The cylinder block is the largest part of the engine. Its upper section carries the cylinders and pistons. Normally, the lower section forms the crankcase, and supports the crankshaft.

All cylinder blocks are made with ribs, webs and fillets to provide rigidity but also keep weight to a minimum. The cylinder head bolts onto the top of the cylinder block where it forms the top of the combustion chamber.

In-line engines of light vehicles have just one cylinder head for all the cylinders. Larger in-line engines can have 2 or more. V-type and horizontally opposed engines have a separate cylinder head for each bank of cylinders.

As with engine blocks, cylinder heads can be made of cast iron, or aluminum alloy. A head made of aluminum alloy is lighter than if it were made of cast iron.

The piston, with its connecting rod and bearing, transfers the force of the combustion and expansion of the power stroke to the crankshaft. The piston itself, its rings, and the piston pin, also known as the gudgeon pin, are together called the piston assembly. The cutaway shape on this piston allows it to clear the counterweights on this rotating crankshaft.

The connecting rod connects the piston to the crankshaft. It is fastened to the piston at its little end, by a piston or gudgeon pin. The big end of the connecting rod has a detachable cap, and carries 2 halves of the big end bearing. The big end is attached to the crankshaft at the crankpin journal.