In most types of

reciprocating engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common featu ...

s, a valve guide is provided for each

poppet valve A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...

in the

cylinder head In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinders and forms the roof of the combustion chamber. In sidevalve engines, the head is a simple sheet of metal; whereas in more modern ov ...

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

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. The valve seat is a critical component ...

. A valve guide is a cylindrical piece of metal, pressed or integrally cast into the cylinder head, with the valve reciprocating inside it. Guides also serve 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 is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids such ...

is commonly used, as is

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

; a balance between stiffness and wear on the valve is essential to achieve a useful service life. The clearance between the inner diameter of the valve guide and the outer diameter of the

stem is critical for the proper performance of an engine. If there is too little clearance, the valve may stick as oil contaminants and thermal expansion become factors. If there is too much clearance, the valve may not seat properly and excessive oil consumption can occur. Over time, the inner diameter of the valve guide and the outer diameter of the valve stem may become worn. In the 1980s, many U.S. production engine remanufacturers began reaming valve guides, rather than replacing them, as part of their

remanufacturing Remanufacturing is "the rebuilding of a product to specifications of the original manufactured product using a combination of reused, repaired and new parts". It requires the repair or replacement of worn out or obsolete components and modules. ...

process. They found that by reaming all the valve guides in a head to one standard size (typically 0.008 in. diametrically oversized), and installing remanufactured engine valves having stems that are also oversized, a typical engine head can be remanufactured in much less time. Since the reaming process leaves the valve guide with a much better surface finish and shape than typical replacement guides, and since the oversize valves often have chrome plated stems, remanufacturers also discovered that

valve train A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...

warranty issues are virtually eliminated. Studies have been conducted which show that through the proper selection of the

reamer A reamer is a type of rotary cutting tool used in metalworking. Precision reamers are designed to enlarge the size of a previously formed hole by a small amount but with a high degree of accuracy to leave smooth sides. There are also non-precisio ...

and reaming process, valves guides can be quickly and efficiently reamed to a consistently repeatable size.

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 A reamer is a type of rotary cutting tool used in metalworking. Precision reamers are designed to enlarge the size of a previously formed hole by a small amount but with a high degree of accuracy to leave smooth sides. There are also non-precisio ...

and honed 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 An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...

and

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'' (from the Anglo-Saxon ''manig'' anyand ''feald'' old and refe ...

s. Modern engines are fitted with valve stem oil seals which can be replaced if oil leakage occurs.{{cite web, url=http://www.nwideuk.com/oil-seals/valve-stern-seal.asp , title=Valve Stem Seals, publisher=Nationwide Bearing Co., access-date=2017-02-26

References

Automobile engines Mechanisms (engineering) Engine valves