225HP wont start, guages dead, trim dead

[panasonic]

Still sounds like you have not measured voltage when doing these tests when hooked battery directly to the motor.
and we have no idea what you are using to hook them together with or where you are hooking them up to. Junk aluminum or even small copper do not work well

as I said before if the solenoid is pushing the gear into the flywheel, it still has to close contacts inside the unit to let voltage and current to flow into the starter. if those contacts are not making up due to burned areas then the starter will not spin.
If you do not hook up to the solenoid properly and energize it properly it will no do what it is suppose to do. But the battery voltage should not be dropping that far if the contacts are not energizing the starter to turn.

maybe if the battery voltage is dropping that far when doing all of this, the motor or lower unit is binding keeping the motor from turning

99Yam40 +1 on everything you said.

Hook it up like shown in attached diagram to get the voltage drop across that starter solenoid. NOTHING from the boats wiring should be connected to the starter for the test.

Let us know what happens

[zenoahphobic]

I'll buy into this.
It was said a couple of times that the starter spins on the bench but does not when installed on engine.
1. One needs to ask what the differences in the two setups are and critically examine and think.
2. I keep wondering about the earth of the starter when attached to the engine block but then there seems to be because that would be easy to clean and you would have done that... but still critically look at the starters mating with it's mount.
3. There is a strong suggestion the starter is binding either through some distortion or tightness towards the flywheel. Maybe the pushing of the gear is restricted at the very end of its travel or indead (excuse pun) the inerds of that part of the starter are being restricted to get its full travel to make the contact to spin the motor.

Critically look at what is different between the bench set up and the on engine setup.

And remember fundamental electricity principles. A voltage drop can occur through resistance in which the intended circuit current becomes LOWER.... the voltage being shared differently. Or the voltage drop can occur because the intended current of the circuit is HIGHER.. such as in a short or stalled motor condition.

[scofflaw]

Big diff. would be a no load condition on a bench test