In the dark days immediately following WW2, Germany and Italy were banned from participating in international motorsport on two or four wheels. The reasons were complicated: the victorious nations couldn’t stomach the thought of losing a race to competitors from a country they’d been at war with for 6 years, and immediately before the war, German and Italian factories built the fastest and most sophisticated car and motorcycles in the world. Since development of civilian vehicle designs was illegal or strongly frowned upon in several Allied countries during the war (to focus ingenuity on winning), motorsports resumed in 1946/7 as it had left off in 1939/40, with the very same machines, although supercharging was banned by the FIM for the motorcycle Grand Prix circuit, so quite a few prewar designs had to be altered for natural aspiration.Banned from international competition didn’t mean racing stopped in German and Italy, and national-level racing resumed by 1946. Freed from the FIM rules against supercharging, the same RS255 BMWs and NSU blown twins appeared on the tracks postwar, much to the fascination of the American troops occupying both countries, especially in Germany. American motorcycle fans had never seen and seldom heard about these sophisticated machines, and soldiers were gobsmacked by what they witnessed, especially if they were racers themselves on Class C machinery: rigid-framed Harley-Davidson and Indian sidevalve v-twins of 750cc.Of course, except for riders with factory connections, most would-be German or Italian racers had nothing so exotic to race! Modifying prewar or military machinery was the only viable path, and such machinery filled the grids in the years when German and Italian industries were also forbidden to build machines larger than mopeds. In Germany, the BMW R75M was a common foundation for racing, with its 750cc OHV motor placed in a civilian R51 or R66 fully sprung, lightweight chassis, and tuned up with bigger valves and carburetors, and hotter camshafts.Some home tuners took the R75M much further than mere tuning, and the most impressive example of a home-made racer with world-class performance is the little-known MFK racer, a collaboration of Franz Mohr, Kurt Friz, and Hans Kleinhenz-Schweinfurt. It’s an awesome machine, that still exists in as-raced condition in Germany, a testament to hard work and ingenuity by ordinary motorcyclists with no engineering training to speak of.
Franz Mohr was interviewed in a German magazine in 1949 about his fantastic creation:
“After the end of the racing season in 1947, during which I drove a refurbished BMW R75M engine in its original Wehrmacht frame, I came up with the idea to convert this engine into a overhead camshaft motor. Thanks to the energetic support of the well-known racing driver Kurt Fuglein, the financial basis for such a project was established.
In December 1947 I began, as a professional motor mechanic, on a primitive drawing board, the sketching and designing of our MFK motor. The basis was the old BMW R75M engine, whose crankcase and crankshaft were carried over to our project. The central idea was to use everything that could be used somehow, in order to keep the costs – which started out huge anyway – as low as possible. The camshaft of the R75 engine was not used; instead, a camshaft with adjustable cams was placed in each cylinder head, driven by bevel gears, a drive shaft and corresponding pieces. At the same time a gear oil pump was coupled with cylinder head, for each individual cam to lubricate the cylinder head via a pressurized system with fresh oil. The motor oil was cooled by an oil radiator, in a continuous system with the crankshaft, bearings, cylinders, cam drive shafts and cylinder heads.The parts required for all this have been produced in my little workshop, and also the machine shop of Hans Kleinhenz-Schweinfurt, a pure master of his trade and pure idealist of motorsport, who works with painstaking craftsmanship.
New parts included two new crankshaft drive housings, screwed to the former magneto shaft; two camshafts connected to the crankshaft by two shafts; four special gray cast iron cylinders with four large square-threaded cylinder head bolts; 4 cylinder heads and valve covers, for which first models were made; 8 rocker arms; the associated axles, bearing housings, valves, and camshafts. The cams, made by hand, had to be ground one degree at a time, since no cam grinder was available for us; 8 output housing for the conical shafts and their bearings, lock nuts, protective tubes with rubber rings for oil seals.
My friend and lubrication master Kurt Fritz and 5-6 companions helped me in their leisure time with all these and other great things; they even sacrificed their vacation for up to 120 hours a week. Of Sundays and holidays, there were no more for months, and after-work was a flexible concept. My poor wife became merely a maid, but bravely held through, to support the big goal. Often I worked up to 100 hours continuously with only 3-4 hours of sleep. I owe this endurance and energy to certain envious people who foretold a full fiasco: they did not trust such a job to a simple engine mechanic, especially not in such a short time, without help or support from any designer or engineer for solving problems. According to their view, even factories worked for years on such a redesign.All this I knew and all this spurred me all the more. The ‘apple cart’ overturned after my wife and I had already sacrificed so many pieces of clothing and furnishings to live, and only just live. In between, I rode off with my old racer and earned money off a race, partly with, partly without success – but we had some money again and continued to work. Who can imagine what it means to do such work without training, without a technical cylinder head design for a bevel drive OHC (with partial views and cuts), so the mould maker can build the casting moulds without a single question? That the heads were cast, and handed off to others with only my blueprints, to be drilled and processed accordingly? Not only the cylinder head: hundreds of parts were needed for this redesign, and everything had to be calculated beforehand. I agree, engineers had done it before – but I had no such experience. So I sacrificed night after night, studied in technical books and moved toward my goal.What this also means, is that the materials used for various parts of the motor – the gears, the shafts, the camshaft, the rockers, etc, had to be processed and hardened as well, in the ways that each required, which were all different: this cost me ‘several’ gray hairs.
But one day it was finally time. Perhaps the readers will be able to gauge what this was for us – that inner joy – all the effort, all the hate was forgotten, and we listened to the engine with devotion. A few days later, on July 4, 1948, was the Garmisch race. With the new engine, I drove the fastest practice time, but then had to replace the bike and race my old loyal R75, as our camshaft shapes had to be changed. Within eight days we had made new camshafts, and in Karlsruhe I easily won the first prize. Another eight days later, we drove in Reutlingen, setting the fastest lap of all sidecar classes with 83.6kmh, but could then hold only second place due to clutch problems. Again, we had to make a new clutch, because the high engine output of our OHC motor resulted in failure.
I had to renounce further races for a time, because our MKF motor exposed other chassis difficulties, and we converted to an R66 plunger rear frame. Result: change the frame, turn a new rear hub, make new set of final drive gears, turn a new shaft, change the tailpipes – in other words: an unspeakable amount of new work had to be done. Those who have not done such work themselves cannot gauge what is involved in not ‘throwing the gun into the grain’.
For the first time, I returned to the Grenzland track and achieved the fastest practice time. By an most unfortunate event, I retired in the second lap of the race: the undisciplined behavior of the spectators, who compared the route to a wastebasket, was to blame. A large bag, without being noticed by me, caught on my left cylinder, blocking its cooling: the piston melted, and molten aluminum stuck on the crankshaft. By the way, Eberlein (a factory BMW racer) fared just as well, – his motor ate a scrap of paper and tore off his cylinder.
Now a short summary of the entire working hours:
Design hours: 580
Planning, making tools: 250
Pattern-making hours: 1000
Machining hours: 2500
Assembly hours: 950
Testing and fettling: 500″
[If you’ve ever considered building your own motor, or wondered at the expense of custom machine work, the above hours are sobering!- ed.]