Many early generation waterboxers suffered from premature cylinder head coolant leaks that gave them a reputation as being a poorly designed, unreliable powerplant. This couldn't be further from the truth. Read on!
The common "leaky head gasket design" problem on Vanagons is neither a head gasket nor a design problem. The part that actually fails and leaks coolant externally is the water jacket seal, not the head gasket.
As you can see in photo #1 (below), the cylinders in the waterboxer design are free-standing. The actual head gaskets are the small all-metal rings that sit between the tops of the cylinders themselves and the bottom of the cylinder head. They seal the combustion gases inside the combustion chamber, one per cylinder. The water jacket seal is an all-rubber, U-shaped cross section gasket that seals the water jacket from leaking coolant out, one per cylinder head, as seen below in photo #2.
When VW introduced the waterboxer in 1983, they did not realize the extent to which the design was susceptible to corrosion. The corrosion problem, particularly between the water jacket seal and cylinder head (see photo #3), was exacerbated by the presence of phosphate in the coolant. On early waterboxers it was not uncommon for the water jacket seals to fail and leak coolant as early as 40,000 miles, or two to four years. VW replaced countless water jacket seals under warranty, and the stigma has plagued the waterboxer image ever since. VW realized the problem and switched to a phosphate-free coolant. The problem was pretty much solved, but not entirely. Even with phosphate-free coolant, the typical lifespan of the water jacket seals were no more than 10 years or 140,000 miles. Out of warranty, for sure, but still not up to modern-day reliability standards by a long shot.
So what's the answer?
The trick to keeping this problem from occurring on your waterboxer-powered Vanagon is to simply flush the coolant every two years with phosphate-free coolant and distilled water. We have taken engines apart after five years with ABSOLUTELY ZERO corrosion. So what started out as somewhat of a design issue has turned into a maintenance issue.
So, what kind of coolant is the best? Contrary to popular folklore, it is not essential to obtain at all costs the genuine German, VW-endorsed, flown-over-from-the-homeland Autobahn (blue) coolant. ANY coolant that is PHOSPHATE-FREE will work fine. That, distilled water, and flushing every other year is the ticket. Here at GoWesty, we have the building plumbed with Exxon brand coolant mixed 50/50 with distilled water. That's the way we buy it, pre-mixed, and it works great. This is a non-issue, people! On to the next problem!