Leyland Engines, lorry, truck and van engines, Wolseley 2200, Austin 18-22, Morris 18-22

A pushrod engine or I-head engine is a type of piston engine that places the camshaft in the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine) and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets reside in the engine block between the camshaft and pushrods.
This contrasts with an overhead cam (OHC) design which places the camshafts above the cylinder head and drives the valves directly or through short rocker arms. In an OHC engine, the camshafts are normally part of the cylinder head assembly, while in an I-head engine the camshaft (rarely more than one) is part of the main engine block assembly.
Pushrod engines are perceived to be "old fashioned" by the modern automotive press, the cause is historical: While both layouts are over 100 years old, the I-head engine came first. OHC engines were developed as more expensive high-performance engines and have largely replaced the pushrod design in countries where cars are taxed based on engine displacement.

The straight-4 or inline-4 is an internal combustion engine with four cylinders aligned in one row. Nicknamed a four-banger, this straight engine configuration is the most common in cars with a displacement up to 2.3 litres. The current "practical" limit to the displacement of straight-4 engines in a car is around 2.7 litres. However, in tractors and other construction machinery, four-cylinder diesel engines with a displacement of 4.5 litres and above can still be found. The straight-4 engine is not a balanced configuration. While a pair of pistons is always moving up at the same time as the other pair is moving down, piston speed - as with all internal combustion engines - is higher through the top 180 degrees of the stroke than the bottom 180 degrees, which leads to a secondary harmonic imbalance. While this is tolerable in a small low-displacement low-power configuration, the vibrations get worse with increasing size and power. Most straight-4 engines below 2.0 L in displacement rely on the damping effect of their engine mounts.

The straight-6 (also inline-6, I-6, or I6) is an internal combustion engine with six cylinders aligned in a single row. The name slant-6 is sometimes used when the cylinders are at an angle from the vertical. Straight-6 engines have perfect primary and secondary balance and require no balance shaft. Usually a straight-6 was used for engine displacements between about 2.5 and 4.0 L. It was also sometimes used for smaller engines but these, although very smooth running, tended to be rather expensive to manufacture and they where inevitably physically longer than alternative layouts.