Now here's a thing. BMW used bushes to support the crankshaft at both ends in the airhead engines.

If by "airheads" you are refering to the /5 and onwards models then, yes, they have all plain bearing cranks rather than the built-up rolling element cranks of their /2 predecessors.

IIRC the bottom end design on the /5s was basically taken from the 1602 car engine. The main bearings being "rings" rather than split shell bearings would just have come about by the crankcase design - basically a box with a larger opening at the alternator end to feed the crankshaft in & closed with a plate, both of which lend themselevs to having a bush-style plain bearing pressed in. The /5s onwards also have a pretty meaty trochoidal oil pump to pressure feed the plain bearings, driven off the clutch end of the camshaft.

Again IIRC, the change was due to a desire to reduce production costs, with the one piece crank & plain bearings being cheaper to produce than the all rolling element built-up cranks of the earlier models. BMW came very close to collapsing due to their high production costs in the '60s. Motorcycle manufacture moved from Munich to West Berlin with the introduction of the /5s - I would not be suprised to find that was a move that coincided with tax breaks or other financial incentives to bolster West German industrial activity in West Berlin during the Cold War period - that BMW already had a former aircraft engine factory in the Berlin suburb of Spandau probably also had a hand in the decision.

The later "oilhead" engines have Japanese-style split crankcases, so I would expect that they use split shells rather than ring form as the crankcase layout would lend itself to that style of bearing.

Nigel B