Backset and Centre Distance: How to Read a Mortise Lock Case Number

A distributor sends a purchase order for four hundred mortise lock cases. The model number is right, the finish is right, the price is agreed. Eight weeks later the cases arrive on site, the installer offers one up to a prepared door, and the handle spindle sits fifteen millimetres from where the hole is. Nothing is defective. The cases were simply made for a different door preparation, and the only clue was two digits in the model number that nobody read.

This happens often enough that it is worth spelling out exactly what those digits mean, because once you can read them you can specify a lock case from its name alone.

Centre distance is the gap between the centre of the cylinder hole and the centre of the handle spindle, measured along the face of the lock. Backset is the distance from the front edge of the faceplate back to the centre of that spindle. Centre distance has to match the handle plate or escutcheon you are fitting. Backset has to match how far in from the door edge the door was bored. They are independent of each other, and a lock case is only usable when both agree with the door.

Across the euro-profile range those two figures are the model number. LC08 85×55 is an 85 mm centre distance with a 55 mm backset. LC21 85×50 is the same 85 mm centre distance on a 50 mm backset. LC8520 compresses the same information without the separator: 85 mm centre distance, 20 mm backset. Once you know the convention, a packing list becomes readable at a glance.

Eighty-five millimetres is the figure you will meet most often. It is the standard centre distance for euro-profile cylinder locks, and it is why a European handle plate drops onto a case from almost any manufacturer without drilling. Seventy-two millimetres is the other one worth knowing — it is common through Latin American markets, and it is why LC34 50×72 puts the backset first: at 72 mm centre distance the numbers read in the reverse order, so the model name follows the market's own convention rather than ours.

Backset is where the real variation lives, because backset follows the door rather than the standard. Narrow aluminium stiles cannot take a deep case, so 16 mm, 17 mm and 20 mm backsets exist for them — that is what LC8520 at 20 mm and the 16 mm sliding-hook cases are for. Timber doors take more, and 45 mm, 50 mm, 55 mm and 60 mm are the usual span. A 70 mm backset like the LC7065 puts the handle a long way in from the edge, which is what a wide entrance door with a heavy rail needs.

Two suffixes are worth recognising while you are reading the number. PS marks a passage case: latch only, no cylinder, for a door that closes but does not lock. And where the model carries a bolt count — LC04 85×60 and LC14 85×50 both run four round bolts alongside the latch — you are looking at a multi-point case meant for a security or entrance door rather than an interior one.

The practical sequence when you specify is short. Measure the door first, not the lock: how far in is the spindle hole bored, and what centre distance does the handle plate you intend to use require. Then read those two figures out of the candidate model numbers. Then confirm handing and faceplate dimensions, which the model number does not encode. If you are replacing existing hardware rather than fitting new doors, take the old case out and measure it — a door that was prepared once has already made both decisions for you, and matching it is cheaper than re-boring four hundred doors.

Where a dimension is not printed on our product pages, it is because we have not verified it for that model rather than because it does not matter. Ask for the drawing before you order and we will send the one that covers the model on your schedule, or say plainly that it is still being prepared.