MACS (Maximum Adjacent Cut Specification)

MACS — Maximum Adjacent Cut Specification — is a per-profile constraint that limits how much the cut depth can change from one position to the next on a key. If position 3 is at depth 2 and the MACS is 4, then position 4 can be cut to depths 0–6 only; depths 7, 8 or 9 would violate MACS even though they’re individually valid depths.

If you’ve ever cut a key on a code machine and then had it refuse to insert into the cylinder, MACS is probably the reason. Every legal bitting respects MACS; bittings that violate it produce keys that physically don’t work, no matter how clean the cuts are.

The geometry

A pin-tumbler key is a metal blade with cuts of varying depth along the top edge. Each cut sits in a chamber of the cylinder when the key is fully inserted, and lifts the bottom pin in that chamber to a height proportional to the cut depth. A “depth 4” cut lifts the bottom pin to position 4; a “depth 7” cut lifts it to position 7.

The depths along the key are physically connected — they’re the same piece of metal. Between two cuts of different depths there’s a transition: a sloped face that runs from one cut depth to the next.

If position 3 is cut to depth 2, and position 4 is cut to depth 8, the transition between them is a 6-unit drop over the width of one position (typically 4–6 mm). That’s a near-vertical face. As the key slides into the cylinder, the bottom pin in chamber 4 — sitting at depth 8 — has to ride down the sloped face from chamber 3’s depth 2.

If the slope is too steep, the pin doesn’t ride smoothly. It catches on the vertical face. The key won’t insert, won’t withdraw cleanly, and over time the pin and the key both wear at the catch point. MACS is the manufacturer’s “this slope is the steepest we tolerate” specification.

Per-profile MACS values

Profile	Pins	Increment	MACS
Lockwood 570	6	0.025″	4
Lockwood 530	6	0.025″	4
Schlage C	6	0.0125″	7
Schlage Primus	6	0.0125″	5
BEST A2 SFIC	7	0.0125″	6
EVVA EPS	6	0.025″	4
ABLOY Classic	9 (discs)	n/a	n/a

Tight MACS (3 or 4) means most depth pairs are illegal — the system has fewer valid TMK candidates and the optimiser has to work harder. Loose MACS (6 or 7) means almost any depth-pair is allowed; allocation is easier but the resulting keys may be more error-prone in production. ABLOY Classic doesn’t have MACS in the same sense — it’s a disc-detainer, the math is rotational rather than depth-based.

Why MACS matters in master keying

If you’re cutting a single change key, MACS is a constraint you check once. If you’re designing a master key system, MACS cascades through every key. The TMK has to respect MACS. Every master key has to respect MACS. Every change key has to respect MACS. And — critically — when you put a master pin in a chamber, both the change-key cut depth and the master-key cut depth have to respect MACS in the chamber on each side.

That last constraint is the one that makes MACS painful in deep hierarchies. Consider:

• TMK has cuts 4-2-6-3-5-1. MACS = 4. ✓ valid (every adjacent diff ≤ 4)
• Change key A has cuts 4-2-6-3-5-1 with master pins giving an alternate cut at chamber 4 of 7. So change key A could be 4-2-6-7-5-1. Adjacent diffs: 2, 4, 1, 2, 4. Max diff is 4. ✓ valid.
• Change key B with master pin at chamber 2 giving alternate cut 9. So 4-9-6-3-5-1. Adjacent diffs: 5, 3, 3, 2, 4. Max diff is 5. ✗ MACS violation.

Change key B isn’t legal even though both depths (9 at chamber 2 and 4 at chamber 1) are individually valid. The combination violates MACS. The master pin at chamber 2 has to be sized differently to keep the bittings legal.

Multiply this constraint across 6 chambers and (say) 30 change keys per master, two masters, and a TMK on top, and you can see why TMK allocation isn’t a casual exercise.

Where most spreadsheets fail

A spreadsheet can validate MACS on the TMK. Some can validate MACS on individual change keys. Almost none validate MACS in the combinatorial sense — across every alternate depth produced by every master pin in every chamber. The bitting in cell C7 looks legal; the bitting in cell C8 looks legal; but the cylinder pinning that produces both legally accepts a third bitting that violates MACS, and that third bitting is the one a code machine will happily cut for you.

Software that walks the access matrix catches this. The MACS check has to run against every cylinder’s full set of valid splits, not just the listed change keys.

Common MACS mistakes

A few patterns that come up regularly:

• Treating MACS as advisory. Code machines will produce a violation; cylinders will accept the key on the bench (the bottom pin isn’t load-bearing during insertion); the key fails three months in when the cylinder body wears.
• Ignoring MACS on master pins. Master pin sizing doesn’t directly violate MACS, but the bitting it implies for the master key absolutely can. Validate the master key’s bitting, not just the pin.
• Tightening MACS by hand. Some shops use a self-imposed tighter MACS than factory (“we use MACS 3 even on Schlage C”) to leave production margin. Reasonable, but only if it’s documented in the system spec — otherwise the next locksmith re-keying the system uses factory MACS and produces keys that violate the local convention.

Why MACS-validation matters at design time

A key that violates MACS will be cut by an automatic code machine without complaint, since most code machines don’t check MACS — they only know depths and increments. The key will either fail at the bench (tested into a cylinder, won’t go) or fail in production (works for a week, then jams). MACS validation has to happen at the design stage, not the cutting stage.

Related

→ Bitting — the depth string that MACS constrains → TMK — the master key whose MACS choice cascades to every other key → Thin pin — what happens when MACS forces problematic master pins