Re: 1974 85hp mercury possibly froze?
If you cannot spin the prop with the motor in neutral then it is time to drop the LU and do some work on it. Water setting in the LU can rust and pit all the bearings and gears and this may be why you cannot turn the prop. If you do not have a manual for the motor, now is the time to buy one. You will need it to work on the LU and doing some reading before hand is advisable. Special tools to set the gears are needed on most of the older motors. Dropping the LU will also tell if the power head is the cause of the prop not spinning. JMO
Oldman570