Possible Clues:

• Automobile engine part
• Device that operates valves in an engine
• Vital bit of gear arrived shortly before opening of mine
• Device that operates valves in internal-combustion engine
• Rod to operate valves in an engine
• Caught hams switching at the back for operating valves
• Return of Scot (don't tell anyone) behind in engine
• Part of an internal-combustion engine
• Short distance behind river has included broken part of engine
• Engine part, half of machine's fast revolution