One very important part of the internal combustion engine is air. You need fuel to combust and the most important part is getting oxygen into the combustion chamber. That is where the intake valve comes in. The valves open and close continuously while the engine is on. They are opened and closed by a camshaft that may be mounted overhead or on the block and connected by pushrods. There are several types of valve train like double overhead cams, single overhead cam, overhead valve, inlet-over-exhaust, and side-valve. Most cars today use an overhead camshaft design with four valves per cylinder. The intake valves specifically open to allow fresh air into the engine for the first stroke of a four-stroke engine. Valves may gradually wear out or burn causing a knocking sound. Often valves can easily be bent if the engine is over-revved or the timing belt or chain fails. Replacing the valves can take a lot of time and may be expensive. The valves are often changed at the same time that the valve stem seals, the valve springs, guides and the cylinder head and intake and exhaust gaskets are changed since the work is easy to accomplish at one time.
The intake valves are a type of poppet valve that has been in use before internal combustion engines, but they are by far the most common type of valve use in today's engines. In the past some cars had a valve-less sleeve design called a Knight engine. They were used because conventional poppet valves created a lot of noise. With modern technology, the valves became quieter and the Knight engine faded from memory. They are usually made from metal but may also be stainless steel or sodium filled. The intake tends to be larger than the exhaust and the exhaust must be designed to handle a great deal of heat since is releasing the hot gases from the combustion chamber. Changing to different types may increase your car's performance. Most vehicles have one or two intake valves per cylinder so make sure that you are ordering the correct number for your car's engine.
Like any automotive work it is a good idea to contact an experience mechanic. Intake valves are an important part of the engine and caution should be taken when installing them. Some basic instructions are shown below to give you a general idea of what needs to be done to install them.