Yamaguchi serial numbers decoded
Yamaguchi was a Japanese bicycle frame manufacturer from Sakai Japan. In the early 1980s some of their frames has a decal at the base of the seat tube:
"AYS" in a diamond, "Yamaguchi Sports Frames Sakai Japan".

Yamaguchi was contracted to make frames for several brand names in North America including:
Baycrest in Canada, Lotus, Manta of Madison Wisconsin, Norco in Canada, Sekai in Settle washington, Shogun, Silver Eagle, SR of California, Steve Bauer, and probably several others.

In late 1979, the Yamaguchi serial number format changed to:

Yytnnnn where
"Y" = Yamaguchi
y = a number for the year of frame manufacture
t = a letter fot the time of year of frame manufacture
nnnn = four digits to uniquely identify the frame for that year.

From the best of my observations this format was used as late as 1989. The serial numbers are always stamped onto the BB shell.

I have done an analysis of the time of year letters. I have found that there were 12 unique letter used.

I presume 12 letters, for 12 calendar months. Here is how I would decode the letter:

A = January
B = February
C = March
D = April
E = May
F = June
G = July
H = August
K = September
L = October
M = November
N = December

The letters I, J, and O to Z are not used.

* * *

Prior to the above format change Yamaguchi serial numbers had the following format:


Yyy-xxxxxx where
"Y" = Yamaguchi
yy = two digits for the decade and year of frame manufacture
nnnnnn = six digits to uniquely identify the frame for that year.

I have seen serial numbers with this format from 1974 to 1979. The serial numbers are always stamped onto the BB shell.


* * *

Prior to the above format Yamaguchi had serial numbers that were not stamped into the BB shell.

The serial number was on a relatively thin strip decal, underneath the seat lug, on the inside of the frame.

On the decal there is an "AYS" logo followed by some numbers.

I assume that these decals are from the 1960s and/or early 1970s, which makes them more than 40 years old.
Now they are mostly faded and difficult to read.
Attached Images
File Type: jpg
Yamaguchi_Sports_Frames_IMG_2171.JPG (18