Solved! It was worn cotter pin shamfer and thread. Only a 1/3rd of part of the thread was missing, but enough to work loose over time. Had hell of a time sourcing a similar item, but eventually found a 'Brompton' cotter with same spec!
left side (non drive). ive checked: bottom bracket, cotter pins, crank arms, pedal threads, pedal bearings, chain/sprocket/chainring wear, chain link seizure etc, the only thing i can think off is maybe a weird 'freewheel mech problem????
I have the same problem on the none drive side, single speed 1950's east german path racer... I've checked cranks bb, cotter pins, pedals (changed for known good 'uns), but didn't consider stiff chain link/s and or worn chain/sprocket teeth. will check and report back..
The entire bike has correct, original OEM parts. Brake lever, callipers, Cranks, saddle, seatpost, Handlebars and grips, wheels rims/hubs all as the 1959 spec from DIAMANT factory… the Modell 109 was a ‘fixie’ path/town bike with unique brazed pannier mounts on seat stays… A former East German...
The saddle was incredibly dry. Not surprising since it was made in early 1950’s!! Just applied 4th coat of neatsfoot, after leaving each coat for 24 hours.. will leave it like that for now until leather has rejuvenated… I don’t go for ‘showroom’ … I prefer leather with a history….
Diamant Decals say Factory Karl Marx Stad, which was in East Germany (now Chemnitz in Germany)... It has wing nut front and back hub spindle locking nuts. I think the brake has AFA stamped on it ??? more photos to follow tomorrow... Frame number is 3469899
My gearing is as follows
Crank: Sugino GT 2 speed, 52t-42t
Rear cassette: shimano 7 speed, 28t-13t
Cassette is a Shimano hub on ‘FIR Vittoria’ rims so not original’heliomatic’ wheels/rear cassette I believe….
Thank you Sir! the kind of reply I was looking for. A few friends on other forums (and facebook classic cycle groups) have suggested it could be Cliff Shrubb's Work ??? There is no frame number (i've had it stripped to bare metal) only stamp of id is on dropouts. the lugged frame is very fancy...