I had to replace the ring gear on my flywheel, and also installed a new starter. I hook everything back up and all I get is a click when the solenoid kicks in. The starter motor never turns over. I have checked and rechecked the wiring and all appears to be in order. I pulled the starter back off thinking it might be bad, and took it to Advance Auto Parts to test. It worked fine. Reinstalled it and the problem continues to exist. It almost sounds like when the solenoid kicks in the starter gear hits the flywheel and stops. Any idea of causes/cure?

I did put all the shims in as when I had the engine on the stand it appeared slightly tight.

Same tooth starter& same tooth flywheel?

Hook a jump pack directly to the positive on the starter, ground to engine, and jump the solenoid or turn the key. If it works then you have a connection issue which is very common after winter storage. Clean all connections and proceed.

Seeing that you had the flywheel off, did you get all the ground wires back on the bellhousing when you reinstalled the engine? Check and recheck.

the mercury slave solonoid is probibally bad,they ane not expensive and fairly easy to change.

Can you turn the engine over manually with the a breaker bar on the crank bolt?

+1

I would try another shim or 2.
Careful turning eng over using the crank snout bolt. It has fine threads. Strip those threads and you got big problems.

After considerable searching, it appears I have narrowed the issue down to the neutral safety switch. It works when I wired the ignition direct to the solenoid bypassing the neutral safety switch. The standard Mercury stuff is gone as I have a custom 555 big block in the boat. It is a 2001 Baja Hammer with twin handle controls. I can't tell who manufacturers the controls. Can I buy a separate neutral safety switch? Are they generic or specific to the control?