Figure 5: A round head rivet is placed in the cam gear. The rivet acts as a contact to complete the circuit from the battery and around through the primary winding. This contact point or switch needs to be adjustable so that the timing of the spark will fire just as the piston comes to the top of the compression stroke. This adjustment can be as simple as just bending the contact finger that touches the rivet head. On large engines it is very desirable to have some means to retard the timing for starting, and then advancing it for running.