6V Bosch "Blue" Coil (they are Silver now though!) Coil, US Version (0 221 124 001) is a factory replacement Bosch Blue Coil with the ~2 ohm impedence compatible with points, and points replacement devices, in 6V electrical systems. There are 5 DIFFERENT "Bosch Blue Coils" last we checked, only ONE is correct for you (This one). If you are running points, or a a points replacement device (Compufire or Pertronix), you MUST make sure your ignition coil has the ballast resistor. Stock Bosch blue coils have the 2 ohm ballast resistor built INSIDE the coil. If your coil or primary ignition winding doesn't have this ballast, the distributor will BURN UP and leave you walking.These are CALLED a "Blue Coil", but Bosch has recently changed the actual color of the coil to silver.
To check your existing coil, first make sure you have 6V going to the terminal 15 on the coil (Terminal 15 is positive, terminal 1 is negative (think of 1 sideways being negative). If you do, then pull the wire out of the center of the distributor with a rubber handled pair of pliers, and hold it about 1/4" from the metal clips that holds the distributor cap on. Have someone crank over the engine, and there should be a nice blue spark that goes from the end of the wire you pulled out of the center of the cap to the metal clip. It should look and sound strong, and should be easy to see, even in daylight. If it looks thin and weak (yellow is weak, blue is strong), then the coil windings may be giving out.
Another test for the Bosch Blue coil, is to use an Ohmmeter and with all the wires taken off the terminals of the coil attach the positive and negative, red and black, wires of the meter to first the terminal 1 and 15 that are printed (stamped) on the coil. You should have a reading of at least 1.5 ohms, which is the resistance reading inside the coil. A bad coil will show a higher reading then 2, you want less resistance.
Next, place the red or black lead from the meter to the center of the coil (secondary post), and to either one of the terminals, again 1 or 15 on the coil. You want 9500-10000 Ohms, sometimes you'll see less, but you don't want to see something like 11,000. Or if you have no reading in either case, the coil is trash.