Balancing act

A boxer engine is a strange beast: Simple in the number of parts but requiring fiendish attention to detail when assembled, they don't require counter-weights to offset reciprocating forces; the opposing pistons and connecting rods do that job. They also have a low center of gravity since the cylinder heads are hung off of the ends of the engine, rather than perching on top of a bored out 'block' of iron.

Obviously, this means that the boxer is a lovely engine for racing (thank you, Mssrs Benz and Porsche) fantastic for handling (Subaru bows here) and the more effort that is poured into making the reiprocating masses equal, the more like a sewing machine the engine runs. In a race engine, this is the difference between the imbalances causing the engine to fight itself and losing the race or blowing through low 9's in the quarter-mile.

For the layman and non-racer, a zero weight bias means an engine that may get forty percent more miles before a rebuild is needed. Not bad, for a little care and feeding.

Volkswagen, working in the mass market of the 1960's and 1970's didn't have the material science to get the pistons and connecting rods to a close tolerance. Just piles of components with a paint daub on them to say what weight range they were in. Just use four components from the same weight range, and you were within 5 grams of balanced, which seems awfully close...right?

But spin that engine at 5000 rpm and suddenly those five grams are multiplied in their reciprocation (Accelerate out to the end of the 71mm stroke and then quickly decelerate it to a stop...then PULL! the piston back in that 71mm, then slow it down to a stop, then PUSH....and so on. Those 5 gram differences are suddenly like an elephant hanging on the end of the connecting rod journal. Literally forty or fifty extra pounds of effective weight imbalance at 5000rpm. So those few grams difference REALLY matter in the final analysis.

If you're rebuilding your own engine, it behooves you to get the tolerance of your pistons and connecting rods to less than one-tenth of a gram. How? Weigh all of them, find the lightest one in the set, and then grind a little off of each of the rest until they all match. Let's go to the video:


At some point, there is a diminishing return of benefit. Yet any boxer engine can benefit but this kind of care and attention. The good news is that the AA 94mm piston's I'm going to be using come out-of-the-box in almost perfect balance. (Who said that Chinese products are all junk. You can be ISO9000 certified and produce junk, as long as you document your process for how to make junk. Origin is no sign of quality. The iPhone is made in China. I don't hear anyone moaning about the quality of the components there.)

So really, it is only the connecting rods which will close tolerance work either on my part, or pay a machinist who knows what they're doing to produce the 'sewing machine engine.'