One of the aftermarket tool manufacturers used to sell a tool that installed into the head after removing the center head bolt (with the engine assembled). By pouring the head bolt hole full of oil, inserting the tool, and smacking it with a big hammer, a clogged oil passage could be cleared with hydraulic pressure. The tool was just a close-fitting punch that allowed the oil to be forced backwards through the oil galleries, hopefully clearing a clog. I made my own tool, using a piece of hardwood broomstick whittled to fit. Worked like a champ!
Now that you have the head off, try squirting some carburetor cleaner down the oil passage and forcing it through with air pressure.
Jerry
