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Pistons for Waterboxer: The Whole Story

When it comes to waterboxer pistons, there are a variety of options available to the engine rebuilder. This article provides some background history on the original Volkswagen piston design and illustrates our personal experience with "off the shelf" aftermarket pistons. While the four pistons we compare below look similar, they are dramatically different in dimension, quality and overall performance and reliability.

All waterboxer engines originally had the same piston diameter or "bore" (94mm). The 1.9 liter engine (1915cc, to be exact) has a 69mm crankshaft stroke, and the 2.1 liter engine (2110cc, to be exact) has a crankshaft stroke of 76mm. The difference between the 1.9 liter piston and the 2.1 liter piston is the design and size of the dish at the top—and, more importantly, the position of the wrist pin relative to the top of the piston. This is referred to in the industry as compression height. The original German-made pistons were constructed so that, when at top dead center (TDC), the piston comes right up to the very top of the cylinder. This is VERY important. See images below:

When rebuilding a waterboxer, one has several options when it comes to replacement pistons:

Option #1) Purchase a new set of German-made, 94mm pistons. However, these are next to impossible to find (and are very costly if you do), so nobody does that. Besides, even if you can find them, all you end up with is a stock, 8.6:1 compression ratio, 2110cc, 90 HP engine... and an empty wallet.

Option #2) Purchase inexpensive and usually readily available Brazilian-made, 94mm replacements. Although theoretically made to the same standards and of similar construction as the original German pistons, these have the top cut down by 1mm (.040 inches). The result is that, at TDC, the piston is 1mm shy of the top of the cylinder. Check out the gap between the top of the piston and the straight edge in the picture below.

This creates two problems:

A) The first is that the compression ratio (the ratio of the total volume above the piston at bottom dead center and the volume above the piston at top dead center) is reduced somewhat. In the case of the 2.1 liter engine, the compression ratio drops from an already low 8.6:1 to an even lower 8.3:1. The result is a loss of power and efficiency. Not good!

B) The piston is 1mm further away from the cylinder head at TDC. The head gasket is about 1mm thick, so now the piston ends up about 2mm away from the cylinder head. All modern engines rely to some degree on the close proximity of the piston to the cylinder head to create turbulence inside the combustion chamber. This is commonly referred to as piston to cylinder head "squish" area. If one uses aftermarket Brazilian-made pistons right out of the box, the "squish" effect between the piston and the cylinder head is reduced substantially, reducing the efficiency of the combustion chamber. Another problem with the Brazilian pistons is that the piston rings that come with them have an oil consumption problem. Most engine rebuilders go this route, however, and their engines often run poorly (can't even make 90 HP), consume oil, and don't last very long.

Option #3) Use a Chinese copy of the waterboxer piston. Up until recently, another way to go was with a piston made in China, imported by two different companies, one in San Francisco and one in LA. This was the piston GoWesty used to use (read our "GoWesty Engine Program" article), but is no longer available. It, too, had the wrist pin in the wrong location (again, see photo above), but we got around that by lengthening the connecting rod by 1mm to compensate. They sent us a new design they were offering that was 95.5mm diameter, and with the compression height corrected, so the top of the piston came right up to the top of the cylinder like it was supposed to. However, they inexplicably changed the design of the dish in the piston, and in doing so the dish volume dropped to only 35cc. The result of using this piston is a compression ratio of 10.53:1! That is simply unacceptable with any standard pump fuel and a cast piston. This change, plus an overall unreliable supply from China, made it impractical to continue to use this option in our engines.

Option #4) Have a custom, forged piston made from scratch. This is an expensive and time consuming proposition, but by far the best solution, so that is precisely what GoWesty has done. In doing so, we were able to solve all the problems at once. We employed a U.S. company to manufacture a piston for each of the engine displacements we offer, made exactly to our standards. All of our custom pistons have a dish volume of 48cc. The piston we use in our 2.2 liter engine option has a diameter of 96mm, the 2.3 and the 2.4 are 96.5mm piston, the 2.5 piston is 97mm. Each piston design has a compression height such that the piston comes right up to the top of the cylinder, ensuring the combustion chamber can work like it was designed to. The results are highly efficient engines with exact displacements of 2200.4cc, 2300.9cc, 2398.9cc, and 2497.8cc. The compression ratios for each are 8.7:1, 9.0:1, 9.35:1, and 9.7:1, respectively.
GoWesty Piston
Wiseco and JE are two divisions of the same U.S. piston manufacturer. We started out with Wiseco in the begining, then switched to the JE division in 2008, when our volume increased to the point  that we could justify having our own, proprietary forging. With our own forging, it was possible to further refine an already excellent piston design to a more modern "slipper skirt" design.

In summary:

Various volumes measured at Lesco Machine Shop:
Original German 94mm piston dish: 47cc
Brazilian-made 94mm piston dish: 43cc
Chinese-made 95mm piston dish: 48.5cc
Wiseco USA-made 96 and 96.5mm piston dish: 48.0
Cylinder head combustion chamber volume: 15cc
Piston/cylinder head clearance volume, per mm: 6.9cc to 7.3cc, depending on bore

Based on these measurements, the following was calculated:

Original German 2.1 liter engine:
German-made 94mm piston, 427 grams
76mm crankshaft stroke
2110cc actual displacement
137mm connecting rod
1mm piston/cylinder head clearance
8.64:1 compression ratio, 87 octane OK

Other rebuilders' 2.1 liter engines:
Brazilian-made 94mm piston, 425 grams
76mm crankshaft stroke
2110cc actual displacement
137mm connecting rod
2mm piston/cylinder head clearance
8.3:1 compression ratio: too low
Chinese-made 95.5mm piston-469 grams
76mm crankshaft stroke
2178cc actual displacement
137mm connecting rod
1mm piston to cylinder head clearance
10.53:1 compression ratio: way, way too high

GoWesty rebuilt engines:
GoWesty standard 2.2 liter engine:
USA-made, forged JE brand 96mm piston, 407 grams
76mm crankshaft stroke-stock
2200.4cc actual displacement
137mm connecting rod
1mm piston/cylinder head clearance
8.7:1 compression ratio, 87 octane OK*

GoWesty 2.3 liter upgrade:
USA-made, forged JE brand 96.5mm piston, 409 grams
78.65mm crankshaft stroke, counter-weighted
2300.9cc actual displacement
137mm connecting rod
1mm piston/cylinder head clearance
9.0:1 compression ratio, 87 octane OK*

GoWesty 2.4 liter upgrade:
USA-made, forged Wiseco brand 96.5mm piston, 379 grams
82mm crankshaft stroke, counter-weighted
2398.9cc actual displacement
forged, "H-beam", 139.7mm connecting rod
1mm piston/cylinder head clearance
9.4:1 compression ratio, 89 octane required*

GoWesty 2.5 liter upgrade:

USA-made, forged Wiseco brand 97mm piston, 371 grams
84.5mm crankshaft stroke
2497.8cc actual displacement
139.7mm connecting rod
1mm piston/cylinder head clearance
9.7:1 compression ratio, 91 octane required

*Note: It is a good idea to run 91 octane fuel on all Vanagon waterboxer engines. The price difference between regular and premium, as a percentage of the total price, has never been lower. We recommend premium fuel in all of our engines.

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GoWesty! Camper Products is a privately owned company specializing in parts, accessories, sales and service of Volkswagen (R) campers. It is not affiliated in any way with Volkswagen of America or Volkswagen AG. "Vanagon", "Bus", "VW", "Volkswagen", and "Eurovan" are registered trademarks and should be considered as such throughout our entire website.


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