Unlike the VW Beetle engine block, the waterboxer engine block is made of much sturdier aluminum (rather than magnesium). They were all manufactured this way, which makes them extremely durable. Additionally, a key change in the crankshaft design of the waterboxer greatly reduced the stresses on the center main bearing, which is an area where Beetle engine blocks commonly failed (see our article on crankshafts for more details). The material choice, combined with the new crankshaft design, resulted in practically-unlimited engine block life, barring corrosion or catastrophic failure due to lack of lubrication (running without oil) or cooling (running without coolant).
There were two types of water boxer engine blocks produced, one from 1983 to 1985 used to build 1.9 liter engines, and a different block starting 1986 to end of production in 1992. These are the differences between the 1.9 engine block and the 2.1 block:
1) Main bearing #1 design: 1.9 is one-piece. 2.1 is three-piece.
2) The 1.9 was not originally fitted with an oil cooler.
3) The 1.9 was not originally fitted with a second oil pressure switch near pulley.
4) Early 1.9s are slightly different on top where the plenum bolts on (pre-85 only).
5) The 1.9 has a slightly smaller inside dimension.
Contrary to what some folks believe, the 1.9 liter block is no weaker than the 2.1 liter design. It is made from the same material and is practically, for all intents and purposes, identical. The 1.9 liter and 2.1 liter engines are identical with respect to piston diameter, and the cylinder heads were almost identical (in fact, interchangeably so). The extra 200cc comes from the crankshaft stroke. The 1.9 has a crankshaft stroke of 69mm (same as the 1600cc beetle). The 2.1 was fitted with a 76mm stroke crank. The 2.1 liter block is a little bigger inside for that reason. As it turns out, to fit the 2.1 crankshaft in a 1.9 block requires practically no modification on the inside at all. The only place where the crankshaft and rods come too close for comfort is in the area of the head stud hole bosses, the other side being where some of the head studs fasten. We are talking less than one millimeter, literally.
That said, need the extra space offered by the 2.1 block to build engines that require larger crankshaft strokes than used in the 2.2 and 2.3. For that reason, our core policy is that if we send out an engine built with a 2.1 liter block, we expect a 2.1 liter block back. But if we send out a 1.9 liter block, either a 1.9 liter block or a 2.1 block will suffice for full core credit (assuming the core is in rebuildable condition—click here to view our core refund sheet).