Abstract: An internal combustion piston engine with horizontally opposed cylinders. Each opposed pair of cylinders fires simultaneously, providing symmetric cranking force on the crankshaft 13. Each cylinder 1 is a simple solid of rotation with air-cooling fins 10. The cylinder heads 5 are modular. Each module covers two adjacent cylinders, and has liquid cooling channels. All cylinders are identical, and all cylinder heads are identical. The cylinders are held inward against the crankcase by the cylinder heads. Each cylinder head is held inward against the cylinders by two tension bolts 42 that span to the opposite cylinder head. One tension bolt crosses above the crankcase, and one below it. In a preferred 12-cylinder engine embodiment, each opposed pair of cylinders is separated from the other opposed pairs in spark timing by a sequence of 105 and 135 degrees that reduces torsional harmonic resonance in the crankshaft, eliminating harmonic balancers.