One of my landscaping customers recently gave me a 1970 Merc 7.5. Was stored in his shed for at least 10 years. Motor looks good, no signs of a lot of use. But! can't turn over the engine. I squirted "PB Blaster" penetrant into the cylinders, and tapped the pistons lightly daily. But, after sitting for 10 days or so, won't budge. A friend suggested that the driveshaft gears may be frozen. Before I take the lower unit off (never did it before) is there anything else I should be trying? Is there any way to determine the source of the seizing with out dismantling engine components? Thanks for any suggestions.