How to check a motorbike frame and engine number

Start at the headstock.

Everything that matters about a bike's identity is stamped into metal in two places, and the whole job takes about four minutes if you know where to look. Work through the four steps below in order, before any money changes hands.

Step 1: Find the frame number

The frame number is a bike's VIN. It is a seventeen-character code on anything built since the early 1980s, stamped directly into the frame rather than printed on a sticker.

Look here first. The right-hand side of the steering head, the thick tube the forks pass through. On the large majority of bikes, that is where it is. You will often need to turn the bars to full lock and get your head down beside the front wheel to read it.

If it is not there, work through these. The left-hand side of the steering head. The frame spar running back from the headstock. On some models, a section of frame under the tank, which may mean lifting or removing the seat. Cruisers occasionally carry it lower on the frame near the downtube.

There may also be a plate. Many modern bikes carry a riveted or adhesive VIN plate in addition to the stamping, commonly under the seat, inside the frame near the headstock, or on the frame rail. Treat the plate as a convenience and the stamping as the truth. A plate can be swapped in an afternoon; a stamping cannot.

Take a photograph. Well lit, straight on, close enough to read every character. Phone torches are better than daylight here because the shadow shows the depth of the stamping.

Step 2: Find the engine number

This is the one people skip, and it is the one that catches ringing.

A bike carries two identifiers, not one. The engine has its own number, stamped into the crankcase, and it is recorded on the V5C alongside the frame number. On a legitimate bike, both match the paperwork. On a bike assembled from a stolen machine and a dead identity, they frequently do not.

Look on the crankcase, usually near the seam where the two halves join. Depending on the model it can be on either side, sometimes behind the exhaust header, sometimes just above the sump, occasionally partly obscured by a bracket. Expect to need a torch and, on a faired bike, a certain amount of patience.

An engine number that does not match the V5C is not automatically sinister. Engines do get replaced, particularly on older and higher-mileage machines, and a legitimate rebuild with a different engine is a normal event. What matters is whether the seller knows about it and can account for it. A shrug is a problem. A receipt is not.

Step 3: Check both against the V5C

Put the logbook next to the bike and compare character by character. Not at a glance, character by character.

Confirm the frame number matches, the engine number matches, and the colour and model description are consistent with what is in front of you. Then look at the document itself. Check the issue date makes sense against the ownership story you have been told, and be wary of a logbook that has been issued very recently on a bike that has not just changed hands.

If the seller cannot produce the V5C at all, stop. There are legitimate reasons for a missing logbook, and every one of them can be resolved before a sale rather than after it.

Step 4: Run the check

With the numbers confirmed against the paperwork, the remaining question is whether the identity itself is clean.

Entering the registration returns the DVLA-held identity for that bike. Entering the frame number as well lets you confirm that the registration and the machine belong together, which is the specific thing cloning is designed to hide.

What the seventeen characters mean

A frame number is not a random string, and knowing roughly how it is built lets you spot one that has been invented rather than stamped at a factory.

Since the early 1980s, VINs on road vehicles follow an international standard of seventeen characters, split into three parts.

Characters 1 to 3. The manufacturer identifier. These tell you who built the machine and where. The same three characters appear at the start of every frame number from that plant.

Characters 4 to 9. The descriptor section, encoding model, engine and body details in a format the manufacturer defines for itself.

Characters 10 to 17. The identifier section, which makes this particular machine unique. Position ten commonly indicates the model year and position eleven the assembly plant, with the remainder acting as the serial number.

The detail worth remembering. The letters I, O and Q are not used in VINs anywhere in the sequence, because they are too easily confused with the digits 1 and 0. If you are reading a frame number and you are certain you can see one of those three letters, either you are misreading a 1 or a 0, or the number is not a genuine factory VIN. It takes two seconds to check and it catches fabricated numbers.

Bikes built before the standard came in will have shorter frame numbers in the manufacturer's own format, which is normal on anything from the seventies or earlier.

What altered stamping looks like

Worth knowing in detail, because this is where a cloned or rung bike gives itself away to somebody paying attention.

Surface that does not match its surroundings. Original stamping sits in metal that has aged the same way as everything around it. A patch that is smoother, flatter or a slightly different colour has usually been worked on. Run a fingertip across it. Filed metal feels different to cast or forged metal that has simply been cleaned.

Fonts and spacing. Factory stamping is done in one operation with consistent character shapes, consistent depth and even spacing. Hand-struck characters wander. Look for digits that sit slightly high or low against their neighbours, spacing that opens up or tightens mid-sequence, or a character shape that does not match the same character elsewhere in the code.

Depth. Re-struck numbers are frequently shallower than original ones, because whoever did it was working on metal that had already been ground back. Angle a torch across the surface and the difference shows.

Paint and lacquer. Fresh finish over the stamping area alone, when the rest of the frame carries its age, is worth a hard look. So is lacquer that has pooled in the character strokes.

The obvious ones. Characters that have been altered rather than replaced. A 3 made from an 8, a 1 from a 7, a 5 from a 6. Compare the suspect character against the same character elsewhere in the sequence.

On plated bikes. Rivets that are brighter than the plate, rivets of a type the manufacturer did not use, a plate sitting slightly proud, or adhesive residue at the edges.

If anything here applies, do not buy the bike and do not tell the seller why. Walk away, then report what you saw to the police on 101.

Imports and bikes with no plate

Grey imports. A bike brought in privately, particularly from Japan, may carry a frame number in a format that looks unfamiliar and will have no UK history before the date it was registered here. That is expected rather than suspicious. What you want to see is a coherent import trail and paperwork that accounts for the gap.

No registration fitted. Project bikes, barn finds and machines bought as non-runners often turn up without a plate. The frame number is then the only identity available, and checking it becomes the whole job.

Age-related plates. A bike wearing a Q plate has an identity the DVLA could not establish with confidence, usually because it was built from parts or its provenance could not be evidenced. Not automatically a problem on a genuine special or rebuild, but it needs explaining properly and it affects value.

Confirming the numbers establishes what the machine is. A full check then tells you what has happened to it: outstanding finance, theft markers, write-off category, keeper history and the mileage trail.

FAQs

Where exactly is the frame number on my bike?

The right side of the steering head on most machines. If it is not there, check the left side, then the frame spar behind it, then under the seat. Your owner's manual will name the location for your specific model.

The engine number does not match the V5C. Is the bike stolen?

Not necessarily. Replacement engines are common on older bikes and entirely legitimate when documented. Ask the seller for the history and the receipts. A mismatch nobody can explain is a different matter.

Can I check a bike using only the frame number?

Yes. That is the right approach for an import, a bike with no plate fitted, or any situation where you are not confident the registration and the machine belong to each other.