JDM to USDM License Plate Adapter

This adapter lets you mount a U.S. license plate to vehicles that use the Japanese medium-size plate mounting pattern, commonly found on imported JDM passenger cars, like my 1989 Mazda Bongo, a vehicle that was designed to haul things down narrow farm roads in rural Hokkaido and is perfectly adequate for my needs, but which makes it borderline subversive in a country where the average Home Depot run apparently requires a vehicle with the frontal area of a townhouse and a hood so tall you could hide an entire kindergarten class in the blind spot.

Anyway I modeled this adapter to match both the USDM and JDM dimensions as accurately as possible, but I quickly learned that the so-called "standard" U.S. plate isn't nearly as standardized as you'd expect. All hardware is M6, because that's what the JDM mount uses and I wasn't about to start buying ¼" or M5 screws for a one-off bracket.

Hardware Used

What I Learned About U.S. and Japanese License Plate Standards (Or: How I Spent Three Hours Being Mad About Bolt Holes)

In Japan, license plate specifications are clearly defined and publicly accessible under the Road Transport Vehicle Law Enforcement Regulations Ordinance (Ministry of Transport Ordinance No. 74 of 1950). Article 11 / Form No. 1 lays out everything: plate dimensions, hole spacing, character sizes, and even the font geometry. Anyone can read it, for free, because it's published on Japan's official e-Gov portal. You know, like a standard.

In the United States, things are a bit more... entrepreneurial. Plate size and hole spacing aren't set by law, but by SAE J686, a technical document created in the 1950s by the Society of Automotive Engineers, a private nonprofit organization dominated by automakers (shocking). Despite being treated as a "national standard" for over 70 years, SAE J686 still isn't freely available. Getting access to the specs about where to drill holes in a metal rectangle would cost me $54. Obviously I'm not paying that and you can readily find this information online anyway, like here: https://www.lookupaplate.com/blog/license-plate-sizes/

The dimensions (12 × 6 inches, mounting holes 7 inches apart horizontally and 4.75 inches vertically) became a de facto standard only because automakers and states agreed to use them. Each U.S. state still controls its own plates, colors, and slogans ("Live Free or Die," but also, pay us to access basic dimensional data), but most follow SAE J686 for compatibility, assuming you've paid to know what J686 actually says.

What I thought would be a quick 15-minute "just make the holes line up" CAD session turned into an evening-long dive into the bureaucracy of bolt holes and the realization that I was more annoyed about intellectual property law than I was about my license plate holes not lining up.

So this model lives at the intersection of two philosophies:

• One where standards are open, transparent, and shared for the public good,
• And one where standards are locked behind a paywall, because apparently, Freedom Isn't Free™ extends to knowing how far apart to space mounting holes.

A small reminder that sometimes the hardest part of engineering isn't the math or the tolerances, it's the fact that someone, somewhere, decided that the distance between two holes should be classified as proprietary information worth fifty-four bucks.

Future Versions

I may add more versions for different international plate sizes in the future, assuming I can find the standards without paying a licensing fee to an industry consortium. If your country publishes its plate specs online like a normal place, let me know and I'll add it to the queue.

Version History

v1.0 (October 8th 2025)

• Initial release
• Supports standard U.S. license plates (12" × 6") to Japanese medium-size plate mounting pattern
• Includes mounting points for M6 hardware
• Tested on 1989 Mazda Bongo (and my patience)