Junk head

Head sealing

It is essential for any piston engine to seal the joint between block and head. This is usually done by means of a head gasket, a flat gasket on the surface of the block. In early engines, the high pressures and temperatures made it difficult for the materials of the day and gasket failure was common. The junk head requires no head gasket and is sealed by piston rings inside the cylinder bore, as for the power piston.

An alternative solution was the monobloc engine, where the block and head were formed as one piece. This solved the sealing problem, but complicated manufacture and maintenance. Particularly when routine maintenance still required frequent head removal for de-coking, on a monobloc engine this required the removal of the pistons from the crankshaft end beneath the engine.

Origins of the term

The term "junk head" originates from these sealing rings or "junk rings" and the dummy piston head. The term was previously in use with steam engines. Whereas a piston ring must slide within the cylinder and so be made of the best quality long-lived materials, where a fixed cylinder cover was sealed by a ring, this stationary ring could be of the lowliest materials or "junk", often a greased or graphited rope or oakum packing. Although the high cylinder temperature of an internal combustion engine requires high quality materials even here, the term stuck.

Sleeve valve engines

The junk head was only widely used with sleeve valve engines. These sealed on the inside of the sleeve, also sealing the sleeves themselves from the combustion pressure. It would also have been difficult to arrange a flat head gasket on the top of the moving sleeves.

Sleeve valves were rarely used in road vehicles, although the Knight engine did enjoy some limited popularity in luxury cars.{{cite book

Radial engines

]] Radial engines, almost universally air-cooled aero engines, almost all use single-cylinder blocks, with a separate head for each cylinder. Most use monobloc blocks and heads, referred to as 'barrels'. The single-cylinder arrangement of each head, together with air-cooling, remove some of the restrictions on the use of a junk head. Accordingly, most of the junk head designs or number of heads manufactured have been for air-cooled radial engines.

Where radial engines have used sleeve valves (notably those of the Bristol Engine Company), the junk head was the usual design of head.{{cite book

Cooling

Notes

References