JAP engine number

[PaulBurton]

Can anyone help decode the following JAP engine serial number?

I O Y/S  54073 /J

I have interpreted this as

I = 350cc
O = OHV
Y = Twin port
S = 1929 build
54073 = Serial number
J = non standard

Am I right and in particular the year number?

Thanks

Paul

[chaterlea25]

Hi Paul,
That about sounds right, if the engine fits in generally with a 1929 appearance,
twin port would be a style of the times
JAP offered several style options on basically the same engines, (different finning shapes and magneto chain case lengths and placements or maglita driven from the drive side)
The year codes ran from September to August according to my JAP book which can lead to "discussion" when it comes to Whats Vintage???

Remember that JAP reissued the engine year code post WW2
This is why lots of late industrial engines that appear for sale are mis described as "vintage"
Usually the SV engines from the earlier period had cylinder and head as one casting

John

[PaulBurton]

Thanks John, reassuring. My concern was whether the Y refers to twin port (which it is) or year. The 'S' certainly fits with everything else about the machine and the date of original registration.