Editing Poppet valve (section)

=== Materials and durability ===
In a typical modern mass-production engines, the valves are solid and made from steel [[alloy]]s. However some engines use hollow valves filled with [[sodium]], to improve [[heat transfer]].

Many modern engines use an aluminium cylinder head. Although this provides better heat transfer, it requires steel [[valve seat]] inserts to be used; in older [[cast iron]] cylinder heads, the valve seats are often part of the cylinder head. A gap of {{convert|0.4|-|0.6|mm|in|3|abbr=on}} is present around the valve stem, therefore a valve stem [[oil seal]] is used to prevent oil being drawn into the intake manifold and combustion chamber. Typically, a rubber lip-type seal is used. A common symptom of worn valve guides and/or defective oil seals is a puff of blue smoke from the exhaust pipe at times of increased intake [[manifold vacuum]], such as when the throttle is abruptly closed.

Historically, valves had two major issues, both of which have been solved by improvements in modern [[metallurgy]]. The first was that in early internal combustion engines, high wear rates of valves meant that a [[valve job]] to regrind the valves was required at regular intervals. Secondly, [[Tetraethyllead#In motor fuel|lead additives]] had been used in petrol (gasoline) since the 1920s, to prevent [[engine knocking]] and provide lubrication for the valves. Modern materials for the valves (such as stainless steel) and valve seats (such as [[Stellite]] and [[inconel]]) allowed for leaded petrol to be phased out in many industrialised countries by the mid-1990s.