The Power of the Lock BACK SET
The Golden Measurement when fitting locks
When your choosing a lock, whether is be for a replacement or a new install its a nightmare sussing out all these different measurements.
So lets try to make some sense for you.
If youre fitting locks new for the first time then one measurement is quite important and thats the back-set!
This is the measurement of the lock from its face to where the key is going to go. This is probably the most important measurement as it will eventually determine where your handle, escutcheons etc will sit on the door.
To narrow and the key/escutcheons or handles may touch the door stop, to deep and the handle will look off centre.
Most door stops these days are roughly 10mm but you may find some a lot deeper. If you have a 10 mm door stop then you could get away with a 2.5 inch lock case where the back set would be 1 ¾ inches (metric 44mm). This means the centre line on the door where your handles are going to sit would be 1 ¾ inches from the leading edge.
The leading edge part of the door is called the door style and this piece of timber is normally 44mm thick for an external door and 38 mm for an internal door. The depth of this door style is normally around 3 inches.
One side of the door would have the door stop covering a bit of the door that would be normally 10mm. (1/4 inch). If you take this into consideration then the handle would sit more centre off the door stile.
Obviously the other side wont, but nether the less it would look centre.
In some cases the door may have a wider stile maybe it is 4 inches, perhaps a bit more, the idea is to get the handles/escutcheons sitting somewhere in the middle for it look good and look in proportion to the doors shape and size.
So on a 4 inch stile you may consider a three inch lock case that would then have a 21/4 inch Back Set, again sitting the handles pretty much in the centre of the door stile.
Another considering factor is the thickness of the door stop. As mentioned earlier, normally door stops are 10 mm or thereabouts, but if the door is a fire door, some old legislation required the door stop to be 1 inch to help with the fire integrity of the door and frame.
In these cases, fitting a 2 ½ inch lock will mean the door handle is going to get in the way of closing the door, Not ideal, so you may have to go for a deeper lock case just to get over this problem. Assuming the stile will allow for a deeper lock to be fitted.
When retro fitting locks with other door locks from different manufacturers the Back Sets are pretty universal (except for the Chubb 3K75 and 3G114 there always has to be one !) so measuring the back set will give you an idea of the size of lock you require, If, however, the lock you are replacing is very old, and I mean old imperial, then you will have no chance of matching a modern lock to the old imperial sizes, these can be out by as much as a ¼ but normally around 1/8 of an inch out.
Unless you are lucky and when your replacing a lock on a door you manage to replace like for like, the chances are other measurements are not going to be the same, the size of the face plate, the size of the lock body, will possibly be different. this we can all cope with as it does not really effect the aesthetics of the door, If the face plate is oversized, then great, a little chiselling and you are sorted, same with the lock case. but if the backset is not the same, now you have to change ironmongery locations on the face off the door, and we don't want to do that!
Finally one more important measurement is the P/Z measurement. That is the distance between the key hole (centre to where the key inserts see pic) and centre to where the follower fits.
The UK will see this measurement being universal as well, at 1 ¾ inches (give or take) and most handle manufactures have ensured there P/Z centres are the same, so you can choose a lock case and handles being confident that they are compatible.
But, be mindful, there is a lot of European door manufactures coming into the UK and new EU standards as well and you may see different measurements hitting the UK markets, these being namely 72 mm and 92 mm.
Hope this helps with your deliberations in choosing a handle and locks to suite your door and application
Head of the tech team
TRUST Security - Trust LocksOnline