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.
The head gasket is the most highly stressed static seal in an engine, and was a source of considerable trouble in early years.
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.
The drawback to the monobloc head is that access to the inside of the combustion chamber (the upper volume of the cylinder) is difficult.
This placed the spark plug further from the combustion chamber, leading to long flame paths and slower ignition.
Aircraft engines at this time were beginning to use high supercharging pressures, increasing the stress on their head gaskets.
Older engines operated on unleaded automotive gasoline as allowed by supplemental type certificates approved by the FAA may require more frequent machining replacement of valves and seats.
It is almost unknown today, but has found a few niche uses, as the technique of monobloc cylinder heads was adopted by the Japanese model engine manufacturer Saito Seisakusho for their glow fueled and spark ignition model four-stroke engines for RC aircraft propulsion needs.
Monobloc cylinders also continue to be used on small 2 stroke-cycle engines for power equipment used to maintain lawns and gardens, such as string trimmers, tillers and leaf blowers.
This was a simpler construction, thus less expensive to manufacture,[15] and the communal water jacket permitted closer spacing between cylinders.
The main reason for this was to improve stiffness of the engine construction, reducing vibration and permitting higher speeds.
Being an air-cooled OHC design, this is possible thanks to current aluminum casting techniques and lack of complex hollow spaces for liquid cooling.