Twin-cylinder four-stroke air-cooled rebirth of a Ducati 500SL







At the moment the spark jumps the Gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs).


Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level.  Most Resistorspark plugs use a monolithic Resistor, generally made of graphite and
glass materials, to filter the electrical voltage as it passes through theCenter Electrode.


Since Resistor type plugs actually “resist” some of the spark energy, non-Resistor type plugs actually deliver a more powerful spark.  It is for this reason that most racing plugs are non-Resistor types.  However, in most automotive applications, a Resistor plug is required for proper vehicle operation.  Use of non-Resistor plugs in
vehicles that call for a Resistor type can result in rough idling, high-rpm misfire, and abnormal combustion.


The stock plugs in the Alazzurra 650 engine were NGK BP7HS spark plugs.

I replaced the stock plugs with Denso Iridium IWF22 plugs. I had these in my 1974 Land Rover, and have them in my Jaguar XJR as well. I really started using them in the Land Rover and they were the only plugs that ever really ran smoothly in my 2.25L. I haven’t done any other experimentation in my Pantah, and I am sure cheaper plugs will work equally well. Let me know if you have had success or failure stories.
Or visit the Official Denso Iridium page.

Other spark plugs that will fit the Alazzurra 650 engine: (these links go to … no affiliation)

  1. Autolite options
  2. Champion Options
  3. Denso Options
  4. NGK Options
  5. Splitfire Options

Begin by removing the spark plugs from the vertical and horizontal cylinders. This is a straightforward procedure unless the plug is corroded in the head.

Ducati provides a spark plug removal tool in the tool kit that comes with each bike. This is a pipe-like tool with a hex formed at one end and a hole through which the handle is placed. It fits, it works, but it isn’t the best choice if you have access to a shop.

I use this socket set from Sears and a short 3/8″ extension on a socket wrench to remove plugs. The spark plug sockets have a rubber insert that holds the plug in place when you remove it. It works equally well when inserting plugs. The main benefits are that you don’t drop the plug and knock dirt and grime into the plug holes and it also prevents changing the plug gap if you drop the plug while inserting it. Both of these things fall into the “I probably won’t fix it” category.

Using the right tools for removing a spark plug may not seem like a big deal. The second you snap off the head of a plug or crossthread the head, you’ll make sure you have the proper tools. I have done both over the years. I am always wary of using a socket extension that is too long. Whether you are loosening or tightening the plug, a 12″ or so extension tends to angle itself on the plug and can result in a snapped plug.

If you are not planning on rebuilding the engine, make sure to plug the holes with a cork or rubber stoppers to prevent dirt from entering. Grime tends to collect aroud the plug holes and removing the plugs usually knocks nasty stuff down there.

Be sure to check out my page on sourcing replacement spark plugs here.