How to fit a mortise lock

1st August 2015 - 0 Comments

How to fit a mortise lock

Tools required

  1. Power drill
  2. 22mm flat wood drill bit
  3. 19mm flat wood drill bit
  4. 25mm flat wood drill bit
  5. 3mm hss drill bit
  6. 1.5mm hss drill bit
  7. 10mm flat wood drill bit
  8. Pencil
  9. Ruler
  10. Stanely knife
  11. Screw driver pozi and flat head
  12. Hammer
  13. Tape
  14. Set sharp wood chisels

The instructions are how to fit a BS3621 motise lock. The width of the lock is just shy of 19mm due to the hardened steel plates mounted on either face of the lock.

  1. With a pencil mark a center line down the center of the door and using the ruler draw a line.
  2. Get the mortise lock and position it against the door. With the pencil mark were the bottom and top of the lock is in relation to the center line.


  1. Using the tape mark the depth of the lock on the 19mm drill bit taking into account the faceplate of the lock.
  2. With the power drill insert the 19mm drill bit and drill along the center line to within the top and bottom marks of were the lock has been marked on the door. 
  3. Trim the access using the wood chisels and insert the mortise to judge correct fit.
  4. Once the body of the lock has been trimmed insert the lock again and mark on the door the top and bottom of the faceplate.
  5. Attach the faceplate to the lock and measure the distance. Should be around 3 or 4mm and mark the 25mm drill bit with the tape.
  6. Using the 25mm wood drill bit drill alond the center line to the required depth and markes top and bottom.
  7. Trim of the access with the wood chisels.
  8. Next place the body of the lock including faceplate against the side of the door and using the 3mm drill bit drill through the keyhole to mark the keyhole position.
  9. Insert the 10mm drill bit and drill a hole.
  10. Insert the lock into the mortised area and check that the keyhole marries up to the hole just drilled.
  11. Using 10mm drill bit and wood chisels finish off the key hole.
  12. Carry out steps 10 to 13 on the other side of the door.
  13. nsert lock in the door attach faceplate and secure in position with the securing screws.
  14. Next place the escutcheon or key hole cover onto the keyhole and insert one of the keys.
  15. Check operation of the key in the lock.
  16. Using the 1.5mm drill bit pilot drill the screws for the escutcheon then secure in position with the flat head/ pozi screwdriver.
  17. Do the same with the other side.
  18. Close the door turn the key so that the lock locks and mark the frame were the bolt is on the frame. You now have the hieght and depth of the bolt.
  19. Next measure the distance from the edge of the door to the center of the bolt. 
  20. From the edge of the frame mark in the measurement taken in step 21. This gives the position were the bolt enters the keep.
  21. Draw a couple of other marks and draw a center line.
  22. Get the keep and place on the frame. The box section of the keep should be placed so the top and bottom of the bolt has equal amount of space. Mark the center line were this happens.
  23. Place the keep on the frame and draw an outline around it.
  24. Using the stanley knife score along the outline and tap in the chisel.
  25. Using the 25mm drill bit drill along the center line ti within 3mm to remove the access and trim with the wood chisel.
  26. Inset the keep into the rebated section and screw in place with the pozi/flat screw driver.
  27. Test the operation of the lock.

The lock should work smoothly. It should not touch the sides of the keep.

Time to fit the lock is about an hour.

Information provided by Steve at A.L.S. Locksmiths in Glasgow.


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