A magnetic pickup is an AC generator.

It is normally installed into the flywheel housing of an internal combustion engine, so that the starter ring gear acts upon it to generate a voltage pulse each time a gear tooth passes the end of the sensor.

Pickup Model Part Number Total Length Threaded Length Thread Size
MP3298  20700162  3 in. (76 mm) 3 in. (76 mm) 5/8-18 UNF
MP7906  20700161  3 in. (76 mm) 3 in. (76 mm) 3/4-16 UNF
MP7905  20700160  4-1/2 in. (114 mm) 4-1/2 in. (114 mm) 3/4-16 UNF