What size diodes do I need when adding the throttle mounted trim switch for a twin engine? I really don't want to pay $30-40 dollars for the diode kit when diodes are very inexpensive.

Thanks.

1n4004

1n4004 5, 6, 7.... all are 1 amp with increasing maximum voltage ratings and any would be fine except for a small flimsy wire size on the leads.
Most of the time you find a more robust package like on a 1n5408 3 amp 1000 PRV (peak reverse voltage)
The Voltage rating is the amount of reverse kick transient voltage produced by an inductive load ( a 12 or 24 Volt solenoid or relay coil produces a few thousand volts for part of a second when the magnetic field collapses on turn off.

If your using a trim switch with something like 22 AWG wire you don't need to use diodes. You need to use relays.

There are two circuits. One is up travel, one is down. Each will have its own relay. Your trim switch sends the signal to the relay to activate another switch (the relay), which handles all the work (high amp load). Only one circuit can be working at a time; you don't need to use diodes.

Relays are cheap, and so are the harnesses they plug into.

While your at it why not just add a double switch to the throttle handle to control tabs/drives.