21 Serial Number/VIN Question

[MikeW]

My project is what I assumed to be a late (solid flywheels) 1921 61cu, magneto V-twin. To the best of my knowledge it is an original bike - not a kitset of parts. Frame No. 21F 5400. Motor No. 21F 5801.

Two questions. (1) Does this seem to the experts out there like a "matching" frame and motor, given the serial nos. are 400 apart? (2) Given that there were only about 2400 21Fs produced, how come the serial nos. on my 21F are over 5000?

If the answer to the second question is that frames and crankcases were stamped initially with a sequential serial number for the year, and then later stamped F/J/FD/JD depending on the model for which they were used in the engine building and motorcycle assembly shops, then my bike looks as if it were not assembled all that late in the programme for the 1921 releases, given that there were about 10,000 V-twins of all types produced for that year.

Apologies if this has been covered before. I scanned previous posts but couldn't find a thread that covered this ground.

[Chris Haynes]

Although I am no expert on the '20's machines I can only say that I have never seen a bike with matching numbers.

[MMasa]

Frame and engine numbers do not match on these models but they tend to be within hundreds of each other in my experience. The same rule follows for transmission numbers too. What is the number stamped into the transmission case? It should be the letter J followed by a number that is within a few hundred of the others.
Actually, the frame numbers on all models, J and F only contain the letter F. The frame number format is TWO DIGIT YEAR the letter "F" and then SEQUETIAL SERIAL NUMBER starting at 1000 regardless of if the model is an F, FD, J, or JD.

In answer to your VIN / production number question, you are essentially correct. All models were lumped together in the serial number progression. Your VIN is 21F5801 and that was the only motor stamped that year with 5801. There was NOT a 21FD5801 nor a 21J5801 nor a 21JD5801.
I hope that answers your questions.
Mark

[MikeW]

Thank you Chis and Mark. That answers my question fully. The gearbox number looks like J5442. (It is clearly J, though one of the middle 4s is difficult to read.) Which is consistent with your response Mark, and confirms, at least to my satisfaction, the advice I received when I acquired the bike 40 years ago that at least the major components of the machine were as originally assembled in Milwaukee in 1920.