Sierra Marine Ignition Switch MP39780 Bayliner — 7/8-inch Short-Body, 3-Position 15A/30A

The Sierra MP39780 Bayliner marine ignition switch is a three-position, marine-grade replacement switch rated for 15A ignition and 30A solenoid control, designed to fit a 7/8" dash hole with a short reach housing.

This compact ignition switch is built for recreational and small commercial boats where space, corrosion resistance, and reliable electrical control matter. Molded from glass-filled polyester and fitted with brass backup hardware, the MP39780 provides a sealed interface between the cylinder and contact plate to minimize oxidation and electrical faults in salty, humid environments. It’s a conventional three-position unit with three screw-tab terminals for simple wiring and straightforward replacement or retrofit installations.

How does this switch fit my boat’s dash and wiring?

The switch is designed for a 7/8" (22.2 mm) mounting hole and a short-length body that accepts panels up to 5/8" thick. It uses three screw-tab terminals that match common marine ignition and solenoid wiring layouts, making it compatible with many Bayliner and similar small-boat installations. You can swap it into existing dash cutouts or use it during refits where space is limited.

• Mounting hole: 7/8" diameter
• Maximum panel thickness: 5/8" (short housing)
• Terminal type: 3 screw-tab terminals for ignition/solenoid/battery
• Drop-in replacement for many Bayliner-style switches

What materials and protections prevent corrosion and electrical shorts?

The housing is made from marine-grade glass-filled polyester to resist moisture and mechanical wear, while a brass backup nut adds secure, corrosion-resistant fastening. An inner seal between the cylinder lock and the contact plate limits water and salt intrusion, and the non-conductive housing reduces the risk of accidental shorts against the dash structure.

• Glass-filled polyester housing for weather resistance
• Brass backup nut and hex black plastic face nut
• Inner seal to limit oxidation at contact points
• Non-conductive housing to prevent stray shorts

What are the electrical ratings and how should I use them?

The ignition circuit is rated at 15 amps at 12 V DC, suitable for starter circuits and ignition systems on small engines. The separate solenoid terminal is rated to 30 amps at 12 V DC, accommodating standard starter solenoids. These ratings guide correct wiring: keep the starter load on the solenoid terminal and use the ignition terminal for accessory/ignition loads within the 15A limit.

• Ignition rating: 15 A @ 12 V DC
• Solenoid rating: 30 A @ 12 V DC
• Three-position conventional operation (off/accessory/start)
• 3 screw-tab terminals for clear wire connections

How easy is installation and maintenance?

Installation is straightforward: push the switch into a 7/8" hole, secure with the included brass backup nut and face nut, and connect wires to the three screw tabs. Periodic checks of terminal tightness and seal integrity keep the switch performing well; if corrosion appears, cleaning contacts and replacing the seal will extend service life.

• Push-in mounting with brass backup nut and hex face nut
• Simple screw-tab wiring — no special connectors required
• Inspect terminals and seals during routine maintenance
• Compatible with standard Bayliner dash panels and wiring harnesses

Who is this for: The Sierra MP39780 is for boat owners, marine electricians, and repair shops replacing worn or corroded Bayliner-style ignition switches on small to mid-sized boats. It suits anyone needing a compact, sealed, and rated three-position switch with straightforward wiring and good resistance to marine environments, especially where panel depth is limited and reliable solenoid control is required.