In most types of reciprocating engine

Reciprocating engine

A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...

s, a valve guide is provided for each poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

 in the cylinder head

Cylinder head

In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...

. Along with the valve spring, it serves to positively locate the valve so that it may make proper contact with the valve seat

Valve seat

The valve seat in an internal combustion gasoline or diesel engine is the surface against which an intake or an exhaust valve rests during the portion of the engine operating cycle when that valve is closed...

. A valve guide is a tube-shaped piece of metal, pressed into the cylinder head, with the valve reciprocating inside it. Guides serve also to conduct heat from the combustion process out from the exhaust valve and into the cylinder head where it may be taken up by the cooling system. Bronze

Bronze

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 is commonly used, as is steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

; a balance between stiffness and wear on the valve is essential to achieve a useful service life.

Aircraft engines

In aircraft piston engines, the valve guides are typically shaped in a tube with a flare at one end, so as to resemble in section, the letter "T". Their replacement involves removing the worn part by driving it out with a hammer and specifically shaped punch. Installation involves heating the cylinder (with its integral head) and freezing the valve guide so as to ease insertion, and then driving the new guide in (quickly) with a hammer. Once the parts return to room temperature the new valve guide will be solidly in place and ready to be reamed

Reamer

A reamer is a metalworking tool used to create an accurate sized hole. The process is called reaming. They may be used as a hand tool or in a machine tool, such as a milling machine or drill press.-Construction:...

 and hone

Hône

Hône is a town and comune in the Aosta Valley region of north-western Italy.-Twin towns:twin towns with the year of its establishing:# Nora Municipality, Sweden...

d to proper diameter...

Oil seals

As valve guides wear, their ability to positively locate the valve to the valve seat decreases. As the valves lose their ability to seal the combustion chamber properly, the engine can lose performance and start to burn oil, leaking from the top of the cylinder head into the intake

Inlet manifold

In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...

 and exhaust manifold

Exhaust manifold

In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...

s. Modern engines are fitted with valve stem oil seals which can be replaced if oil leakage occurs .