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After the proper size hole is bored into the guide, a steel or bronze insert is then hammered in and cut to length. If inserts are not going to be used, then the guide can reamed oversize, and a valve with an oversize stem installed. Valves are usually offered for most engines with 0.003-, 0.005-, 0.015-, and 0.030-inch oversized stems. The head shop will also be used to perform any necessary machine work for the installation of valve stem seals. There are a number of different types of seals. Some rest over the guide while others attach to the valve stem itself. The most effective are the "positive insert" style. These are held securely to the valve guide with a snap ring and contacts around the valve stem.