Japanese Diamond Back BMX Frames

Koizumi is a Japanese bike frame manufacturer. Diamond Back used Koizumi to manufacture their early frames. These are some of the most classic, iconic – and therefore desirable – Diamond Back frames.

Around 1983, due to costs, Diamond Back began moving their production to Taiwan and the Fairly bike company. Higher-end frames such as the Harry Leary Turbo and Formula 1 continued to be produced in Japan by Koizumi until 1984.

Koizumi frame numbers appear on the inside of the rear, non-drive side dropout.

Please note that Koizumi made frames for several other BMX brands in this period, including Kuwuhara and Raleigh, so if you’re using our frame identifier tool, ensure your frame has a Diamond in the gusset to make sure it is a Diamond Back.

Decoding Diamond Back Koizumi frame serial numbers

They have the following format which is made up of 6 digits, followed by what looks like a baseball symbol. In Japanese, this is known as a Kanji. (https://en.wikipedia.org/wiki/Kanji) like so:

1X2345⚾︎

The Koizumi serial number does not describe which model of bike the frame is, rather, it describes the year and month of manufacture and the unique number for the frame produced and can be broken down as follows:

1. The first digit, which is a number represents the year. This is as follows:
   1. 9 = 1979
   2. 0 = 1980
   3. 1 = 1981
   4. 2 = 1982
   5. 3 = 1983
   6. 4 = 1984
2. The second digit, which is a letter from the alphabet, represents the month of manufactuer as follows;
   1. A = January
   2. B = February
   3. C = March
   4. D = April
   5. E = May
   6. F = June
   7. G = July
   8. H = August
   9. X = September ( The letter ‘I’ was replaced with an ‘X’ so as not to be confused with a ‘1’ (One) )
   10. J = October
   11. K = November
   12. L = December
3. The last 4 digits, which are numbers, simply represent the number of that frame that month. Where 0001 is the first, 0002 is the second etc.. (prior to 1980 this system was different, and reprented the unique frame number in that year)
4. Lastly. The Kanji symbol or baseball, is essentially the logo for the Koizumi company.

Decoding Diamond Back Koizumi fork serial numbers

Koizumi also made the forks for the Diamond Back framesets. These have a short serial number and can be found on the inside of the drive side dropout. They are identified as follows:

3ET⚾︎

Where;

1. The first digit is the year (81,82,83)
2. The second letter, is the month (as per list above)
3. The third letter stands for Tange (https://www.tange-design.com/) who supplied the tubing
4. Kanji

Koizumi made Diamond Back BMXs

This is a list of framesets made by Koizumi for Diamond Back

• Large Pro (79-81)
• Mini Pro (81)
• Senior Pro (81-83)
• Medium Pro (81-83)
• Silver Streak (81-83)
• Formula 1 (83-84)
• Harry Leary Turbo (82-84)

Koizumi Serial Number Reference Sheet

In this section, you can find a handy serial number reference sheet in PDF format.