By the 1980s, it was only BMW and Harley-Davidson that were making solely air-cooled engines. BMW had at one point considered building a water-cooled flat-four, but risked copying Honda’s Gold Wing, so the Germans came up with a much more creative solution which ideally suited their shaft-drive system.

These new designs were the BMW K100 four-cylinder and K75 three-cylinder water-cooled DOHC engines, laid over on their right hand sides. The K100 launched in 1983 with the K75 triple two years later. Both models utilized the automotive-style dry clutch and shaft final drive found in the boxers, while the fit and finish remained solidly Bavarian.

They were initially criticized for their un-approachability when it came to working on them. Boxers had always been the realm of tinkerers, but these new engines seemed practically sealed. The K100/K75 owner’s manual even glossed over such details as ignition adjustment and fuel injection system tuning, directing owners to take the machine to a BMW dealer for service. Boxer purists shunned the early K’s, but later grudgingly admitted that the K-bikes were quite reliable.

K75 and K100 engines featured long-stroke, water-cooled engines (67mm bore x 70mm stroke), hemispherical combustion chamber with two valves per cylinder and dual overhead chain-driven cams. K100s displaced 987 cc (90hp/63 ft lb), and the smaller K75s displaced 740 cc (75hp, 50 ft lb). The K75 also featured a rotating counter-balance which made it a bit smoother than the K100.

Valve actuation was a bucket and shim arrangement, and valve clearances were extremely stable. After a break-in service, K-bike valves could go tens of thousand of miles before needing any adjustment. Cylinder bores were Nikasil-coated aluminum for tight clearances and long life. The engine was connected to the five-speed transmission via a dry clutch.

Like so many previous BMWs, the K-bikes lasted because they were overbuilt. The K100 was hefty at 526 lbs wet, while the K75 was 500 lbs, but both handled well and had abundant torque to make up for the weight.

Some of the smarter thinking included a monoshock, single-sided aluminum swing arm, a steel frame with fully stressed engine member, aluminum tank, stainless exhaust system and open loop fuel injection. Later models were among the first bikes to use antilock brakes and the K-bike side stand retracted as the clutch lever was pulled in.

K-bike plastic hard bags were lockable, could accommodate a helmet and a week’s clothes, and were easily removable. Thanks to low maintenance, excellent luggage and several windshield options, K’s were among the best sport tourers of their day.

The same K100 motor was used in the “RS” and “RT” variants. The RS had a wind tunnel-tested sport fairing that created an effective bubble of still air around the rider, while the K100RT had a large touring fairing.

The K100RT later evolved into the K100 LT designation (“Light Truck”) with a full fairing, integral radio and other extras. The K75 came in a “naked” variant, a “café” faired K75C and the desirable sporting K75S.

The K-bike engine turned out to be even tougher than the legendary boxers and 200,000 miles is not uncommon. Provided the owner has kept up with tires and vital fluids, a K-bike should have no serious problems. In fact, K-engines are quite common in wrecking yards because nobody needs them.

The only weak link in early K-bikes is the splined transmission input shaft. The shallow splines can wear through, which means a complete a transmission teardown. Best bet is to partially disassemble the bike every 20,000-30,000 miles and grease the splines.

One feature that the K-bike designers failed to consider is characteristic. When you park a K-bike, oil pools in the combustion chambers, so restarting the bike creates a big puff of smoke.

Eventually, the K100 engine grew to 1100cc and got four-valve heads from the flagship BMW K1. The K75 engine, meanwhile, remained largely unchanged throughout its production run. K100s eventually reached 1200 cc and the K motor still powers the K1200 LT.

The K100 was built from 1983-92, while the K75 ran from 1985-95. There are plenty of survivors, but look for long-term loyal ownership with complete records and make sure the owner is familiar with the spline issue and has addressed it.