[supereal]

Many of the replacements now sold use a plastic tip. With today's gas, the utility of the friction retard system is hardly necessary. Years ago, when "knock" (detonation) was heard on hard pulls, the cure was to increase the spring pressure via the adjuster on the top of the distributor. Intake manifold vacuum was supposed to hold the piston back from contact spring plate until an open throttle reduced the vacuum and allowed the piston to be pressed down by the spring. The high octane content of today's gas makes this crude device obsolete, and often interfere with engine efficiency.