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Choose a screwdriver bit by matching the screw’s recess family first, then its size. A cross-shaped recess may be Phillips (PH) or Pozidriv (PZ); those are different bit families even when they look similar. A bit that sits deeply and does not rock is the correct starting point. If it sits shallowly, wobbles or cams out, stop and recheck the profile and size instead of applying more force.
This guide shows how to identify the common recesses, verify fit, select a practical assortment and avoid the mistakes that damage screw heads.
Start with the recess, not the driver handle
The driver or power tool only supplies rotation. The bit is the part that must match the geometry cut into the screw head. Inspect the recess under good light, brush away debris and compare its shape with the profiles below. Manufacturer catalogues, including Wiha’s catalogue, treat these as separate drive families rather than interchangeable names.
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| Recess family | What to look for in the screw | Bit marking to seek | Important distinction |
|---|---|---|---|
| Slotted | One straight slot across the head | Slotted, flat or SL; the size may be expressed by blade width | The blade should fill the slot without bottoming on the head outside the slot. |
| Phillips | A simple cross with four tapered arms | PH, followed by a size such as PH2 | Do not assume every cross recess is Phillips. |
| Pozidriv | A cross with additional smaller ribs between the main arms; the extra marks can be hard to see when the head is dirty or worn | PZ, followed by a size such as PZ2 | Pozidriv and Phillips are separate families. A PH bit is not the intended match for a PZ screw, or vice versa. |
| TORX | A six-point star-shaped recess | T, TX or TORX with a number such as T20 | The number identifies a size within the Torx family; it is not a Phillips-style cross size. |
| Tamper-resistant TORX | A Torx star with a pin in the center | Security Torx, tamper-resistant Torx or a bit marked for the matching size and center-hole design | A standard Torx bit has no center hole and cannot pass over the pin. |
| TORX PLUS | A Torx-Plus-style star with a different, more substantial lobe shape | IP, Torx Plus or the manufacturer’s Torx Plus designation | Use the Torx Plus family specified for the fastener; do not treat it as automatically identical to ordinary Torx. |
| Internal hex | A six-sided socket recessed into the head | HEX, H or a size such as HEX4 | This takes a hex bit, not a slotted or cross-head bit. |
Some catalogues also list external hex nuts and other specialized profiles. Those require a socket or a profile-specific driver rather than a bit chosen from the common cross, star and slotted groups.
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Phillips versus Pozidriv: the mistake that causes many poor fits
Both profiles have a cross appearance, which is why they are often confused. Phillips arms taper toward the center. Pozidriv adds secondary ribs between the main arms, producing a more complex cross when the head is clean and viewed straight on. Look for the small extra marks before selecting a bit.
Bit markings are the quickest confirmation: PH means Phillips and PZ means Pozidriv. Bosch’s product documentation lists PH and PZ bits separately, reinforcing that they are not one interchangeable range. If a PH bit seems almost right in a PZ recess, that is a warning rather than a reason to press harder. Switch to the PZ size indicated on the fastener, packaging or manufacturer instructions.
How to choose the correct size
After identifying the family, match the size. The inspected manufacturer material does not provide a complete universal screw-to-bit sizing chart, so there is no reliable rule that assigns one bit size to every screw diameter or head shape. Use the marking on the screw, its packaging, an installation manual or the fastener manufacturer’s instructions whenever one is available.
Use the recess as a fit test
- Insert the bit straight into the recess without turning it.
- Press lightly so the tip reaches the bottom of the drive features.
- Check that the bit sits squarely and does not rock from side to side.
- Try a gentle hand rotation. The bit should remain engaged instead of climbing out of the recess.
A bit that sits shallowly, rocks, leaves visible gaps at the sides or cams out under light pressure is not a confirmed match. Test the adjacent size in the same family and recheck that you did not confuse PH with PZ, ordinary Torx with security Torx, or Torx with Torx Plus. Never compensate for a poor fit by leaning harder on a drill.
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Why a size can look close but still be wrong
Wear, paint, dirt and manufacturing variation can obscure the recess. A worn screw may accept a bit loosely even when the family is correct. Conversely, a larger bit may appear to fill the opening while failing to seat at the bottom. The practical objective is full, stable engagement of the intended surfaces, not merely finding a tip that enters the hole.
Choose a bit set by coverage and compatibility
A mixed set is useful when your work includes several drive families. Evaluate it in this order:
- Family coverage: confirm that it contains the profile on your screws, such as PZ rather than only PH.
- Size coverage: check the exact sizes, not just the family name. A set with Torx is not necessarily useful if it lacks the T-size you need.
- Access: verify bit length and whether a long or short bit can reach the screw without the holder striking nearby surfaces.
- Shank and holder compatibility: make sure the bit’s shank matches your hand driver, ratchet adapter or drill chuck.
- Retention: a magnetic holder can help keep a bit in place, but it does not correct a mismatched profile or size.
Documented mixed-set example
Bosch’s UK DIY product page, retrieved 9 October 2026, documents a 12-piece assortment as an example of broad household coverage. It contains PH2, PZ2, T10, T15, T20, T25, two slotted sizes, HEX3, HEX4 and HEX5, plus a magnetic universal holder.
| Specification | What the documented set provides | How to interpret it |
|---|---|---|
| Phillips | PH2 | One Phillips size; it does not cover every PH size. |
| Pozidriv | PZ2 | One Pozidriv size; useful only when the screw is PZ2 or the fit is otherwise confirmed. |
| Torx | T10, T15, T20 and T25 | Four ordinary Torx sizes; no security-pin or Torx Plus bit is listed. |
| Slotted | Two slotted sizes | The page lists two sizes but does not establish a universal screw-size mapping. |
| Hex | HEX3, HEX4 and HEX5 | Three internal-hex sizes. |
| Bit length | 25 mm | This is the product’s stated length, not a requirement for every job. |
| Shank | 1/4-inch external hex, marked ISO 1173 C6.3 | Check that your holder or driver accepts this shank standard. |
| Holder | Magnetic universal holder | Provides retention and connection; it does not replace profile and size matching. |
This set demonstrates category coverage, not a guarantee that it fits every household screw or that it is available from a particular retailer. If your projects involve security screws, Torx Plus, larger slotted blades or several PH and PZ sizes, look for those additions explicitly rather than assuming a 12-piece assortment includes them.
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Check access, length and shank before you start
Bit length
Length determines whether the tip can reach the screw while the driver clears the surrounding material. A short bit is convenient in open spaces; a longer bit may be necessary in a recessed cabinet, bracket or narrow channel. The Bosch example uses 25 mm bits, but that specification belongs to that product and should not be generalized to all bit sets.
Shank and holder
Many interchangeable bits use a 1/4-inch external hex shank. Bosch specifies the ISO 1173 C6.3 designation for the documented set. Confirm the marking on your bit and the capacity of your holder, ratchet adapter or drill chuck. A holder that accepts a 1/4-inch hex does not make an otherwise incompatible profile fit the screw.
Hand driver or power tool
For an unfamiliar screw, begin with a hand driver or a drill set to low speed and low torque. This gives you feedback if the bit slips. Keep the tool aligned with the screw axis and apply only enough forward pressure to maintain engagement. High speed can spin a poorly matched bit out of the recess before you recognize the error.
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A repeatable selection procedure
- Clean and illuminate the head. Remove dust, paint flakes and loose corrosion with a suitable brush or pick, taking care not to enlarge the recess.
- Identify the family. Decide whether the recess is slotted, PH, PZ, Torx, tamper-resistant Torx, Torx Plus or internal hex.
- Read available markings. Check the screw head, box, installation sheet or manufacturer instructions for a family and size code.
- Select the marked bit. Choose the same family and stated size; do not substitute a visually similar family.
- Seat it by hand. Insert the bit straight and verify that it reaches the recess without rocking.
- Check the working clearance. Confirm that the bit length reaches the screw and that the holder or driver can stay square.
- Drive slowly at first. Turn by hand or use low power until the screw starts cleanly. Stop immediately if the tip rises, slips or begins to damage the recess.
- Reassess rather than forcing. Try another size in the same family, then revisit the family identification if neither adjacent size fits securely.
Common failure modes and fixes
The bit keeps camming out of a cross recess
First check whether the screw is PZ rather than PH, or PH rather than PZ. Then test the neighboring size and inspect for paint or damage. Keep the tool square to the screw. If the recess is already rounded, a new matching bit may not restore reliable engagement; replacing the fastener is often safer than escalating force.
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The screw is likely tamper-resistant Torx. Use the matching security bit with a center hole and the correct T-size. Do not drill or pry the pin as a first response, because that changes the fastener and can damage the surrounding material.
The star-shaped bit seems close but still feels loose
Check whether the screw is Torx Plus rather than ordinary Torx. Confirm the number marking and try the specified family. A loose fit is not evidence that a larger ordinary Torx size is appropriate.
The bit enters but the driver cannot reach the screw
Change the access arrangement: use a longer bit, a suitable extension or a right-angle holder if the workspace permits. Verify that the replacement still has the correct shank and that the extension does not prevent the bit from seating fully.
The slot is wider than the available flat bit
Use a blade that fills the slot’s width and reaches its depth without contacting the head outside the slot. If the blade is too narrow, it can concentrate force on the slot edges; if it is too thick, it may not seat.
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When a single bit is better than a mixed set
Buy or select a single bit when the fastener specification is known and you need repeated work with one profile and size. This avoids searching through an assortment and ensures that the bit you carry is the one required. A mixed set makes more sense for maintenance across unknown hardware, provided it contains the families and sizes you actually encounter.
For a general household assortment, prioritize at least one clearly marked PH size, one PZ size, several common Torx sizes, slotted options and internal hex sizes. Treat that as a coverage checklist, not a universal standard: the right contents depend on the screws in your equipment and the access constraints of the job.
Safety and care while driving
- Wear eye protection when a damaged screw, bit or workpiece could shed fragments.
- Keep fingers away from a rotating bit and disconnect powered equipment before changing bits.
- Use the lowest practical speed and torque until engagement is proven.
- Replace bits with visibly rounded, chipped or twisted tips; a damaged tip makes fit assessment unreliable.
- Store bits so their markings remain readable and profiles do not become mixed together.
- Do not use a screwdriver bit as a punch, pry bar or improvised drill unless the bit is specifically designed for that purpose.
Quick decision checklist
- Can you name the recess family from its shape?
- Have you distinguished PH from PZ?
- Do the bit markings match the screw or its documentation?
- Does the tip seat deeply, stay square and resist rocking?
- Is the bit long enough for the location?
- Does its shank fit the holder or driver?
- Have you stopped at the first sign of cam-out instead of forcing the screw?
If all seven answers are yes, you have made a sound bit selection. If any answer is no, resolve that uncertainty before turning the screw.
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Frequently Asked Questions
Can I use a PH2 bit in a PZ2 screw?
They are different drive families. Use a PZ2 bit for a PZ2 recess and verify the fit by seating the bit before applying torque.
What does a security Torx bit look like?
It has the six-point Torx shape plus a hole through the center to clear the pin in a tamper-resistant Torx screw.
Is a 25 mm bit length suitable for every screw?
No. Twenty-five millimetres is the stated length of the documented Bosch example set. Choose a length that reaches the screw while keeping the driver aligned and clear of surrounding material.
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