SFIC core pinning for the A2 system

A small format interchangeable core has two shear lines in every chamber. The operating keys turn the plug at the lower one. The control key turns the plug and the control sleeve together at the upper one, ten steps higher, and that lets the core come out. You can't see the shear lines while you pin a core, so it's all arithmetic. This tool does the arithmetic for the A2 system, the one used by Best style cores and by Schlage, Arrow, Falcon and KSP cores keyed to A2.

The A2 rules

1. Bottom pin = the shallowest operating cut in that chamber.
2. Master pin = the deepest operating cut minus the shallowest. With a grand master you can get two master pins in one chamber.
3. Build-up pin = the control cut plus 10, minus the plug total (bottom plus master, which is the deepest operating cut).
4. Top pin = whatever brings the stack to 23.

That last step always lands on 13 minus the control cut, so a control cut of 5 always takes an 8B top pin. Bottom pins are A pins, .110" plus .0125" a step. Master, build-up and top pins are B pins at .0125" a step. Schlage works one chamber at a time, start to finish, and says beginners make more mistakes loading all the bottom pins first.

A worked example from the Schlage manual

Control 509427, grand master 361605, master 123605, change 123029, bittings read tip to bow. In chamber 1 the operating cuts are 3, 1 and 1. The shallowest is 1, so the bottom pin is 1A. The deepest minus the shallowest is 2, so the master pin is 2B. The control cut is 5, so the build-up is 5 + 10 minus 3, which is 12B. That's 15 so far, and an 8B top pin brings it to 23.

Check chamber 2 of the manual's example yourself. Its text says to add a 10 top pin, but its own stack of 2 + 4 + 4 is 10, so the top pin has to be 13 to make 23. The manual's drawing shows 13, and so does this tool.

What the warnings mean

• Operating cuts one step apart need a 1B pin. Schlage makes none, and Best uses it only in rare systems. A2 is a two-step system.
• A build-up of 15B, 17B or 19B isn't in Schlage's kit. Best A2 kits have them.
• If every control cut matches an operating cut, the control key would also work as an operating key. Check that against your system's rules.

A2 small format keyways have no MACS, so the tool doesn't check one.

Sources

Last checked October 2026.

• Schlage, Small format cores service manual: A2 depths 0 to 9, the four rules, total stack 23, pin lengths and the worked example.
• Allegion, What is the difference between A2 pinning and A4 pinning?: .0125" steps, two-step progression, cuts 0 to 9 and a 23 stack; and the MACS article: no MACS for small format keyways.
• Locksmith Ledger, Mastering IC Pin Stack Mathematics: the same A2 method step by step.
• Tyler J. Thomas, Key Bitting Specifications (2025): the Best A2 pin table, including the rare 1B.

One small difference: Schlage's table prints the 9A pin as .225", while .110" plus nine steps of .0125" is .2225", which is what the spec book prints. The pin numbers don't change. Check against your manufacturer's pinning chart before you cap a core.