Cam Backlash: The Whole Story

Getting the correct fit (backlash) between the camshaft and crankshaft gears has always been a challenge when building a VW horizontally opposed engine, and the water-boxer is no different. The problem is this: the distance between the crankshaft main bearing bore and the camshaft bearing bore in the engine block varies from block to block. In order to compensate for this, VW offered crankshaft gears in three size variations: standard, +1, and -1. Camshafts were available in nine different sizes, ranging from +4 to -4. If you do the math, you come up with 27 possible different combinations. 

During the engine-building assembly line process, the technician building the engine would simply grab a standard cam and place it in the engine block. If the backlash between the cam and crank gears was too much, he would grab a "plus" cam (+), and if it was too tight, he would grab a "minus" cam (-). This was an iterative process until the tech was able to get just the right backlash. You thought Goldilocks had it tough, right?

Exactly what these "plus" and "minus" notations actually measure in terms of gear size did not matter back then—it was just a relative size. Today, it matters even less, because all of those notations were relative sizes on brand new gears. No new camshafts or gears have been available for the water-boxer for over a decade. Instead, good used camshaft/gear assemblies are re-ground and reused. Since the gears are used, it is actually possible for a gear stamped with a larger number to be smaller than a gear stamped with a smaller number—due, of course, to wear. Thus, it is no longer possible to simply ask for a larger or smaller gear based on what is stamped on the gear. Nowadays, in fact, the size variation between gears is random and practically infinite. 

Therefore, what we do at GoWesty is almost exactly what the factory did when they assembled engines from new parts, except that we pay virtually no attention to what is stamped on the gear; we just grab one at random and try it. If the backlash is not correct, we grab another one and repeat the process until we find one that fits just right. When building an engine, we have dozens of cams to choose from, and usually one fits just perfectly. Occasionally, we go through all the cams we have with none of them fitting just right, so we simply remove the crank/gear assembly and grab another one—we usually have at least a dozen or so of each stroke, as well. And so the process begins anew. As a last resort, we can always buy a new crankshaft gear (the same as a VW Beetle) in a "plus" or "minus" size and start over. Basically, the real problem is the engine block, and we are just trying to come up with the right combination of gears to make it all work—just like the VW factory did. 

You might be asking yourself, "So how can a shop (or individual) building only one engine at a time possibly get the backlash just right?" Good question. The answer is this: They don't. They just run what they have available and call it macaroni. 

Your next probable question might be, "Wait a second... won't the engine blow up?" The good news is that, no, an engine without the exact correct cam/crank gear backlash will not just blow up. It might make a little bit more noise, and it probably won't last quite as long—but it will definitely run. Many shops around the country understand this problem, and they appreciate the immense volume of parts one needs to have to properly build a water boxer engine. And that's why many shops choose to buy a GoWesty engine: we have what is needed to build the best engines possible. 

The bottom line is that you can drive yourself crazy trying to get this part of the engine assembly just right. Short of having a true assembly line—like the one GoWesty has—it is impractical to even try. Either you have to leave good enough alone... or buy a GoWesty engine!

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