Forum bmwbike.com

Hi Mark:

I am considering purchasing a 1939 R51. The machine is in excellent restored condition and complete. On closer inspection the carbs appear to be from a Chinese Chang Jiang and not the correct Amal 5/423 or Bing 1/24.

This situation made me concerned about the originality of the frame. The engine is very correct and numbered properly but the frame numbers are odd and the machine is missing it's builders plate. The frame number appears on the tank tube under the seat, before it butts into the plunger. Could this frame be from a Soviet M-72? It also does not appear to be from a current Chang Jiang. How can I authenticate the frame as BMW. It looks very much like my R51/2 but the large frame numbers on this pre-war bike seem odd. Where should the frame numbers on this model reside?

Thanks, I just bought a complete R50S in need of a total restoration. I will be ordering parts soon.

Hi:
I was able to inspect the 1939 R51 frame again and found it to be a almost certainly a genuine 251/1
frame. The quality of welding and materials can only be from BMW. The frame is missing the front cross tube that BMW added after the war on the 251/2 frames.

My only question now: Did frame and engine numbers match on pre-war BMWs, even in 1939? In the Hartel Book "BMW Motorrader Typen und Technik" he lists engine numbers 505001-515164 and frame numbers 503001-506172 for this model.These numbers do not match. The engine number is 505387 on the machine in question and the frame number is missing on the plunger casting. A very unlike BMW number appears under the seat tube.

Thanks

Dear Jeff, I am the owner of a 1958 Chang Jiang w/sidecar and I can tell you it runs like a true tank. It almost never breaks down and is happy to lug around the heavy sidecar. As it is a replica of a 1937 R71 the Chinese stamp the frame # on the tube appear exactly where you say. It could very well be. The give away would be the condition of the frame itself. The true BMW frame has much more finesse in its construction than the Chinese vareitn which is heavy and spotted with slag. If ithe framing is smooth a, well, "German" looking than you can be sure it is not the Chinese knock-off.
Hope I was able to help.

Rhodes
Los Angeles

Greetings
As the proud owner of both an original R51( Frame and Engine) it is easy to make
comparisons. The two most obvious differences between the Chineese Tank and an
R51frame are:
There is no center post under the seat
The driver seat front pivot is metal the Chineese use rubber

Like your bike my frame and engine #s don't match ( 511214 & 505261). The plunger casting
numbers are readable and correct. If there are any numbers below the seat multiple repair
welds and repaintings have completely covered it over. Many Chineese parts will fit your
frame but with few exceptions the substitution will be obvious.
Good luck