What size engine do I actually have?

The problem it solves

Owners routinely discover their machine is not the size they were told. Titles get issued wrong, stickers get swapped, speedometers read to speeds the machine cannot reach, and a seller's description is a seller's description. Every one of those is softer evidence than the number stamped into the crankcase.

There is a documented case of a machine titled as 150cc whose VIN lookup said 50cc and whose engine stamp proved it was a 50cc two-stroke — and the conclusion the forum reached was that the paperwork was simply wrong. That is not unusual enough to be a story.

It matters for more than pride. Displacement determines what licence and registration class the machine falls into in most places, what insurance is available, and which parts fit. Buying on the seller's word and finding out later is an expensive way to learn.

The good news is that this is settleable without dismantling anything. The bore digits in the crankcase code, cross-checked against a couple of physical observations, give an answer that no sticker can override.

The mistakes this sheet exists to prevent

• Believing the speedometer. A dial that reads to 80mph is a parts-bin dial, not a specification.
• Believing the sticker. Stickers are the easiest thing on the machine to change.
• Believing the title without checking it against the engine. Titles get issued from whatever the paperwork said.
• Assuming a bigger-looking machine has a bigger engine. Body size and engine size are unrelated on these.
• Reading the bore digits as the displacement. 39 means a 39mm bore, not 39cc.

How to use it

1. 01Start at the crankcase code. Everything else is a cross-check on it.
2. 02Note what each source of evidence claims, including the ones that disagree.
3. 03Record your confidence honestly. "Probably 150" is a more useful note than a wrong certainty.
4. 04If the title disagrees with the engine, sort that out before it becomes somebody else's problem.