I am going to try to cut through some of the confusion here——from memory, lacking an official 1957 parts book or wiring diagram.
1. 1957 was kind of an oddball.
2. There are two kinds of starter solenoids, looking exactly alike. But they are different inside. A car (Ford) type sends 12V to the coil, bypassing the ignition resistor while cranking. That is not the one you want because that 12V cranking whallop will burn the wire off the mercury switch. The type used on OMC outboards has the same two small terminals, but they are connected to the two ends of the pull-in coil inside. When you operate the key or button to start, 12V is sent to one of the small terminals, goes through the pull-in coil, and out the other small terminal, finally to ground through the closed mercury switch. The coil resistance provides a voltage drop so the mercury switch does not “see” a full 12V.
The mercury switch is either open or closed, according to throttle position. If closed, the solenoid wire is grounded, and the motor will crank. If open, the solenoid wire is not grounded, and the motor will not crank. Simple as that. Note: because the mercury switch is mounted on a moving part the ground is not reliable. That is why the mercury switch body is grounded by a separate wire.
All this was changed in 1958, and the one diagram posted above is not correct for 1957.
