Great assortment of interesting comments - BUT, looking at my opening comment you'll see that I stated specifically CAR engines.
I know nothing about Diesels or radials other than the fact that they're not car engines (with a few Diesel exceptions) and furthermore I was curious about the complexity of the car engine itself, not the accessibility and/or serviceability.
OK, now my story.
The most complicate engine I’ve ever worked on was a Ferrari type 141 engine, serial number 0646. The engine was a marvel of engineering and reliability and quite complicated.
V-12, aluminum block and heads, steel liners, 4 cams, two plugs per cylinder, two distributors, each having four sets of points, six twin-throat down draft Webers, dry sump and no flywheel.
245.5 cubic inches, 447 HP @ 8,500 RPM (we had the dyno sheets).
It was originally fitted to a Ferrari 4100 Sport (I cannot recall the serial number) in 1956 and set up for road races, including the Mille Miglia.
Driven by Alfonso Antonio Vicente Eduardo Angel Blas Francisco de Borja Cabeza de Vaca y Leighton, 11th Marquess di Portago, AKA Alfonso de Portago, he had a horrific crash, killing himself, his co-driver and 10 spectators. The cause was a tire failure at 150 MPH. This ended the Mille Miglia.
The engine was then used in a single seat roadster for the Monza-Indianapolis “Race of Two Worlds” in July of 1958.
After the race, Ferrari commissioned a new Testa Rossa style body and installed the engine, coupled to a Lancia D-50 transaxle. Serial number 0744, it was nicknamed the 412 MI and sent to the USA to be raced by people like Phil Hill.
My friend acquired it in 1978, ran it in FCA club events and occasionally on the street. He let me drive it at Watkins Glen and it’s the only car that ever truly scared me. Applying full throttle at 120 MPH would spin the tires.
The photos show the engine of this remarkable car. The oil filler was moved to the right side for direct access to the oil tank. Originally, it was on the left side, going directly into the crankcase (sump). If the engine was revved before the scavenging pumps could empty the sump, the front crank seal would blow out.
That’s my story, comments welcome!