A window lock installs by positioning the hardware so the bolt engages fully, marking and drilling the manufacturer’s specified pilot holes, and testing the sash closed and open.
Most lock failures trace back to one mistake made before drilling: buying hardware that doesn’t match the window. Replacement locks must match the manufacturer, series, sash configuration, and the geometry of the existing lock — otherwise the bolt path lands nowhere near the receiver.
Where the Lock Goes Depends on Your Window Type
Casement, awning, and sliding windows each put the lock in a different spot, per Yale’s Presslock instructions.
On a casement window, mount the lock in the middle of the sash. On an awning window, mount it at the bottom. On a sliding window, mount it at the end of either sash. With two sliding sashes, the upper portion of the lower sash can lock into the upper sash; when the upper window is fixed, the lock goes on the lower sliding sash.
Yale’s Pushlock instructions add clearance rules: the bolt must not contact the frame while unlocked, and the lock body needs at least 10 mm between it and the frame so the sash travels freely.
Step-by-Step: Drilling and Fastening
Mark fixing points with the window tightly closed, then drill pilot holes sized to your lock and frame material. Confirm screws clear the glass line before insertion.
- Close the window firmly and hold the lock in position.
- Check the lock body sits at least 3 mm from the sill or frame (Presslock minimum) and the button or bolt clears other hardware.
- Mark the fixing holes. A center punch keeps the drill from wandering.
- Drill two 3 mm pilot holes in the sash for a Presslock. For a Pushlock, drill two 4 mm pilot holes for the lock body.
- Fasten with timber fixing screws suited to the frame material.
The receiver side tightens tolerances. With a Pushlock, close the window firmly, use the extended bolt to mark the drilling point, then drill a 13 mm diameter hole deep enough for full bolt travel. Without a ferrule, that hole drops to 10 mm. Ferrule fixing uses two 2.5 mm pilot holes. The bolt must engage completely before the key turns.
Andersen’s concealed lock on 400 Series and A-Series casement and awning windows reverses the logic: remove the lock handle, lock bezel, operator handle, and operator cover, keeping screws; score the paint or stain seal between interior stops and frame; loosen and pry off the interior sash stops; pull remaining nails from the back of the stops to keep visible faces clean; then remove the three mounting screws from the lower sash lock and two from the upper lock where one exists. Replacement goes in with original or supplied screws; stops return with 4d finishing nails; nail holes get wood filler. Reinstall the operator cover, operator handle, lock bezel, and handle last. Andersen’s concealed lock installation guide lists this order because stops must come off before the lock clears the sash.
French casement hardware uses two stages: a 90-degree turn locks the bottom and center; a full 180-degree rotation locks the top. Check mushroom-cam operation and hardware connections before finishing.
Before committing to a brand, measure your sash and confirm receiver geometry. Our tested roundup of window locks covers which models fit which sash types.
Clearance, Safety, and Pre-Drill Checks
Verify position, bolt path, glass clearance, and frame condition before drilling — never install into decayed, corroded, or unsound material.
- Safety glasses on. Required for drilling, prying, or pulling nails.
- Screw-to-glass clearance. Check screws won’t contact the glass line.
- Full bolt engagement. A partially seated bolt is a lock that looks installed and isn’t.
- Test both states. Confirm the lock operates closed, then open and confirm the bolt stays clear through full sash movement.
- A packer helps on casement and awning windows needing extra clearance or strength.
A ventilation limiter is not a security lock.
| Lock | Pilot Holes | Key Clearance |
|---|---|---|
| Yale Presslock | Two 3 mm in sash | 3 mm from sill/frame |
| Yale Pushlock body | Two 4 mm | 10 mm from frame |
| Pushlock receiver | 13 mm hole (10 mm without ferrule) | Full bolt travel |
| Pushlock ferrule | Two 2.5 mm | Bolt clears when unlocked |
| Andersen concealed lock | Original mounting screws | Stops removed first |
Common Questions
Can I install a window lock on vinyl frames?
Yes, if the lock is made for vinyl and screws won’t crack the frame. Drive pilot holes sized to the screw and frame material, keep screws clear of the glass line, and never fasten into a hollow or unsound section. Confirm the bolt reaches the receiver before drilling.
Why won’t my new lock’s key turn?
The bolt isn’t fully engaged. The key only locks once the bolt seats completely in its receiver, so check alignment between lock and receiver hole first. Drill the receiver deeper if the bolt bottoms out, and confirm the sash closes fully without the bolt contacting the frame while unlocked.
How do I know which lock fits my window?
Match the original hardware’s manufacturer, series, and sash configuration before buying. Measure the existing lock’s mounting hole spacing and bolt geometry. On Andersen 400 Series and A-Series casement and awning windows, replacement locks are model-specific. When in doubt, match the old lock rather than adapting a universal one.
Sources
- Andersen Windows. “Concealed Lock Installation for Casement and Awning Windows.” Source for the 400 Series and A-Series replacement procedure and required supplies.
- Yale Home. “Presslock Installation Instructions.” Source for window-type positions, the 3 mm gap, and 3 mm pilot holes.
- Andersen Windows. “Service Guide — Complementary French Casement, Clad.” Source for the two-stage 90-degree and 180-degree locking sequence.