Re: Finding TDC
BLUF: I'm not an expert. With that said, I can't answer your specific question on the keyway, but here's a couple of other things to try.
I was taught to find the stroke where the exhaust rocker returns and the intake just starts to open, mark pulley at the top. Rotate the pulley 360 to give you the tdc between compression and power stroke.
Nigel Calder's book says to "slowly rotate the engine in its normal direction of rotation. Watch the inlet valve's pushrod as it moves up and down. When it is almost all the way down, the engine is at the bottom of its inlet stroke. Mark the crankshaft pulley, and turn the engine another half a revolution. Now the piston will be close to TDC on its compression stroke."
Should give the same spot. You can do this with a large breakover bar and a socket on the front pulley. Be absolutely sure you only turn the engine the direction the engine runs/starter turns.
How about the flywheel? I don't remember if Yanmar has them, but many manufacturers put an observation port in the back and marks on the flywheel to show each cylinder TDC and each cylinder injection timing mark. Those should be very accurate.
Sailing the "Knot Yoars," a Hunter 31, from the Middle Chesapeake around Solomons Island.
I can keep a secret, it's the people I tell who can't.