Phillips bit size chart — numbers differ between KS and ISO
The same M3 can take a different recess number depending on the standard. KS vs ISO chart, PH vs PZ, why recesses strip and how to choose a bit.
"Use a #2 bit for M3" is true for our screws, but not for every M3 screw. The cross recess number is not set by screw size alone; it depends on the applicable standard and the head type. An M3 cross recessed machine screw made by JKF to the KS B 1023 annex takes No. 2, while an ISO 7045 pan head takes No. 1. Different standards can also mean different head diameters and heights.
Cross recess numbers by standard
Before choosing a bit, check which standard the screw was made to. Every product page on our site states the recess number.
| Size | JKF products (our specification) | ISO 7045 pan head |
|---|---|---|
| M2 | 1 | 0 |
| M2.5 | 1 | 1 |
| M2.6 | 1 | Not in the standard |
| M3 | 2 (truss M3: 1) | 1 |
| M3.5 · M4 · M5 | 2 | 2 |
| M6 | 3 | 3 |
| M8 | 3 | 4 |
| M10 | 4 (our countersunk products) | 4 |
Our values: in-house rules confirmed against actual parts and reference tables (Aug 2026). Our precision series (M1.4–M2) is No. 0 even at M2; hexagon heads with a combined cross recess are No. 2 for M4–M6 and No. 3 for M8 and M10. ISO values: ISO 7045:2011 pan head (type H and Z).
The numbers 0–4 are recess size numbers. No. 4 does not mean "for high torque"; it is simply the larger recess for larger screws. This table shows the difference between standards; ISO-standard or special-specification products may differ.
Before the number — PH or PZ
ISO 4757 specifies type H and type Z as the representative cross recess forms.
- Type H — Phillips recess, driven with a PH bit
- Type Z — Pozidriv recess, driven with a PZ bit
A PH2 and a PZ2 share a number but not a shape, so they do not seat properly in the other's recess (ISO 8764-1). Our standard cross recessed machine screws are type H, so use a PH bit. For precision and special-specification products, check the recess form and number of the individual product.
Why cross recesses get chewed up
A stripped recess can have several causes:
- A bit of the wrong size or form (PH vs PZ)
- A worn bit, or excessive tightening torque
- A tilted bit or too little end pressure
- Working conditions such as tool speed and impact
- The recess form and depth, or the material strength
A cross recess tends to push the bit out when torque is excessive (cam-out); with the wrong size or form the bit slips and cuts the recess even at normal torque. So the first thing to check is that the bit's form and size match the actual recess.
- Check the applicable standard and recess number on the drawing or product page
- Choose a bit with the same recess form (type H → PH, type Z → PZ) and number
- Before working, make sure the bit sits in the head without wobble, keep it upright and press firmly while tightening
We check cross recesses with a penetration gauge under each product's inspection criteria, controlling recess form and depth (how we inspect) — a bit can only perform when the recess is right.
FAQ
Which bit should I use for an M3 screw?
It depends on the standard. Our M3 screws (following the KS B 1023 annex) take No. 2 (truss M3: No. 1); an ISO 7045 pan head M3 takes No. 1. Check the recess number on the drawing or product page first; our product pages state it.
What is the difference between PH and PZ bits?
PH is for type H (Phillips) recesses and PZ for type Z (Pozidriv). Even with the same number the shapes differ and do not seat properly. Our standard cross recessed machine screws are type H, so use a PH bit (check precision and special products individually).
Why do my screw heads keep stripping?
There are several causes: a bit of the wrong size or form, a worn bit, excessive torque, a tilted bit or too little end pressure, and the recess form. First check that the bit's form and size match the recess, then keep the bit upright and press firmly.
What is a torsion bit?
A bit whose waist is designed to twist and absorb shock. With impact drivers it helps reduce recess damage, but it does not cure a mismatched bit.
References
- ISO 4757:1983 Cross recesses for screws — type H and Z cross recesses, sizes 0–4 and penetration gauge
- ISO 7045:2011 Pan head screws with type H or type Z cross recess — Product grade A — recess number by size (ISO column)
- ISO 8764-1:2004 Assembly tools for screws and nuts — Screwdrivers for cross-recessed head screws — Part 1: Driver tips
- KS B 1023 annex / JIS B 1111:2017 annex JA — recess number by size for the Korean/Japanese conventional range
Last revised 2026-10-12