There are two basic types used on classic Italian scooters. The main difference between the two types is the reach of the threaded section into the combustion chamber. If you use the wrong plug you could cause serious damage to the engine. (A good rule of thumb is older scooters pre rally use B#HS-short, later model scooters and Lambrettas use B#ES-long plugs.)
To de-code a NGK plug look below:

B: Indicates 14mm thread diameter in the cylinder head. This diameter is used on all Vespas & Lambrettas

# = 1-9: Indicates temperature (1 Hottest - 9 coldest).

H: Indicates 12.7mm thread reach (approx 1/2") Short
E: Indicates 19mm thread reach (approx 3/4") Long

S: Standard electrode


Depending on how you ride and your distance should be the factor on what temperature plug you should use.
B9: This is a cold plug recommended for long highway riding.
B8: This plug because of the lower tempare is good for highway rides and longer city rides.
B7: This plug being hotter then the B8 is recommended for quick rides.
B6: This is sometimes referred to as a hot plug. Many kitted bikes will use this plug.  If you are having problems with your carb running rich this can be a short term fix.


Cross Reference Guide. (Please double check the plug first with your owners manual/service guide)
NGK B9ES – Champion N2C - Bosch W3C
NGK B8ES – Champion N3C - Bosch W4C
NGK B7ES – Champion N4C - Bosch W5C
NGK B6ES – Champion N5C - Bosch W7C

NGK B9HS – Champion L77C - Bosch W2A
NGK B8HS – Champion L78C - Bosch W3A
NGK B7HS – Champion L82C - Bosch W5A
NGK B6HS – Champion L86C - Bosch W7A