Strange stamp mark on Blackburne crankcase

[johnc]

I have owned a 350cc Blackburne for many years.  A strange mark on the crankcase has always puzzled me. It is on the rear of the case, opposite to the CP1075 engine number.

The engine has screw-in rocker posts, so I believe it's 1927 or earlier. It also has a slightly smaller outside flywheel than I have seen.  I bought the bike as a methanol-burning grasstracker

Has anyone seen a mark like this before?  Its very neatly stamped.    Can anything be gleaned about the age from the CP1075 engine number?

[100percentcotton]

The first letter 'C' from the Blackburne engine code denotes a 350 OHV, the second letter is the year of manufacture in this case 'P' for 1928. Do you know the number of your frame? this should be stamped on the top tube lug of the offside rear frame casting and would help in dating the frame.

[johnc]

Thanks for your message. Interesting that its 1928 engine. The rockers are threaded on posts, and I believe that is slightly earlier. I boulgt the bike as a grass-tracker, so no doubt made from bits and pieces. The frame is much later. Unfortunately the rusty-est bit of the frame is where the frame number is. I think its 16*8* or 18*8*. I always thought it was mid 30's frame. The pipe ends are squashed, so dont have proper lugs. I assume that this was an economy measure done later on. Its a saddle tank frame with narrow lower pipes.
I would love to get a proper early frame for the 1928 engine, but I guess they are impossible to find now.

[cottonjo]

I asked Rick Parkington the Blackburne specialist about the strange marking on JohnC 's crankcase, below is his reply.

Yes I know exactly what you mean, it appears on all Blackburne engines around the 1925/26/ 27 period and not just Cottons of course. I believe it is an inspector’s mark, ie factory quality control. This was confirmed by the late Pete Gasson who said when out on his Cotton local to Bookham, he had once met a guy who pointed at the stamping and said ‘That was my mark!’ having been a factory inspector, sadly Pete didn’t think to get his name.
Hope this helps
Cheers Rick

[cottonblackburne]

Hi, yes I'm agree: The C is 350 OHV and P is the firstl model with the new Head, that is much stronger then the old CK Head. This new head has 5 fixing bolts and was used with different valves of corse, on 350, 500 and 600 OHV engines from 1928 to the end of Blackburne production. The barrel on the CP engines are as well special, because 5 stud on top and 4 stud at the bottom. Just made in 1928 with single or double port. I have them both. fitted for example in a french Motoconfort of 1928.