Pistons Get Hot, Oil Departs, And Metals Stick Together

You can learn a lot by just cruising the paddock at the races. People who know real stuff are there, and a surprising number are bursting to tell someone who might understand how cool it is. After all, are they going to discuss deflagration of monodisperse sprays with wife and children over dinner?

In this case, I was at Laguna Seca in July for the US round of World Superbike when I encountered former Harley-Davidson racing director Steve Scheibe, who is running a BMW S 1000 RR supported by Hayes brakes in MotoAmerica Superbike using a 2018 engine he bought off eBay.

Scheibe remarked that engine durability has come a long way in recent years; the BMW is able to rack up racing miles that 20 years ago would have required not one but multiple high-quality rebuilds to survive. Also, what used to be considered trick high technology now appears on production stuff, an example being the use of diamond-like carbon coating (DLC) on finger followers.

The topic on Scheibe’s mind was wrist-pin scuffing, the pick up on the wrist pin of the softer aluminum from the wrist-pin bores in the piston. In four-strokes, the piston end of the connecting rod carries a bronze bushing, and the wrist pin is a careful thumb push into the piston and through the rod bushing until it’s centered in the piston. Then it is held in place by retaining clips that fit into grooves in the piston’s wrist-pin bosses.

Scheibe’s speaking of this reminded me of talking with Ducati tuner/builder Eraldo Ferracci in the 1990s, when he told me that he and Ducati had chosen two different ways of overcoming the scuffing problem. Ducati, employing Pankl titanium con-rods, decided to run oil up the rod shank through a drilling, just as Packard did long ago. Scheibe said he has high regard for Pankl but didn’t much care for that solution, as that oil is taken from the con-rod’s crankpin bearing, one that never has any surplus!

Ferracci’s solution was a different source for piston material, as he worked with Wiseco. Scheibe described to me his discussion of scuffing with people at Mahle, the respected German piston specialists. When he told them he wanted grooves machined in the loaded zone in the wrist-pin bores, they objected that this would reduce the bearing area. Scheibe told Mahle, “Just do it, and I’ll take responsibility for whatever happens.”

Scheibe Racing is campaigning a BMW S 1000 RR in MotoAmerica Superbike with 2015 Superstock 1000 champion Jake Gagne. With seven of 10 rounds complete, the former World Superbike pilot is 10th overall in championship points. (Brian J. Nelson/)

In the present era, pistons consist of a thin disc forming the piston crown and carrying the sealing rings, braced underneath by four crisscrossing webs that carry the two wrist-pin bosses and also support the piston’s very short skirts.

In a thermal sense, what is really different from past practice is that now the pin bosses are like stalactites attached directly to the hot piston crown. In the past, they instead grew inward from the sides of the piston skirts. Because the heat path from combustion to wrist-pin bosses is much shorter now, higher boss temperature has made wrist-pin scuffing a chronic problem. Hotter wrist-pin bosses reduce oil viscosity and may even evaporate the light base oil of a multi-grade, such as 5w or even 0w. As the oil thins or departs, friction increases, generating even more heat. Eventually scuffing—metal transfer—occurs.

Mahle found that Scheibe’s grooves did indeed serve to make pin-to-boss scuffing less likely to occur.

Scheibe noted that life is worse for wrist pins in two-stroke engines because there is no load reversal on the pin, meaning that oil squeezed out from between pin and pin bosses is not replaced. That’s why the original bronze bushings I saw in the con-rods of early Yamaha and Tohatsu two-stroke bike engines were quickly replaced by less oil-dependent needle bearings. But in a four-stroke, the piston’s inertia at the top of the exhaust stroke does reverse pin loading, allowing any oil in unloaded parts of the wrist-pin clearance to be redistributed. So how does machining a groove help? Does oil accumulate in the groove? Does the groove somehow assist or helpfully alter the redistribution of oil each time the load reverses?

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If you Google “wrist-pin scuffing,” you will eventually find a paper entitled, “An experimental investigation of scuffing mechanism of piston-pin/bore contacts.” In their conclusion, the authors wrote, “The addition of a circumferential groove is found to have a remarkable influence on the improvement in scuffing resistance.”

Normally in such papers I expect to see formidable-looking equations that promise to explain everything. In this paper, there is only the bland suggestion that the grooves help to “improve lubricant circulation and bearing cooling.”

That suggests the authors used the same methodology that racers do: Try a bunch of stuff, and something might work. And it did. Once there’s a solution—grooves!—the applied-math folks can get right on the problem of explaining why it works.

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