Motorcycle Blogs: BUILDING A 'FOUR' IN THE 1960s

1973 Penthouse Pet of the Year, Patricia Barrett

In the mid 1960s an overhead-camshaft four cylinder engine was the object of fantasy and an ideal of red-blooded motorcyclists everywhere. None were available commercially, barring a few ungainly 600cc MV Agustas, prohibitively expensive and with virtually no support from the MV factory, almost an apology or misdirection to fans eager for a roadster version of their all-conquering World Championship racing 4s.

Showing off the finned oil sump, and width of the motor

Honda had not yet translated their remarkable RC161 racing four-cylinder to the public, that would happen soon though. It's hard to visualize today that while four-cylinder motorcycle engines had been around since the first FN's pioneering machine of 1905, no factory in the early 1960s offered a 'four' worth its salt. Creative motorcyclists responded to this decades-old desire as they always had - they made their own, by lashing together a pair of twin-cylinder engines, or, for the truly inspired, fabricating the completely home-designed 'cammy' fours (Nougier, Marsh, etc), which cropped up in magazines like exotic flowers, typically built for racing.

NSU Prinz 1000, the rusty donor car.

Another route to a 'four' was to pull the engine from a small car and stuff it into a motorcycle frame, as several 'special' builders did, some even in series, like Friedl Münch. The most capacious frame in the 1960s was the Norton Featherbed, in production for 10 years already, meaning plenty of 'loose' frames were laying in salvage yards or with blown engines at home. An engine with good 'spec' was actually hard to find, as most 'fours' in cars were both water-cooled and made of cast iron - guaranteeing a very heavy motorcycle. A few small engines were of more advanced spec, with the two most likely candidates (in Europe at least) with full and deep motorcycle connections built into their DNA.

Looking like a Gilera spy photo, almost!

The engine originally used twin Solex carbs

The Hillman 'Imp' had a lovely and powerul engine, albeit watercooled, designed by unsung Norton hero Leo Kusmicki, the man who touched the 'Manx' with his magic wand and kept it competitive for 10 years after its 'sell-by' date. Kusmicki went to work for the automotive industry after Norton shuttered its race shop, and the ohc 'Imp' engine was well-known as strong and capable of being tuned to a high pitch. The water-cooling dissuaded many solo motorcyclists, although plenty of racing sidecar outfits found the need to carry water a small price to pay, given the cheap power an Imp provided. Who needed the money of Count Agusta when a wrecked Hillman provided a readymade power unit?

Sitting comfortably within the Featherbed frame...

Another car-engine donor came from venerable motorcycle manufacturer NSU. The engine from their 'Prinz' automobile, which served long years as a rally competitor, was successively tuned by the carmaker to ever sportier iterations, with the '1200 TTS' the ultimate model, appearing at the right moment to inspire plenty of dreaming two-wheel tinkerers. And it was air-cooled and all-aluminum, with handsome finning, and few awkward casting shapes to spoil its looks. The Prinz engine fit into the Norton frame without cutting metal, although the bolt-on sump needed a revamp... heaving the lump into a Norton frame looks simple, at first. The reality of mating this engine with a gearbox, primary chain, and clutch, and a functioning oil sump, was far more complicated than that seductive 'looks easy' mental image. Skilled fabrication was essential for everything to line up and function properly; a talented stylist was needed to make the result look like a proper motorcycle.

The 1058cc NSU engine, with chain-driven OHC.

Mick King (from the 'Norton Not Mammut' post) was kind enough to share his process, in photos, of taking a rustbucket NSU Prinz and a 1967 Norton Atlas chassis, and creating a successful hybrid. The photos hint at the measuring, drawing, and fabrication time required to bring the elements together, something easy to forget when looking at the finished product. That is the magic and drawback of a successful job - it looks easy! The Italians call this 'Sprezzatura' - making the difficult look effortless; the mark of mastery.

No airbox yet to fill the 'gap' - a mockup shot.

The build took him long enough that both the Honda 'CB750' and the Norton 'Commando' emerged on the market in the meantime, but as his machine was never meant as a production exercise, the Commando contributed useful bits to his Norton/NSU: the front forks and disc brake, mufflers, seat, and clutch.

From the rear, almost a Norton Atlas, barring the extra Peashooter mufflers!

The two 'big' jobs in translating the engine from car to bike were the sump, which Mick cast in shapely aluminum to fit between the Norton frame rails, and the clutch/transmission interface, which he solved via an extended, demountable coupling between the gearbox and clutch, using a 'simple' steel box attached to the engine plates, which holds an outrigger bearing for the extended clutch shaft. This also meant installing the Norton gearbox backwards! Yes, it works fine both ways, but Mick had to reverse the 'pawl' on the kickstart shaft. The photographs should explain his thinking, which seems sound enough - the clutch no longer runs on the gearbox mainshaft but its own stub shaft, connected to the gearbox via a mated pair of pegged plates, similar to BMW shaft-drive practice. All very clever and relatively simple.

The extension 'box' for the clutch; note clutch release pushrod in the center.

The donor NSU model was the 'Prinz 1000', and had Mick King read the specifications for this model, he might have thought twice about the engine! While an impressive 'spec' the standard Prinz only produced 40hp @ 5500rpm, which is about 10hp less than the Norton Atlas engine which he abandoned to make his 'special'... no wonder then that he was shocked to find his finished hybrid a 'gutless wonder'. If Mick had access to the latest model (1968) NSU TTS, he would have found a 70hp engine, using 10.5:1 compression pistons (not much room for increase there!) and sporting camshaft. After Mick's engine tuning work, the Norton/NSU was capable of 125mph, so it seems he equaled the NSU factory in hotting up the engine.

Vancouver 1973, winning 'Best in Show'

The finished machine did well on the 'show bike' circuit in the early 1970s, garnering Mick many 'Best of Show' wins, and the snap (top) with the 1973 Penthouse Pet of the Year, Patricia Barrett. Which you may think is irrelevant, but consider that the 1971 Pet of the Year, Stephanie McLean, was racer Barry Sheene's wife.