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Other than being a custom frame or it was stolen is there a explanation for the frame to be missing the serial numbers?

For instance do they use the same identification marks over seas?

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    The "overseas" remark - can you expand on that please? I'm not understanding your question there. – Criggie Jun 3 at 6:24
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    If it was a one-off hand built frame, the builder may not have felt the need for a serial number – Chris H Jun 3 at 6:24
  • I'm guessing the frame was an experimental model produced by a regional manufacturer. They might have produced ten of them for testing, ran a couple in local races, etc, then decided to ditch the design. – Daniel R Hicks Jun 3 at 12:35
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Even if you find the serial number, its absolutely no use in identifying the bike without a brand/manufacturer. There is no global index like auto VINs.

Serial numbers are used for several different purposes.

  • Identifying a frame through production, for logistical and tracking reasons
  • Identifying a completed bicycle through a sales process
  • Uniquely identifying a bike for warranty purposes
  • Identifying a bike for insurance purposes.

At no point is a bicycle frame serial number used to identify the bike's model or age or manufacturer. A frame's serial number is not related to a car's Vehicle Identification Number (or VIN.)

For a frame to lack a serial number could be any or

  1. The frame was hand made by a small-run frame builder, or home made. Most frame builders would use a serial number of some format though, even though their monthly output could be single digits.
  2. The frame has been repaired and the serial number was lost by replacement.
  3. The frame has been stolen and resold, with the serial number ground off or filled-and-painted-over.
  4. The frame was legitimately repainted and the serial number is obscured or hidden (my bike's like this - the S/N is visible but illegible.
  5. The serial number is covered by an accessory (very unlikely)

But most likely, you simply haven't found the serial number. Common places for serial numbers:

  1. Under the bottom bracket
  2. On top of the bottom bracket housing
  3. Up the side/front/rear of the seat tube
  4. Up the side/front/rear of the down tube
  5. Up the side/front/rear of the head tube
  6. Inside/outside the chainstays
  7. Inside/outside the seatstays
  8. Inside/outside the rear dropouts.

Less common areas to find a serial number

  1. INSIDE the bottom bracket housing
  2. Down inside the seat tube.

Places you won't find the serial numbner

  1. On any mechanical part - they may have their own serial number, but it won't be a frame serial number
  2. On the fork - again it may have a fork S/N or replicate the frame's SN or it may not have a serial at all
  3. Inside any closed tube - the headtube, BB and seat tube are the only tubes in the frame with some level of visibility, so it won't be anywhere else.

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