Produced between 1938 and 1940 with 3171 units (N ° 503 001 to 506 172).
This model replaces the R5 which it differs notably from its rear suspension. The R51 seems the first BMW with a rear suspension.
Equipped with a dual overhead cam, it has a displacement of 494 cm3 (bore / stroke of 68 x 68) and a power of 24 hp at 5500 r / min with a weight of 182 kg. Top speed is given to 140 km / h solo.
The model presented here has the Engine number 505004 and frame 508330. The same frames were used interchangeably on R51, R61, R66 and R71 in the order of the production (# 505001 to 515 164 for BMW R51, R61 et R71).
This model went out of the Munich factory on June 12th 1939 to be delivered few days later to the BMW dealer Mr Brenner the June 23rd.
BMW R51 (1939)
At the left the BMW R51 from BMW archives, at the right as I get it. Let go to work.
Dismantling in progress
Disassembly of the clitch and its spring disk
Flywheel and its 36 mm screw
Oil pump rod
Piston and cylinder removal: metrology
View of the distribution, chain removed
And its two camshafts
The two heads are well damaged externally, the fins will be welded and the springs changed for new ones.
One of the rockers is damaged, it will be changed
Back sandblasting and painting
The brake linings as well as those of the steering brake are changed
Reassembly, the fork seals are replaced by SPI seals
Installation and removal of the crankshaft are tricky: Only 0,5 mm free space.
The engine number is also on the cylinders and the heads. OZ-80 indicates the octane number (Oktan Zahl).
The engine finished. New pistons and cylinders bored.
This lady is 74 years old.
Reassembly of the transmission.
The S engraved on the crankcase indicates the method of sand casting (Sandguss) (Thanks Paul).
Blank assembly of the missing parts: mudguards and package holder.
Rebuiding of the gearbox, bearings changed.
The selection grid is polished and the crankcase seal is remanufactured.
The lady is finished and can leave the workshop.