When to Specify MMC Bonus Tolerance

MMC bonus tolerance is the extra position room you get when a hole opens up from its smallest allowed size. Put the circled M on a clearance hole pattern and assembly gets easier to prove. Put it on a bearing bore and you just gave away location.

When Should You Specify MMC on a Position Tolerance?

Use MMC when the job is clearance fit and interchangeability, such as bolts through covers or pins that must pass without binding. Engineering Technology walks through the geometry in The Bonus Tolerance: with MMC on, the position zone grows as the hole grows, because a larger hole still clears the mating pin. At the smallest hole (MMC) you get only the stated position tolerance. That growth is the bonus.

MMC Bonus Tolerance: When It Helps and When It Hurts

Helps on fastener patterns where function is "parts go together." GD&T Basics covers when to use MMC and MMB the same way: modifiers belong on features of size whose job is assembly. Functional gaging with a virtual-condition pin is then possible, which is why shops like MMC on clearance holes.

Hurts when location itself is the function. Press fits, bearing bores, dowels that set alignment, and sealing lands usually stay at RFS (no material modifier). Extra position error when the bore is large is not a gift there. Threaded holes are a separate trap: true MMC on threads is awkward to verify, so many drawings keep tap position tight without leaning on bonus. See tapped hole position tolerance.

If you are still arguing plus/minus coordinates versus true position, start with plus/minus vs position tolerance for hole patterns. Position first. Then decide whether that position callout carries MMC.

How Shops and Quotes Read It

A CMM can report position at actual size and still miss a functional fail. Why a part can pass positional tolerance on the CMM but still not assemble is the classic mismatch. MMC on the drawing lines the inspection method up with assembly. Our CNC work holds ISO 2768-m unless you call tighter features; MMC does not invent prices, but a functional gage or a tighter RFS bore does change inspection and programming time. Ask us when you quote if you want a hard-gage approach called out with the print.