Sunday, November 19, 2006


The bubble shivers in the bullseye of the level as I tap the slider of the Ohaus triple-beam scale with the eraser-end of a pencil. I'm kneeling on a low box before the work-bench in a sweltering shop, both fans turned off, doors closed, my eye aligned with the scale's pointer, parallax eliminated by a second alignment mark twelve inches behind the scale and at exactly the same height as my eye, givertake a thousandth of an inch.

Another tap and the pointer finally aligns. The piston weighs 444.2 grams, making it the lightest of the set, the others tipping the scale at 446.3, 446.6 and 448.1.

There's 28 grams to an ounce, according to Miss Rose Saghetti, my Fourth Grade teacher. It's actually 28.3497 but the Fourth Grade was nearly sixty years ago and an error of 1.2% is allowed. Indeed, the spread of 3.9 grams across a set of cast 94mm Mahle pistons is allowed, the VW spec being five grams. (Okay, ten for repair parts.) And if close enough is good enough, you'd go ahead and slap the thing together. After all, we're talking about a measly one-seventh of an ounce, fer crysakes. What's the big deal about all this balancing stuff?

In the case above, about 1.6 hp, at the rpm you swing a propeller. More, if you wanna spin it faster. Then too, that means you have to burn an additional 1.6 horsepower's-worth of gas to balance the books. That is, if you want to off-set the 'unimportant' 1.6 hp-loss resulting from the imbalanced mass of your jugs. (So what happens to that 3.2hp? It appears as additional heat, friction and fuel consumption. None of it appears as torque.)

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If you've got a lathe and know how to twirl the knobs, and if you've built a big-bore stroker or two, you'll already have a set of heavy copper collars for your 4-jaw chuck, each sized to accept a particular diameter of piston, along with a matching 'test-plug' that looks suspiciously like an old piston of that diameter, cut down to leave just the crown. The test plug has a small but distinct center-punch. To build a good engine I must precisely align that punch mark with the lathe's axis of rotation.

Four-jaw goes onto the lathe, the collar goes onto the four-jaw and the test-plug goes into the collar. You set up your wiggler on the tool post and adjust the four-jaw until the wiggler isn't wiggling any more. I've got a 12" lathe and my wiggler happens to be sixteen inches long with a ratio of 14:1 between the tail stock - - which is the 'true point' I want to match - - to the center-punch on the test-plug. Blip the switch, the chuck begins to spin and the wiggler becomes a blur of motion indicating the center of the collar is not aligned with the center of rotation. So you adjust it, loosening one jaw and tightening another, nudging the center into truth as you rock the chuck back & forth with your hand. Once you've got it right, you know. No need for any measurements because the wiggle vanishes when the disparity between the alignment of the centers becomes something less than a thousandth of an inch, more than close enough for the task at hand. Which is to shave precise amounts of metal from inside the three heavy pistons.

The wiggler goes back in the tool-box and is replaced by a tool holder. A curiously shaped cutting bit is mounted in the tool holder. Nothing fancy, just a hunka 3/8" square tool steel ground to a shape that allows me to reach inside the skirt of a piston and make a nice clean cut about a quarter of an inch wide. The depth of the cut is based on experience, in that advancing the tool so many thousandths of an inch will remove so many grams of metal.

To make the cut I use a dial indicator clamped to the bed of the lathe. The saddle - - the thing on which the tool post is mounted - - butts up against the plunger of the dial-indicator allowing me to measure the depth of the cut in thousandths of an inch as I gently advance the carriage. Each jug has to be zero'd of course but the collar puts me within striking distance and a piece of Zig-Zag cigarette paper used as a feeler gauge tells me when I'm there. Once I've zero'd-in each piston, a bit of arithmetic tells me how deep of a cut I need for that particular piston and the dial indicator tells me when I'm there.

Overall, balancing a set of pistons is about as difficult as making a good pot of coffee. And while it may sound hi-tech my particular method isn't all that precise. My lathe is almost as old as I am and despite having rebuilt the thing a few years ago its repeatability isn't that good.

I chuck the jugs, do the math, twirl the knobs and when I'm all done I end up with a spread of two-tenths across the three jugs: 444.1, 444.2 and 444.3. (The original 444.2 sits aloof atop the big red carton, lording it over his fatter cousins.) I take a die grinder to the heavy jug and bring it down to 444.2 and studiously ignore the 444.1gm piston. I've reduced the imbalance from 3.9 grams to 0.1grams and decide that's good enough. I know from experience that I can spend an hour or more chasing that last tenth of a gram and I've already spent an hour on this batch of jugs and have two more sets to balance before supper time.

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Although balanced, the pistons are not ready for assembly. The crowns will get a zirconium-based ceramic-metallic coating that serves as a heat barrier. The insides of the pistons get coated with a thermal dispersant and the skirts get a coating of moly-based solid lubricant to combat scuffing. The process will increase their mass by a couple of grams but their balance usually remains unchanged. If adjustment is needed, it's done with a die grinder, taking a tad of metal from the balancing pads inside the skirts.

The balancing and the Thermal Barrier Coatings are 'unimportant' details of course - - no one builds engines that way, other than me and a few other fools you'll meet at the finish line. But having built several hundred engines in my life I've found the aggregation of such unimportant details to be difference between a smooth-running, long-wearing reliable engine and the other kind. And a point most tend to forget is that it isn't the fastest car (or plane) that wins, it's the one who finishes first.

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Which is jus' swell if you happen to have a shop full of tools and pick your teeth with a micrometer.

The truth is, even if you don't have all that stuff - - even if you've never built an engine in your life - - you can still build yourself a better engine than you can buy. The reason for that apparent conundrum is that while you may not be able to achieve a tenth of a gram spread across a set of four jugs, using nothing more sophisticated than a Dremel tool and a reasonably accurate gram-scale you can sure as hell do better than four grams.

Although the imbalance in this case was a scant four grams I've seen a spread of sixteen grams from Mahle and more than twice that - - more than an ounce of imbalance - - in sets of jugs from other makers. The bigger the imbalance, the greater the losses... and the shorter the service-life of the engine. If you only reduce the imbalance by half you'll still have a better engine.

So you buy one of those electronic scales, the kind that are only accurate to two grams. Then you remove the rings, wash the pistons, dry them good and weigh them. The lightest piston becomes your gauge, the one whose weight you want to match. You chuck a coarse rotary file bit into your Dremel or hobby tool or even into your quarter-inch drill, pick up the heaviest jug and start removing metal from the balancing pads or ribs. No mystery there because the jug will already have been 'balanced' at the factory and you can see where they removed some metal. Take your metal from the same areas. Periodically, you weigh the thing. When you get close to your goal you wash the piston in solvent to get rid of all the tiny metal particles you've been throwing around, and weigh it again. When you get to within about a gram of your goal, stop. You've done well enough. Your set of pistons is now better balanced than it was.

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Across a set of four, the individual piston pins and the set of rings will usually weigh within a few tenths of each other. Any major imbalance is usually the fault of the piston pin and can be adjusted by a bit of judicious grinding on the interior of the pin. An alternative is to return the pins to their bores and weigh them with the pistons, adjusting their combined mass by removing metal only from the piston (which isn't always possible, hence the need to know how to lighten a pin).

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After-market VW pistons & cylinders come complete with rings, making each P&C a unique set. That means you have to make sure the same rings go back onto the same piston and into the same jug. Because you've got to take them apart. As received, the bores have not been cleaned and rings are often clotted with Cosmoline. So the first thing you do when you receive a carton of pistons & cylinders is to apply 'work marks.' That's a fancy name for numbering them, one through four. But you need to mark them in a manner that will not be obscured by paint (on the jugs) nor coatings (on the pistons). I use a file or die grinder to cut notches in the top fin of the cylinder, over on the flat side. The same number is put inside the skirt of the matching piston using indentations (gently!) or an electric scriber. Since coating the crown of the piston will obscure the arrow indicating the off-set, I orient the piston so that my marks can be used to show the off-set at assembly time.

To keep track of your rings and the pin, you put them in baggies with a note showing the work-number of their piston. Since the geometry of the rings tends to vary I often include a sketch showing the orientation of the top rings. Now you can take them apart with the reasonable assurance things will be returned to their proper place at assembly time.

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I'm not set up for dynamic balancing. The crankshaft and any concentric part attached to it is sent to a balancing shop where it gets spun-up & balanced as a complete assembly. But static balancing is different. It doesn't take much to do pistons & rods. In fact, the way gas prices are going it would cost me almost as much to pick up & deliver the parts as it does to have them done by a professional balancer. (You'll need a support fixture for the rods since you must also adjust their center of mass, a chore usually referred to as 'big-end vs little-end' balancing.)

If you aren't into Thermal Barrier Coatings there's shops that are, some of which are familiar with aircooled Volkswagens.

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The hand-maiden of mass balance is volumetric balance and the reward is equally profound. Adjust the chambers of your heads to within a fraction of a cubic centimeter and the specific impulse - - the amount of power produced by each cylinder - - will be more uniform. Because when it isn't - - when one jug is producing less power than the others - - it represents a pumping loss that must be made up from the output of the other cylinders before any usable torque can appear in the crankshaft. This situation is similar to the losses that result from mass-imbalance because the power needed to overcome the imbalance literally doubles the loss.

Fortunately, volumetric balancing is just as easy as balancing your pistons and the same principle applies, in that any improvement will result in a better engine.


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