriv was developed from Phillips to transmit more torque with less cam-out, which is the bit slipping out of the recess once torque passes a limit. Pozidriv screws carry tick marks at 45 degrees between the main slots, and a PZ bit has matching extra ribs. A Phillips bit only loosely fits a Pozidriv screw and cams out under torque, while a PZ bit fits a Phillips screw poorly and is likely to slip. They are not interchangeable, despite the popular belief, so a PZ screw needs a PZ bit.

Another widely repeated claim is that Phillips was designed to cam out on purpose to prevent overtightening. There is no good evidence for that, and the original patents do not mention it. Albany County Fasteners regards Torx and square drives as more reliable than Phillips at preventing cam-out.

08When does a bit not fit or not last in an impact driver?

A bit fails to fit when its shank is not a 1/4 in hex, and it fails early when it is not built for impact torque or does not match the screw. The cases covered by the sources on this page are:

• Round or three-flat shank. It will not lock into the quick-change holder; use a hex-shank chuck adapter or a drill.
• Larger hex shank. Hole-saw arbors with a 3/8 in hex are made for jawed chucks.
• Insert bit without a holder. A short insert bit has no power-bit groove and needs a bit holder.
• Standard bit under impact. Wikipedia notes that standard bits tend to fail early in impact drivers.
• Wrong tip for the recess. PH in a PZ screw cams out under torque.

Shank systems for other tools follow the same logic of one interface per tool type. Routers grip round shanks in collets, explained under metric router collets, while saw blades use shank standards such as the universal reciprocating blade shank and T-shank and U-shank jigsaw blades.

09How do you check that a bit will fit your impact driver?

Four checks confirm whether a bit or accessory will lock into an impact driver and survive the work.

1. Check the shank shape. It must be a hex of the standard driver-bit size. Round, three-flat and larger hex shanks need a chuck.
2. Check the length type. A power bit with a groove locks in directly; a short insert bit needs a magnetic holder or screw-lock sleeve.
3. Look for an impact rating. Packaging marks such as SHOCKWAVE, IMPACT READY, Impact Tough or Impact XPS indicate a torsion zone.
4. Match the tip to the screw. Read the bit marking and the screw head, especially PH against PZ, before driving.

10What else matters when choosing impact bits?

For cordless drivers, the battery platform decides which bare tools you can add later; the battery platform cross-reference compares the main systems. To see what fits a particular driver model, use the tool accessory lookup, and the remaining fit rules on the site are grouped under the compatibility guides.

11Questions readers ask