Exchanging an L1A1 SLR Rear Sight

Swapping out the rear sight is a simple task requiring no special tools. The rifle will require re-zeroing afterwards of course, but this can be minimised with a little care.
Ensure that the rifle is unloaded and clear, and if possible clamp the rifle in a vice with soft jaws. The rifle should be horizontal and clamped by the right wall only of the magazine housing. As long as you have soft jaws on the vice so as not to mark the metal, the vice can be done up as tight as you like on this side wall without damaging the rifle.
Undo one of the rear sight zeroing screws and remove it from the rifle. Do not move the other screw! Depending on how tight the sight mounting dovetail is, the sight can now be slid out of its housing, or if it is tight, tapped out using a small hammer and suitable drift (brass is best). Tap as low as possible on the sight, immediately above the remaining zeroing screw is ideal. There will be a small 'S' shaped spring under the sight (which gives you the 'clicks' when you move the zeroing screws). It should stay in the sight when you remove it, but be aware it may drop out. The same applies to the ‘Stop Pin’ which protrudes through the range scale at the front right of the ramp. This is a push fit into the sight and is only held in place by the sight seating when the sight is fitted to the rifle.
Replacement is the reverse procedure, but note before;
a.    Ensure that the dovetail on the rifle is absolutely clean before fitting the new sight and a smear of fresh general purpose grease in the dovetail before fitting the replacement is always a good idea. Ensure that the 'S' spring is in position flat against the base of the new sight before fitting it. Care must be taken to ensure that the spring does not become dislodged when fitting the sight otherwise damage may result to the spring and/or the dovetail.
b.    The same goes for the 'stop pin'. If this keeps wanting to drop out use a dab of grease to hold it in place while the sight is fitted.
c.    The sight should be a sliding fit in the dovetail without excessive play for best accuracy. If the dovetail is excessively tight or loose it can be adjusted. If you are uncertain of how to do this then consult an experienced Armourer or engineer.
d.    Push the sight fully home against the remaining zeroing screw and refit the second zeroing screw or replacement screw. Do this screw up until it begins to click then do it up one click at a time and test the side movement of the sight each time using finger pressure. Once there is no appreciable side play that is not taken up by the 'S' spring STOP. The sight is designed to be held in equilibrium by the 'S' spring between the zeroing screws, not held solidly by the screws. Overtightening the screws will damage the sight and/or the screws and make it difficult to zero.
Once the sight is in place the second zeroing screw can be exchanged if required by removing it and carrying out the same procedure as with the first.
Re-zeroing should be a matter of adjusting by no more than a few clicks if you stick to this procedure. Remember when zeroing to undo the screw in the direction of adjustment first and then tighten the opposite screw by the same number of clicks. Each zeroing screw has 24 ‘clicks’ in a full turn. An adjustment of half a turn in either direction (12 clicks) will adjust the fall of shot by approximately 50mm at 100m, or 13mm at 25m.
Rear sights are adjusted out of the error, that is; if the rifle is firing off to the right (right error) the sights should be adjusted to the left to correct the fall of shot.