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The right screwdriver bit is the one that matches both the screw’s drive family and its size. Read the recess first—slotted, Phillips (PH), Pozidriv (PZ), Torx, tamper-resistant Torx, Torx Plus or internal hex—then choose a bit that fills the recess without rocking. If the tip sits shallowly, wobbles or begins to cam out, stop and recheck the profile and size instead of applying more force.
This guide shows how to identify the recess, distinguish easily confused cross-head profiles, verify fit, choose a bit length and holder, and decide whether a mixed set is useful for your work.
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Start with the screw’s recess, not the bit drawer
Look straight at the head of the fastener under good light. Brush away paint, dirt or metal chips so the bottom of the recess is visible. The shape in the head determines the drive family; the markings on the screw, packaging or manufacturer instructions may identify the size.
Wiha’s catalogue treats slotted, Phillips PH, Pozidriv PZ, TORX, tamper-resistant TORX, TORX PLUS and internal hex as separate drive families. A bit that looks broadly similar can still be the wrong match, so do not select a tip solely because it appears to enter the opening.
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Slotted (flat) drive
A slotted screw has one straight groove across the head. Use a flat-blade bit whose width and thickness fill that groove. A blade that is too narrow can twist in the slot; one that is too thick will not seat fully. Keep the driver aligned with the screw so the blade does not ride up the sides.
Phillips (PH)
Phillips recesses have a four-arm cross shape. Bits are marked PH followed by a size, such as PH1 or PH2. Read the size from the fastener or its packaging when possible; there is no universal screw-to-bit chart that covers every manufacturer and application.
Pozidriv (PZ)
Pozidriv also looks like a cross, but the recess includes additional smaller ribs between the main arms. PZ bits carry markings such as PZ1 or PZ2. Treat PH and PZ as different families: a Phillips bit is not automatically the intended match for a Pozidriv screw, and vice versa. If the tip contacts only the main cross and leaves the smaller ribs unused, recheck the profile before turning.
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Torx
A standard Torx recess is a six-pointed star with rounded lobes. Bits are marked with a T size, for example T10, T15, T20 or T25. The bit should reach the bottom of the star and contact all six lobes evenly.
Tamper-resistant Torx
Tamper-resistant, or security, Torx has a small post in the center of the recess. It requires a bit with a matching hole through its tip. A standard solid Torx bit cannot pass over that post and should not be forced.
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Torx Plus
Torx Plus is a distinct profile, even though it resembles a six-lobed star. Use a bit specifically marked for Torx Plus when the fastener calls for it; do not assume a standard Torx bit is interchangeable.
Internal hex (hex socket)
An internal-hex screw has a six-sided socket. Bits are commonly marked HEX followed by a metric size, such as HEX3, HEX4 or HEX5, or by an imperial fraction. Match the measurement system and the stated size. A near match can round the socket, so stop if the bit does not sit squarely on all six sides.
Hex nuts and other specialized profiles
Some fasteners use an external hex head rather than an internal recess; these need a nut driver or socket, not a screwdriver bit. Manufacturers also offer less common profiles for particular equipment. If the recess does not clearly match a standard family, check the fastener maker’s documentation instead of guessing.
Phillips vs. Pozidriv: the distinction that prevents many mistakes
Cross-head screws cause the most selection errors because PH and PZ tips appear similar at a glance. Inspect the recess under a bright light:
- Phillips: the visible shape is a simple four-arm cross.
- Pozidriv: the main cross is accompanied by four narrower secondary ribs, producing an eight-point appearance when viewed closely.
- Bit markings: PH and PZ are separate markings. Use the letters printed on the bit rather than relying on color or appearance.
Insert the candidate bit by hand before using a powered driver. A correctly matched tip drops fully into the recess and stays centered when you apply light axial pressure. If it rocks, contacts only the corners or rises out as you turn, remove it and test the other family or a different size.
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How to choose the correct size
After identifying the family, match the size. The inspected manufacturer material does not provide a complete screw-to-bit sizing chart, so avoid rules that claim every screw of a certain diameter takes one universal bit.
Use the information supplied with the fastener
- Look for a drive marking on the screw head, box or installation sheet.
- For a product supplied with its own screws, follow the maker’s stated bit size.
- When several sizes appear plausible, compare tips in the recess by hand and choose the one that seats deepest and most securely.
Check the fit before applying torque
- Clean the recess and the bit.
- Insert the bit straight into the head.
- Press gently and confirm that the tip reaches the bottom rather than balancing on the opening.
- Try a small side-to-side movement. There should be minimal visible play.
- Keep the driver aligned with the screw axis and make a cautious partial turn by hand.
- Stop immediately if the bit climbs out, slips or leaves fresh marks on the recess.
This fit check is more reliable than choosing a size by eye alone. Do not compensate for a poor match by pushing harder; extra force can damage the screw, the bit or the surrounding material.
Choose the bit length and holder for the access you have
Profile and size determine whether the tip fits, but access determines whether you can use it. A short bit is convenient in open areas; a longer bit may be needed when the screw sits inside a recess, behind a panel or beyond an obstruction.
Bit length
Measure the clearance from the driver or holder to the screw head. The Bosch example discussed below uses 25 mm bits, but that length is specific to that product and is not a universal standard for every job. Select a longer bit or an extension only when the extra reach is necessary, and keep the bit aligned so the added leverage does not bend it.
Shank and holder compatibility
Many interchangeable bits use a 1/4-inch external hex shank. Bosch specifies ISO 1173 C6.3 for its documented mixed set. Check your driver, ratchet adapter or magnetic holder for the matching shank standard before buying; a correctly shaped tip is unusable if it cannot lock into the holder.
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Magnetic holders and retention
A magnetic universal holder can retain a standard hex-shank bit and make changes faster. Verify that the holder accepts the bit’s shank and that the bit locks fully. Magnetism helps retain the bit, but it does not correct a wrong drive family or size.
When a mixed bit set makes sense
A mixed set is useful when your projects involve several drive families and you want the common sizes in one case. It is less useful when you repeatedly service one product that specifies a single specialized profile; in that situation, buying the exact replacement size avoids a drawer full of unused tips.
Coverage to look for
- Drive-family coverage: confirm that the set includes the profile on your screws—PH, PZ, Torx, slotted, hex or a security variant.
- Size coverage: check the printed sizes, not just the number of pieces.
- Access: compare bit length with the spaces where you will work.
- Holder compatibility: verify the shank standard and locking method.
A documented 12-piece example
Bosch’s regional UK product page, retrieved 9 October 2026, documents a 12-piece assortment with a magnetic universal holder. Its listed contents are:
| Drive family | Pieces listed |
|---|---|
| Phillips | PH2 |
| Pozidriv | PZ2 |
| Torx | T10, T15, T20 and T25 |
| Slotted | Two slotted sizes |
| Internal hex | HEX3, HEX4 and HEX5 |
| Holder | Magnetic universal holder |
| Bit length | 25 mm |
| Shank | 1/4-inch external hex, ISO 1173 C6.3 |
This is an example of category coverage, not evidence that the assortment fits every screw or that it is available from a particular retailer. The set includes PH2 and PZ2, but that does not make those sizes suitable for every cross-head fastener; inspect each screw and verify its marking.
A practical selection workflow
- Expose the recess. Remove dirt, paint or debris and use a light that shows the bottom of the opening.
- Name the family. Decide whether the shape is slotted, PH, PZ, Torx, security Torx, Torx Plus, internal hex or another profile.
- Read the size. Check the head, packaging or instructions. For an unmarked screw, compare likely bits without turning.
- Check the bit. Confirm that its printed family and size match your identification.
- Confirm seating. Insert the bit straight; it should sit deep, centered and stable with light pressure.
- Confirm reach. Choose a bit length and holder that allow a straight approach to the screw.
- Turn cautiously. Start by hand or at low power. Stop at the first sign of slipping and reassess.
What to do when the bit slips or cams out
The tip rocks before turning
Rocking usually indicates a family or size mismatch, contamination in the recess or a worn bit. Clean both parts, try the correctly marked family and compare the next size only if the fastener documentation supports it.
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The driver rises out under load
Keep the driver on the screw axis and apply only enough axial pressure to maintain contact. If the tip still climbs out, stop; forcing it can deform the recess. Recheck PH versus PZ, Torx versus Torx Plus, and standard versus tamper-resistant Torx.
The bit enters but does not reach the bottom
A tip that stops high in the opening may be too large, the wrong profile or blocked by debris. Remove the bit, clean the recess and test the designated size. Do not use a smaller bit merely because it reaches deeper if it leaves visible play.
The screw has an external hex head
Switch from a screwdriver bit to a correctly sized socket or nut driver. An internal-hex bit cannot drive an external hex head.
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Write down the drive families and sizes you encounter in your own projects. Buy exact replacements for recurring fasteners, then add a mixed set only when you genuinely need several families. Keep bits in a case with the markings visible; separating PH from PZ prevents the most common mix-up.
Inspect tips before each job. Replace a visibly rounded, chipped or twisted bit rather than trying to rescue it with more pressure. Store bits dry and wipe away dust or metal particles so they seat fully the next time.
Quick checklist before you turn the screw
- Have I identified the recess family rather than guessing from its general shape?
- Does the bit marking match the family and stated size?
- Does the tip sit fully and remain centered with light pressure?
- Is the bit long enough to reach while keeping the driver straight?
- Does the shank fit the holder or driver?
- Am I starting gently enough to notice slipping before damage occurs?
The Bottom Line
Identify the recess, match its marked family and size, verify a firm full-depth fit, then choose the length and holder that let you keep the driver straight. When in doubt, stop and recheck rather than forcing a bit that rocks or cams out.
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