Single: Called a “thumper.” Most dirt bikes, many small motorcycles, and the BMW F650.

This one's air-cooled. Notice the fins and the single exhaust pipe.

Inline Twin: two pistons side by side.

Another example of an air-cooled engine. The near cylinder's exhaust pipe is prominent; the far cylinder's is hiding behind the black frame tube.

V-twin: Harley Davidsons, Ducatis, Honda Hawk and Superhawk, Moto Guzzi, and a couple of Suzukis including the wonderful SV650. The 90-degree twin is very well balanced; other angles may or may not be, depending on whether the crankshaft has one pin or two.

The top one is an air-cooled Harley-Davidson V-Twin mounted laterally; the bottom one is a air-cooled Moto Guzzi V-Twin mounted longitudinally.

Inline-three: Three pistons in a line. Typical examples are Triumph and BMW K75.

The top one is an air-cooled inline-three mounted laterally; the bottom is a water-cooled inline-three mounted longitudinally.

Inline-four: Staple of the UJM or “Universal Japanese Motorcycle,” found in everything from the 400cc Honda CB-1 to the 1200CC BMW K1200. In the UJM it’s mounted with the crankshaft sideways; in the BMW K it’s mounted with the crankshaft aimed north-south and the pistons off to the left.

The top one is a water-cooled inline-four in the common lateral mount. Notice the lack of cooling find. The pipe at the top brings coolant to the radiator, that shiny black box at the front of the engine.

The bottom one is a BMW K100 water-cooled inline-four mounted sideways.

Flat-Four: Older Honda HoldWings had these.

V-four: Honda VFR 800 (sideways) and ST1300 (longitudinal): smooth like a four and balanced like a twin.

This one's out of an ST1300. You can see two exhaust pipes for the near bank of cylinders. By the lack of fins, you might guess it is water cooled—and you'd be right.

Flat-six: Honda Gold Wing and Valkyrie: smooth torque monster with low center of gravity.

Honda flat-six engines are apparently all transparent. I could not find any other pictures.