Monobloc cylinders

A monobloc or en bloc engine is an internal-combustion piston engine where some of the 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 also improving the reliability of the sealing between them.

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

In most cases, any use of the term describes a deliberate single-unit construction, opposed to the more common contemporary practice. Where the monobloc technique has later become the norm, the specific term falls from favour. It is now usual and un-noteworthy 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 avoiding 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 and burning. 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) becomes difficult. Access through the cylinder bore is restricted for machining the valve seats, or simply for inserting angled valves. An even more serious restriction is that for the maintenance task of de-coking and re-grinding the valve seats, a regular task on older engines. Rather than removing the cylinder head from above, the mechanic must now remove pistons, connecting rods and the entire 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. 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. However this also placed the spark plug further away from the main combustion chamber, leading to long flame paths and slower ignition.

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Wikipedia