Engine knock

Unknown

Hi,

 

My recently purchased 1921 Super 6 engine has a knock when the engine is idling and not under load.  When I short out the No4 spark plug, the knock stops, so I assume it is something to do with No4 piston, conrod, little end or big end. I do not hear the noise when the engine is under load because either the knock stops, or the knock is masked by the general engine noise.

 

Is it safe to carry on driving?

 

Assuming the repair is essential, can I draw out the piston and connecting rod through the sump, or do I need to take off the head and draw it that way?

 

What are the tolerances required for big end white metal bearings and for little ends? 

 

What are the generally accepted maximum revs for a Super 6 engine? (I am thinking of discretely installing a 6 volt aircraft type rev counter to make sure I don't get too enthusiastic with my right foot.) 

 

If any of you knowledgeable folk out there can help, I would be most grateful,

 

Robert