Monobloc refers to a component that is made in one block or casting. A monobloc engine or en bloc engine is an internal-combustion piston engine some of whose major components (such as cylinder head, cylinder block, or crankcase) are formed, usually by casting, as a single integral unit, rather than being assembled later. This has the advantages of improving mechanical stiffness, and improving the reliability of the sealing between them.

Monobloc techniques date back to the beginnings of the internal combustion engine. Use of this term has changed over time, usually to address the most pressing mechanical problem affecting the engines of its day. There have been three distinct uses of the technique:

In most cases, any use of the term describes single-unit construction that is opposed to the more common contemporary practice. Where the monobloc technique has later become the norm, the specific term fell from favour. It is now usual practice to use monobloc cylinders and crankcases, but a monobloc head (for a water-cooled inline engine at least) would be regarded as peculiar and obsolescent.

The head gasket is the most highly stressed static seal in an engine, and was a source of considerable trouble in early years. The monobloc cylinder head forms both cylinder and head in one unit, thus averting the need for a seal.

Along with head gasket failure, one of the least reliable parts of the early petrol engine was the exhaust valve, which tended to fail by overheating. A monobloc head could provide good water cooling, thus reduced valve wear, as it could extend the water jacket uninterrupted around both head and cylinder. Engines with gaskets required a metal-to-metal contact face here, disrupting water flow.

The drawback to the monobloc head is that access to the inside of the combustion chamber (the upper volume of the cylinder) is difficult. Access through the cylinder bore is restricted for machining the valve seats, or for inserting angled valves. An even more serious restriction is de-coking and re-grinding valve seats, a regular task on older engines. Rather than removing the cylinder head from above, the mechanic must remove pistons, connecting rods and the crankshaft from beneath.

One solution to this for side-valve engines was to place a screwed plug directly above each valve, and to access the valves through this (illustrated). The tapered threads of the screwed plug provided a reliable seal. For low-powered engines this was a popular solution for some years, but it was difficult to cool this plug, as the water jacket didn't extend into the plug. As performance increased, it also became important to have better combustion chamber designs with less "dead space". One solution was to place the spark plug in the centre of this plug, which at least made use of the space. This placed the spark plug further from the combustion chamber, leading to long flame paths and slower ignition.

During World War I, development of the internal combustion engine greatly progressed. After the war, as civilian car production resumed, the monobloc cylinder head was required less frequently. Only high-performance cars such as the Leyland Eight of 1920 persisted with it. Bentley and Bugatti were other racing marques who notably adhered to them, through the 1920s and into the 1930s, most famously being used in the purpose-built American Offenhauser straight-four racing engines, first designed and built in the 1930s.