How do I fit a hollow wall anchor properly?

A hollow wall anchor — often called a Molly bolt — is the cavity fixing that stays in place when you unscrew it, which makes it the right choice for anything you will take off and put back: a bracket, a cistern panel, a curtain track, a bathroom fitting. The diagram shows all four stages, and the sequence is the part people get wrong.

What you'll need

• Hollow wall anchors of the right board thickness
• A drill bit of exactly the stated diameter
• A cable and pipe detector
• A setting tool, or a screwdriver and pliers (optional)

Step by step

1. Choose the anchor for the board thickness.Anchors are made in grip ranges — the thickness of material they will clamp. A short-grip anchor in thick board will not set at all, and a long-grip anchor in thin board never pulls tight.
2. Check for a stud first.A screw into a timber stud beats any cavity fixing by a wide margin. Use an anchor because there genuinely is no stud there, not because it is quicker.
3. Drill exactly the diameter stated on the packet.This is the single most important step. A hole a fraction oversize lets the whole anchor rotate instead of setting, and no amount of turning will then fold the legs.
4. Push the anchor in so the collar sits flat.The flanged collar with small teeth under it must be hard against the face of the board. Tap it lightly with a hammer if needed. Those teeth are what stop it spinning.
5. Set the anchor.Hold the collar with a screwdriver or pliers if it wants to turn, and wind the screw in. You will feel resistance build as the legs fold. A setting tool grips the collar and does this in one squeeze, which is worth it if you are fitting several.
6. Stop as soon as it goes firm.The legs are now flat against the back of the board. Continuing to turn crushes the board around the collar and destroys most of the holding power — the anchor will feel tight and hold very little.
7. Remove the screw and fit the item.The anchor stays in the wall. Offer the fitting up, put the same screw through it and back into the anchor. This is the opposite of a toggle, where the fitting must be threaded on first.
8. Do not overtighten the final screw either.Firm, then stop. The clamping is done by the legs, not by the screw's last quarter turn.

Tips

• A setting tool costs very little and pays for itself the first time you fit more than three, because it sets them squarely without turning the collar.
• Once set, an anchor cannot come back out through its own hole. If you need to remove one, drive it into the cavity with a punch and fill the hole.
• In a dot-and-dab wall, check the void depth before buying. Some anchors need more space behind the board than a dab gap provides.

Common mistakes

• Drilling one size up because the anchor is tight — The teeth on the collar and the fit of the body are what stop the anchor spinning. An oversize hole means it turns with the screw and never sets.
• Setting the anchor with the bracket already on the screw — That is the toggle procedure. On a hollow wall anchor it stops the collar seating on the board, so the legs fold against the bracket instead of the wall.

If it doesn't work