Bushed Vs. Floating Piston Pins

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You don't have to be Ed Pink to see how wristpins might be the most heavily stressed part in any high-performance engine. Wristpins must deal not only with thousands of pounds of constant acceleration and deceleration, they must also cope with the scourges of friction-induced heat and wear. Following the logic that race stuff is better than street stuff, many hot-rodders and manufacturers turn to full-floating pins to give these steel tubes a fighting chance.

Semi-Floating, Pressed Piston

  • The wristpin -- or gudgeon pin, as Her Majesty says -- connects the small end of the connecting rod to the bottom of the piston in one of three ways: semi-floating with a pressed piston, semi-floating and pressed rod, and full floating. The first approach, semi-floating with a pressed piston, is to make the hole in the rod's small end slightly bigger than the pin bore in the piston. After heating the piston to expand it slightly, machinists push the pin through with a hydraulic press. The pin pivots on a bushing or bearing in the rod's small end and remains fixed in the piston. This approach is simple and inexpensive to execute but falls short because it places all the frictional force on the small rod bushing. This isn't necessarily a bad thing in terms of horsepower, because it reduces surface contact area. But a pressed-piston pin will quickly self-destruct of it gets too hot, runs dry of lubricant or if the bushing fails.

Semi-Floating, Pressed Rod

  • The semi-floating pressed rod, as you might imagine, is just the opposite of the pressed-piston arrangement. Instead of using small piston bores and a bushed rod, the pressed-rod wristpin uses a small rod hole and larger bushed piston pin bores. In this case, the rod gets heated and the piston pivots in the piston bores. This method reduces wear and heat buildup on the pin and extends bushing life because the load spreads out over twice the area. Spinning the pin in the piston also meas that you can use a narrower rod end, which saves weight and increases rpm potential. This arrangement has a couple of drawbacks, though. First, the piston gets a lot hotter than the rod, which can cause clearance and lubrication problems when the engine is either very cold of very hot. Cost of installation is higher than with the pressed-piston configuration because the pistons have to be designed for the bushing sleeves, which themselves cost money.

Full-Floating Pins

  • A full-floating pin uses identically sized bushings in both the piston and the rod small end, allowing the pin to spin relative to either. Because mechanical force takes the path of least resistance, the pin's ability to "chose" whether to spin in the pin or the rod will help to reduce friction, wear and horsepower loss. The alternating back-and-forth motion of the pin also tends to increase lubing around the pin, which further reduces wear on the bushings. Full-floaters are the pin of choice for engines that have large bore-to-stroke ratios and short rods that cause an extreme sideways load on the piston. The floating pin helps to more efficiently and quickly transmit that lateral load to rotational movement, which is good news for the cylinder bores and piston rings.

Floating Pin Considerations

  • While full-floaters are the pin of choice for most racing applications, they do come with a couple of drawbacks. Floating pins require clips or locks on either side to keep the wristpin centered in the piston. Where locks are concerned, you've got a choice of either wire retainers or flat steel retainers. Wire retainers are lighter and cheaper, but steel retainers are stronger and do a better job of centering the pin. You can expect to encounter a bit of piston slap -- noise from the piston hitting the bores -- when using floaters, because the clearances between all those bushings will allow the piston to wiggle around some after warming up. Piston slap is normal for forged piston when they're cold, but full-floaters can cause it when running hot after the bushings get a bit of wear on them. But the good news is that the floating arrangement lends itself to quick and easy service, which is helpful if you ever have to rebuild half your engine in Pit Lane.

References

  • Racing Engine Builder's Handbook; Tom Monroe
  • Photo Credit Comstock/Comstock/Getty Images
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