I believe from what you've said that when you laid the boat up for a couple years, that one or more of the valves were open enough to allow air, which has moisture in it to Rust the cylinder wall of a cylinder or two.
As you cranked it a bit at a time, the piston moved up enough to scrape the rust off the wall(s), enough to add enough rusty debris to block any further movement up the cylinder, as the space between the cylinder head
and piston is very tight at the top.
That's why the compression
is high in a diesel
because of this very tight space.
So that rust may have plied up in there preventing further upward movement of the piston.
How to remedy this, or inspect is a bit of a challenge.
I would think that you could try to get it to move back to where it was, by reversing the rotation with a socket and ratchet or breaker bar on the crank bolt.
If it's stuck tight, pull the valve cover and inspect, you might see some rust in this area as well.
If so, pull the head
, clean out and replace the gasket
If that isn't the cause, drain the oil
and look for metal flakes in the drippings, could be a spun bearing. Hope not.
Possibly a partially seized piston, or rings.
You can try to inspect thru the glow plug
area, with a boroscope, but don't think there is direct access to the cylinder there,
As most diesels have a pre combustion chamber.
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