Spec sheets say 124.9 cc, 155.1 cc, 156.9 cc - never the round number on the badge. The reasons are regulatory thresholds, engineering reality and marketing convention, and understanding them makes you harder to mislead.
What displacement is
Engine displacement is the swept volume of the cylinders - bore area times stroke times cylinder count. It is fixed geometry, measured to a decimal in engineering documents, and it never lands exactly on a round number because bore and stroke are chosen for packaging and performance, not for the badge.
Why the decimals sit just under thresholds
Manufacturers deliberately size engines just under regulatory class limits - a licence class, tax bracket or homologation category - so the bike qualifies for the friendlier rule. The verified Vietnamese examples (retrieved 23 September 2026): 124.8 cc for the Honda SH Mode 125 and 124.9 cc for the Yamaha Janus, FreeGo, Gear and Latte 125 - all just under the 125 cc boundary; 155.1 cc for the Yamaha NVX, NMAX and Lexi 155; and 156.9 cc for the Honda SH160i, Vario 160 and Air Blade 160.
Why this matters for licences
Vietnamese licence classes are set by engine capacity, and the boundary cases are exactly where those decimals matter - a 124.9 cc bike and a 125.0 cc bike can fall on different sides of a rule. Our licence classes guide covers what each licence lets you ride, and our A1 guide covers the class most 125s sit within. Never rely on the badge - the registration papers carry the true figure, and our documents guide covers what to carry.
And for buyers
Displacement correlates with, but does not determine, performance - a two-valve 124.9 and a four-valve 124.9 publish very different outputs (see our valve count and power explainers). Compare verified output figures, not capacity, when ranking models - our spec-sheet guide shows where each number lives.