Like the VW Type4 engines, the waterboxers used pistons sized for 24mm wrist pins, needlessly. Many engines of all different brands and types making many times the power run 22mm or even smaller pins. Whatever, just add this one to the long list of VW design decisions lacking any coherent rationale.
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The factory connecting rods in both those boxer engine types were also extremely overweight for the usage. I built exactly one wbx, my first, using stock rods; after that one, all of the 100 or so engines I built used some of the many replacement Type1 connecting rods on the market made from lighter weight but exceedingly tough chrome-moly alloys, with superior fasteners too. But, being aimed at the Type1 aircooled market, they all have 22mm small ends, while the few aftermarket piston makers with wbx offerings stuck with the 24mm size.
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To mate these parts, the usual approach is enlarging the rod small end, either by boring out and honing the existing 22mm pin bushing to 24mm, a very easy operation that leaves the bronze dangerously thin, or by boring the rod small end out to 26mm in order to fit the wbx 24mm bushing. With most of the aftermarket rods, this left thick enough material in the small end after boring, but chromoly is tough stuff to machine, so achieving this far better result required far more and fairly difficult work.
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There is a rather obvious and far easier solution that makes for a massively superior assembly, though: installing 22mm bushings in the pistons themselves. Imagining how I might approach this, while I slowly and painstakingly modified another set of rods on my mill, I wondered about its viability; would I be breaking fresh ground, or had this been done before? It took a bit of searching, because it was largely lost to time, but sure enough, I would be no pioneer, this was commonly done back when pistons were cast iron (and builders poured their own lead bearings!). The method just didn’t survive the switch to all-aluminum pistons.
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So I had all the courage I needed to proceed, the rest is just the nuts ‘n’ bolts of it. So here’s a series of pictures showing the process I developed. As to its superiority? I did get to see inside some of these, my own hard-working dailys and a couple customer jobs, and even with 50, 60 thousand miles of hard use, with all of them I could just stick my little finger into the pins and draw them right out like I just slid ’em in. No ridge, no visible wear, ready to go right back together for however many more years and miles. It is unquestionably a superior assembly.
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So here’s the tooling that has to be made:
1. A press jig with an upright 24mm wrist pin and a second pin sized a little under 22mm so a new, un-honed Type1 bushing will fit over it.
2. A dimpling die with two interior diameters underneath, to set centered on top of either of the jig pins.
3. A driver sized for a raw Type1 bushing, shouldered to drive it, with a cross-rod “timed” to drive the piston body itself onto the dimpled die when the bushing is at the installed depth.
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Below is the business end of the dimpling die. Note the raised “dimples” at a bit more than the bushing OD.
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The underside of the die is stepped, to sit flat and centered on top of both the 22-minus mm pin and the 24mm pin.
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First the piston is slid over the 24mm pin and the dimpling die set on top of the pin.
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The driver with bushing is inserted. Note that I took these shots with steel bushings I made to test the process, before I ordered a bunch of Type1 bushings, so picture a normal tinned bronze bush on the nose of the driver. The Type1 bushings also may need to be shortened, for these pistons I had to cut them to about 5/8″ length on the lathe, in bronze very quick work with a parting tool. Note the shoulder above the bushing to drive it, and the steel cross-rod to drive the piston body itself once the bushing is at the final depth. Some green bearing retaining compound would be smeared on the actual bushing and in the bore before driving in.
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Here the piston sits on the dimpling die, which will take the press force. This stack will be put on the hydraulic press stand. Here the bushing is already almost home.
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When the bushing hits its final depth, the cross-rod starts pushing on the piston body proper, in the valley adjoining the pin bores. Different pistons will have different architecture around the pin bosses, so custom drivers would typically need to be made for each style of piston, but being only turned aluminum, the drivers are easy to make.
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Once the bushing is home and the cross-rod engages the piston body, some extra press is applied to dimple the inner pin boss. The dimples are visible encircling the bore and bushing (which here is my steel test piece). This slightly deforms the pin boss so the bushing can’t migrate inward.
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So with the first bushing in place, the piston is flipped over and put over the smaller pin, the die set on top, and the process repeated.
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Here is a finished piston with actual Type1 bushings installed and honed to size.
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Honing the two bushings concentric requires a tandem hone for the Sunnen machine, a longer hone with two long stones specifically designed to true up non-contiguous holes just like this. You hone them just like you would a rod small end, to the same “light press fit” at room temperature, or 5-7 ten-thousandths diametrical clearance.
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For pins, I bought sets of regular Type1 22mm pins, but the wbx pin length is a lot shorter, so the pins were shortened with a chop-saw then the end ground flat and clean.
So, then it’s what to do to retain the full-floating pin? Well first of all, if the connecting rods have any bend and twist, whatever is used to retain the wrist pins ends up doing a lot of work, whereas it shouldn’t really be doing any at all. Straightening rods after machining is often neglected, though, and that’s when you see things like pin buttons worn down, or clips hammering out their grooves. With straight rods there’s none of this kind of damage, the pin does as it’s supposed to do, floating balanced in the midst of the reciprocating assembly. Obviously imbalanced forces that cause the pin to want to chronically float one way or another is also wasting work and causing extra wear in the engine, so start by checking and correcting the rods for bend and twist. Both aspects are actually very easy to check and correct, but that’s outside the scope of this article.
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Ok, so what did I do? I shopped the TruArc clip catalog for a wider clip, but there wasn’t anything suitable, so I actually bought some very thin, hardened 24mm OD shim washers to fit under conventional circlips, and I calculated the bushing installed depth so the clip when seated pressed down on the shims. Despite that, in my first test engine the shims did oscillate back and forth and actually wore themselves oval, leaving fine wear debris behind which did some light scratching of the cylinder. But the normal clips did cover the pin itself at the ends and midpoint, and knowing that there is nearly no force acting on the clips when the rod is straight, I tested just using the stock clips alone, which worked perfectly, and I went on to build scores of engines that way.
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It’s tempting to think that there must be a useful weight savings as well, but when it’s all said and done it’s only a few grams with conventional wall thickness pins. Some thin-wall Type1 pins, though, would save more weight, but there’s enough other benefits that weight being a wash is pretty immaterial.
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So this is really the best way by far to solve the 22 vs. 24mm dilemma, because it’s so much easier and faster than reworking rods, but more importantly it produces an assembly that’s far stronger and longer-wearing than the pin floating in aluminum alone. Ever notice how rod small end bushes, with just over half the total bearing area, are barely worn, while the aluminum piston pin bores have ridges pushed up and require a puller to get the pins out? That by itself should put paid to the whole question. It sure did for me!