Time Traveler: 1939 BMW R51

The R5’s engine, a design that carried over to the R51 platform, was BMW’s first opposed-twin that didn’t use split engine cases; the R254-model 494cc overhead-valve engine utilized a single-piece tunnel-type case, with the crankshaft installed and removed from the front. Timing chains drove the two camshafts positioned slightly above and to each side of the crank, allowing the valve gear in each cylinder head to use shorter tappets and pushrods than previous BMW opposed twins. The valve gear was actuated by rockers moving on needle roller bearings and tensioned with hairpin springs.

Mounted on top of the unit engine case sat the Bosch generator; the distributor and coil were tucked with Teutonic efficiency neatly under the front engine cover. The R5’s 22mm Amal carburetors were positioned on either side of the engine, each feeding its own cylinder head. The 4-speed transmission had its foot-shift linkage on the left side; on the right was a hand-lever used primarily for quickly shifting the transmission into neutral for stops. The positive-stop foot gearshift design was based on a system developed by Englishman Harold Willis in 1928 for Velocette. This basic engine and transmission package remained a part of the R51 design that bowed in 1938.

The R5 also sported an all-new tubular steel frame, although there still was no rear suspension. That would come later with the R51. Up front, BMW engineers employed a telescopic fork, a rarity for the time, and the sum total of these components lent to the R5 qualifying as an uberbike of the 1930s. Indeed, much of the R5’s overall design was influenced by BMW’s Grand Prix road racing model, the 500 Kompressor, a racer powered by a supercharged 500cc opposed-twin engine and campaigned with great success by famed BMW factory rider Georg Meier. In fact, Meier eventually won the famous Isle of Man TT’s Senior class in 1939 riding the 500 Kompressor.

Bringing up the rear

Falloon touted the R5 as a true landmark design. Wrote Falloon in his BMW book: “The R5 was a milestone motorcycle for BMW, finally challenging the British in performance and handling. One of the standout machines of the decade the R5 also provided the basis for BMW twins for the next 20 years.”

Those “next 20 years” began with the R51 that replaced the R5, as did the R61 replacing the R6, which was powered by a sidevalve engine. Those two models were joined in 1938 by the R66 and R71. The common denominator for all four models was the addition of the plunger telescopic rear suspension at the rear axle, which necessitated the addition of universal joints for the driveshaft to compensate for the up and down rear wheel travel. This design was a carryover from the 500 Kompressor racer’s rear suspension, originally developed by Alex von Falkenhausen.

Another tidbit of history before we continue: Most of the new 1938 and later models rolled out of the factory sporting black-finished mufflers. It seems the Führer and his gang needed the precious chromium for the rifle barrel bores that the Third Reich had ordered for its military, and so the good guys — that would be any BMW civilian customer — had to do without the shiny stuff on their motorcycle’s mufflers.

Despite such inconveniences and limitations, sales of BMW models continued to grow as the 1930s decade rolled to its ill-fated conclusion. According to sources, overall BMW production climbed from 10,005 units in 1935 to 11,922 the following year, creeping up to 12,549 bikes in 1937. Overall sales spiked noticeably the following two years, reaching 17,300 in 1938 and 21,667 during the last peaceful year the world would enjoy until late 1945.

During those final few years before war broke out, BMW, probably spurred by the Third Reich itself, realized the importance of racing in terms of engineering progress and public relations, and in 1937 offered the R5SS, a production racer for privateer riders. The R5SS was stripped of street gear such as lights, horn and mufflers, and its engine boasted four more horsepower to the rear wheel.

As you might guess, the R5SS was followed by an R51SS production-based racer. The newer model touted more than just new rear suspension, too. The engine utilized a pair of angry 6/432 Amal/Fischer 24mm carburetors that fed an engine with a slightly higher compression ratio of 8:1. The 4-speed transmission also had higher gear ratios to help attain a higher top speed on the racetrack, and the sum total funneled 28 horsepower to the rear wheel, a combination that all but pushed the competition out of the way on the racetrack. Interestingly, though, the R51SS was delivered with lights.

In stock trim, the R51 weighed 401 pounds, and its engine brought 24 horsepower to the party. Although it shared the same sleek silhouette as the R5, the noticeable difference was the addition of rear suspension. Interestingly, some BMWites today categorically claim that the R5, with its rudimentary rigid-frame design, handles better than the fully sprung R51. While the plunger rear suspension’s 2 inches of up-and-down travel might help smooth the ride over some roads, logic dictates that the springs’ stationary vertical plane of movement, rather than the sweeping arc that’s found with bikes using rear swingarms, might actually work against itself when negotiating some bumps in the road.

By 1940 BMW was forced to cease R51 production, and we all know how things turned out during the subsequent five years for Nazi Germany and the rest of Europe. After the war, BMW production was initially limited to the single-cylinder R24, but by 1950 BMW engineers had located a few surviving R51s that they used to reverse engineer for production of an updated model, the R51/2. The new model’s engine had coil valve springs that replaced the more archaic hairpin valve springs originally used in the 1938-1940 models, and engine oil circulation was improved. Revised hand controls on 7/8-inch-diameter handlebars (replacing the fatter 1-inch bars) also found their way onto the postwar model.

The following year, 1951, saw the introduction of the R51/3. Foremost, its unit-case engine sported single-cam cylinder heads using spur gears that eliminated the trouble-prone cam chain found on the R5 and R51, and an improved oil pump was located beneath the crankshaft. Despite these improvements, the R51 series became a footnote in BMW’s glorious past, as newer models including the R50, R60 and R69 sporting more up-to-date technology found their way into the lineup.

War refugee

So what about this particular R51 that was supposedly dispatched to the SS? Good question, and while we don’t have an answer regarding the bike’s war-torn years, one thing’s for certain: At some point in time the bike made its way across the Atlantic Ocean and onto these shores. Today, it’s owned — and was restored — by Mike Dunn, proprietor of Vintage German Motorcycles in Riverside, California.

“For some reason,” Dunn says, “this BMW survived the war and ended up in the U.S. after the war.” Dunn, who acquired the bike from a collector on the East Coast, also chased down documents confirming the bike’s SS connection.

By the time the R51 found its way to Dunn’s shop in California the bike was, in his words,”a pile of rusted parts.” As evidenced by the photos, the R51’s restoration is over the top. Authentic BMW hardware, including Ribe, Verbus and NSF fasteners are used throughout. “All of the engine nuts and bolts are cadmium plated,” Dunn says, adding, “the frame bolts are all black oxide per the manual, and nothing was overlooked during the documented restoration process.”

Look closer and you’ll notice the electrical wires are wrapped in cloth, just as they originally were in 1939 — “no plastic modern stuff,” as Dunn puts it, and the fuel lines are silver cloth, also like the originals. Ditto for the cloth-wrapped control cables, and all of those classy cast aluminum trim parts that make vintage BMWs look so cool are neatly finished, too, not overly polished as some restorations mistakenly offer.

The paint is authentic, as well: There is no powder coating to be found. Another nice touch is concealed within the headlight nacelle that houses the bike’s only gauge. “The original speedometer was restored without removing the original finish on the face plate,” Dunn says.

To make sure he got things right with the restoration, Dunn said that he pored over hundreds of period photos to determine the correct parts and their necessary finishes and trim. Finally the bike’s yearlong restoration culminated shortly before the 2016 Quail Motorcycle Gathering in Carmel, California, where the R51 was runner-up in Best of Show judging. Fittingly, it was another BMW, the immaculate R37 racer owned by collector Robb Talbott, that scored first place.

First-, second-, even third-place, when all is said and done accolades and trophies really don’t matter when your eyes feast on historical bikes such as this R51. Gaze at the bike for only a few seconds and it’s clear that when BMW’s designers laid down the lines for this motorcycle so many years ago, they also penned their own place in history.

Simply, the R51’s classic, genteel, R5-derived silhouette is breathtaking. Indeed, the R51’s long, low profile is as elegant now as when it first appeared in 1938. And the historical perspective aside, Mike Dunn’s example remains a jewel that further links chosen bikes of all ages to a single thread, which is that of timeless beauty. MC