...because it allows a longer hammer stroke and therefore more hammer kinetic energy for any given hammer spring force. The length of the valve stem is seldom what determines lift -- hammer energy + valve seat reslience + valve closure spring force versus pressure do.
Mainly, a long valve stem is a safety/reliability feature, because it limits the likelihood of (in the case of 22xx style power plants) the cocking pin striking the end of the bolt.
That's why, in 22xx's, if you shorten the stem you should also consider shortening the bolt.
This message has been edited by pneuguy from IP address 22.214.171.124 on Jun 28, 2011 9:59 AM