Cam-out

Plain-English definition

Cam-out is what happens when a screwdriver or driver bit rides up and slips out of the screw’s recess while you are turning it. You hear a sudden rasp or click, the bit skates across the head, and the recess gets rounded or stripped. The name comes from the way the bit acts like a cam: under torque, the tapered faces in the recess push the bit outward instead of keeping it seated.

Most DIY cam-out trouble involves cross-head screws. The photo above shows the two families you are most likely to mix up, arranged left to right:

• Phillips (PH) — the screw on the far left has a simple cross with slightly rounded corners and no extra marks on the face. The PH bit next to it has four main flutes that taper to a fairly sharp point. Those tapered sides are the main reason PH is prone to cam-out when torque gets high: the bit is gradually forced out of the recess.
• Pozidriv (PZ) — the screw on the far right looks similar at a glance, but look closely and you will see four small radial tick marks (extra indentations) between the arms of the cross. The PZ bit has four additional thin ribs set at 45° between the main flutes, and the main flutes are straighter and less pointed than on a PH bit. Those extra contact points help the bit stay engaged and resist cam-out when bit and screw are both Pozidriv.

Bits are usually stamped PH or PZ, often with a size number (PH2 and PZ2 are common on general fixings). The recess shape and the bit shape are a matched pair — not interchangeable “cross-head” tools.

Why it matters

A cammed-out screw is annoying at best and costly at worst: you may not be able to drive it flush, remove it later, or finish a job without drilling out or using an extractor. On painted or plastic surfaces, a slipping bit can scratch or chip the workpiece. On a ladder or in a tight cupboard, losing engagement once often means losing patience and applying even more force — which strips the head further.

Phillips and Pozidriv behave differently:

	Phillips (PH)	Pozidriv (PZ)
Typical cam-out risk	Higher when torque is high or the fit is poor	Lower when a correct PZ bit is used
Recess shape	Simple cross; tapered internal walls	Cross plus four tick marks; straighter walls
Bit shape	Four tapered flutes, pointed tip	Four main flutes plus four smaller ribs at 45°
Common on	Many consumer packs, some appliances, older hardware	Many trade screws and wall plugs in the UK/Europe

PH was not invented to ruin your Sunday — early patents aimed for secure engagement — but the tapered geometry still makes cam-out common in DIY, especially with cordless drivers on the wrong speed or a worn bit. PZ was developed to allow more torque without the bit walking out, which is why Pozidriv screws often appear on chipboard, wall plugs, and flat-pack fixings where installers need reliable driving.

Compatibility (this is where most damage happens):

• PH bit in a PZ screw — the bit can sit in the recess and feel as if it fits, but it only contacts the main cross, not the PZ ribs. Engagement is loose; the bit cam-outs easily and rounds the head. This is one of the most common causes of “stripped” screws on site.
• PZ bit in a PH screw — the extra ribs do not seat properly in the shallow Phillips recess. The bit sits too high, contact is poor, and you are likely to damage the screw and the bit while still camming out. Do not assume “cross is cross.”
• Correct match — PH with PH, PZ with PZ, in the right size (PH1/PH2/PH3 and PZ1/PZ2/PZ3 are not interchangeable even within the same system).

Practical ways to reduce cam-out:

• Identify the head (PZ tick marks vs plain PH cross) before you pick a bit.
• Use a fresh, correctly sized bit; rounded or chrome-coated bargain bits slip sooner.
• On a power driver, use a lower speed for final seating and steady downward pressure — let the tool do the turning, not your whole body weight.
• In hardwood or near board edges, drill a pilot hole so the screw does not need excessive torque to pull itself in.
• If the bit starts to slip, stop immediately — another half-turn usually finishes off the recess.

Torx and other drive types have their own rules, but for everyday screws the lesson is the same: match the system, read the markings on the bit, and treat cam-out as a warning sign, not something to power through.

Common mistake

The classic error is treating every cross-head screw as Phillips and reaching for the nearest PH2 bit. On a Pozidriv screw (tick marks visible), that mismatch invites cam-out even if the bit seems to fit. The opposite mistake — forcing a PZ bit into a PH screw because “it’s a more positive fit” — is worse: the ribs prevent full seating, the contact patch is wrong, and you can wreck a new screw in seconds.

Other frequent slip-ups: using a worn or wrong-size bit (a PH2 in a PH3 head has almost no side contact), driving at high speed with no pilot hole in hard wood, and continuing after the first slip instead of changing bit, technique, or screw. If the head is already damaged, stop and switch to a manual screwdriver for control, a rubber band or valve-grinding paste trick for removal, or proper extraction — not more trigger time on a stripped recess.