There are a couple of copper tubs with small holes in them
that run the length of the head. The cooling water enters
the ends of these tubes and comes out the sides through
some relatively small holes. This distributes the cooling
water properly throughout the length of the head.

Volvo Penta 2001 (9 hp diesel) has the same problem.

These tubes become clogged and the only way to really cure
the problem is to remove and replace them or clean them
manually if enough metal remains to be cleaned in the
first place.

But with 2001 it is enough to take a 20 cm long wire and
scratch the hole open. I do it once every three years.

Markus Sadeniemi