The ideal way for water circulation is the thermosyphon system, as used in DKW cars, solar water systems, and some old stationary engines on farms. No pump is needed there. Hot water rises to the top, and with correct pipe routing, cold water takes its place from the bottom. The bigger the temperature difference, the faster it circulates.
For this to work successfully, the radiator must be mounted higher than the engine. And most importantly, the water outlet from the cylinder-head must go upwards all the way to the top of radiator. Straight up, not down underneath the chassis first before going to the top of the radiator in front. That is why the coolant exits the DKW cylinderhead right at the top, in a funnel-type shape if i remember correctly. Upwards to the radiator wich is mounted high on the firewall side, above the gearbox. No chance of air pockets there, it rises straight to the top. Excellent system, no waterpump used by a DKW.
The opposite happens with VW conversions. Hot water and air bubbles/steam rises to the top of the engine, and fights against efforts of the pump to move it downwards to underneath the chassis. The hotter the engine gets, the harder the water/steam fights against the pump trying to circulate it in the opposite direction. So when the car is started from cold, all seems fine, until it gets hot. Then suddenly things changes fast, circulation slows, air/steam accumulates in the highest parts of the engine. Once air/steam enters the waterpump, circulation comes to a halt...and we all know what happens then. (It is called a waterpump, it does'nt pump air).
Most important is to mount the water-expansion tank much higher than the engine. From all the highest positions of the cooling system, (like the heads), pipes must go straight upwards to this tank. Then hopefully air/steam will leave the engine, making space for water...
The wasserboxer should not be as bad, because the heads are lower than upright engines. Vw still did the expansion-tank thing there, and at one stage recommended that the front or rear of vehicle had to be jacked up high when filling water. Think the 2.6 kombi used two waterpumps, one electrical.
With a beachbuggy, one can take the top water-outlet from the engine around te side ,(beach buggy has no doors) to the top of the radiator in front.
Maybe you already have your expansion-tank connected correctly. If you still have a problem, all i can think
is upgrade to an aircooled engine, or get a DKW car. (Just joking).
This is a german set so that explains the high price.Rings i dont know yet as i need sizes to confirm stock and price.
Let me know will ya, please?