Radial. Extending from a center to the circumference, having the
direction of a radius.
Reciprocating. Having a back-and-forth or up-and-down linear motion,
such as an engine piston.
Reclaimer. An apparatus in which dirty lubricating oil, which often
is discarded is treated and made usable, reclaimed.
Relief valve. A valve held closed by a spring and forced open when
the pressure in the system rises above the desired height.
Resistance. Mechanically, a force opposing the motion of a body measured
in pounds. Electrically, that which opposes the flow of an electric current
measured in ohms.
Rheostat. A device to regulate the flow of electric current by transforming
part or all of it into heat.
Ring grooves. Grooves cut in the piston barrel to hold the piston
rings.
Rocker arm. A lever that transmits the action of the cam, usually
by means of a push rod, to the stem of the intake or exhaust valve, sometimes
also to the starting-air valve and fuel valve.
Rocker-arm shaft. The shaft, usually at the top of the cylinder,
that serves as a fulcrum for the rocker arms.
Rotary. Turning on an axis.
Rotative. Pertaining to rotation.
Rpm. Abbreviation for revolutions per minute.