What exactly is a "big single?"  First, it's a single-cylinder motorcycle with an engine displacement of 350 cubic centimeters (cc) or more.  That's about 21 cubic inches.  Some would argue it's 500 cc, but I go with 350.

    Next, it must be a four-stroke engine, which has four distinctly separate motions of the piston for each power stroke.  They are: intake of fresh air with fuel, compression of the mixture, ignition and the power stroke, and the exhaust stroke.  Four-stroke engines have mechanically actuated valves to open the cylinder either to the carbureter (for intake) or to the exhaust pipe.  A two-stroke engne, on the other hand, has a compression stroke and a power stroke, and no mechanically operated valves.  The exhaust gases escape from the cylinder, and the fuel-air mixture comes in, through ports in the cylinder wall uncovered by the piston as it reaches the end of the power stroke.  Four-stroke and two-stroke engines both have advantages and disadvantages.  In their modern forms, however, they are roughly equal in most respects.  There are some really awesome 360 cc and 380 cc two-stroke engines, but by custom we still reserve the term "big single" for four-strokes.

    Another way to think of a big single is that it is like half a Harley Davidson V-twin.  The nickname "thumper" brings this comparison to mind.  With fewer power strokes than a multicylinder motor of the same size, the sound and feel of a thumper are quite different.  One would never describe the sound as a screech or whine or the vibration as a buzz, a common feeling with the multis.

    In the1950s and 1960s 350 cc and 500 cc were the most common sizes, although there was a 700 cc single made by Panther, and a 400 cc the name of which I can't remember.  The ultimate thumper then was the 500 cc BSA Gold Star.  What a sound it made in races with an open megaphone exhaust system!  In all these years I've never seen one on the road.

    I learned to ride a motorcycle on an AJS 500 cc single.  It was comfortable, reasonably powerful, and easy to drive.
 

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