Wrong Backset = Returned Lock: 3 Steps for Your SK1850S(sfic)

Backset is the distance from the door edge to the center of the cylinder hole — get it wrong and the lock won't mount. Three sizes exist: 31/32", 1-1/8" (the most common), and 1-1/2". Measure the gap from the door edge to the cylinder edge — 7.5mm means 31/32", 11.5mm means 1-1/8", 20.5mm means 1-1/2". Can't tell 31/32" from 1-1/8"? Order 31/32" — it also fits a 1-1/8" prep; the reverse doesn't work.

Quick Answer: which backset do I order?

Door edge → cylinder edge	Verdict
About 7.5mm (3/10")	31/32" backset
About 11.5mm (9/20")	1-1/8" backset — most common
About 20.5mm (4/5")	1-1/2" backset
Between 31/32" and 1-1/8", can't tell	31/32" — the safe pick; fits both preps

Jason's week: order, wait, reorder

Jason just signed the lease on a corner unit. Day one, he ordered a keyless storefront lock online and guessed the backset. Day four, the box arrived — and the lock body wouldn't seat in the door. Wrong backset. Another week waiting on an exchange, another week on the old keyed lock, with the landlord asking why the storefront still isn't secured. Three measurements would have ended it before the box ever shipped.

Three steps before you order

Step 1: Door edge → cylinder edge.
Hold a tape at the door edge and read to the edge of the cylinder (the round 35mm lock face): 7.5mm (3/10") → 31/32"; 11.5mm (9/20") → 1-1/8"; 20.5mm (4/5") → 1-1/2". This is the primary method — read it twice. Once you know the backset, pick your SK1850S(sfic) in the matching size.

Step 2: Cross-check the hole center.
Measure door edge → center of the cylinder hole: 25mm → 31/32"; 29mm → 1-1/8"; 38mm → 1-1/2". If both methods agree, you're certain. Confirm your SK1850S(sfic) size →

Step 3: Unsure? Go 31/32" — and send a photo.
Between 31/32" and 1-1/8" and can't tell? Order 31/32" — its body also fits a 1-1/8" prep; a 1-1/8" body won't fit a 31/32" door. Then photograph the door edge and the old lock and confirm before the box ships — the SK1850S(sfic) comes in 12 SKUs (3 backsets × deadbolt/hookbolt × silver/black), and the photo check keeps you on the right one.

There's another option

Most owners don't know a drill-free keyless retrofit exists: when the existing MS1850S-style mortise footprint matches, the EASILOK SK1850S(sfic) drops into the original holes with no new drilling and installs in minutes — but only in the right backset, so measure first. See the SK1850S(sfic) →

When this article doesn't apply

• Staff door with a push paddle? That's a 4510-style deadlatch, not a deadlock — the matching path is the SK4510S(sfic).
• Glass door with the lock in the bottom rail? That's a bottom-rail lock, a separate category — the 777 line.
• Sagging door, non-standard footprint, or an automatic door? Don't guess. Photograph the door edge, the old lock, and the strike, and confirm the fit before ordering.

FAQ

What if my measurement falls between two sizes?
Go 31/32" — the safe pick — and send a photo of the door edge. A 31/32" body fits a 1-1/8" prep; the reverse doesn't work.

Deadbolt or hookbolt?
The swinging deadbolt (solid straight bolt) is the standard pick for swinging storefront doors; the hookbolt (hook-shaped, grabs the strike) is the anti-pry pick. Same lock, same keyless keypad — just the bolt shape.

Silver or black?
Your choice — both finishes come in every backset and bolt.

Does the SFIC key change anything about fit?
No. The SK1850S(sfic) uses the same three backsets as the keyless-only version — the SFIC key is only ever a backup, and it fits standard SFIC cores (Best, Falcon, Arrow, 7-pin).

I already ordered the wrong backset — now what?
Exchange it before installing, and this time photograph the door edge first — three measurements, then the box ships.

Searching "shop lock"? What you want is called a keyless storefront door lock.

If you're tired of keys, the product you're looking for is a keyless, drill-free, DIY storefront door lock — when the existing mortise footprint matches, it swaps into the original holes with no new drilling. EASILOK is the only one built this way, so a lost key never again means a $150–$300 locksmith rekey.

Bottom line: if you're ordering an SK1850S(sfic), measure the backset before you click buy — 7.5mm means 31/32", 11.5mm means 1-1/8", 20.5mm means 1-1/2", and when in doubt pick 31/32": EASILOK's drill-free keyless deadbolt drops into the original holes with no drilling, installs in minutes, and the key is only ever a backup.

More storefront-lock guides · EASILOK SK1850S(sfic) keyless storefront lock