Exotic Piston Finish Features With MAHLE Motorsports

The modern aftermarket is evolving – this evolution comes out of necessity and is in response to growing OEM participation in what would be considered specialized racing and performance technology just a few years ago. Engine designs have become more and more advanced to serve the demands of consumers who want speed and economy, but are also self-serving to manufacturers challenged with ever-increasing emissions scrutiny and CAFE standards.

Pistons may just be a solid block of aluminum carved into shape to suit an engine, but there is so much more to them than meets the naked eye. MAHLE Motorsports is one of the manufacturers keeping pace with technological leaps and bounds, usually reserved for the vast resources of the OEMs. How do they stay relevant? It’s a question of methodology, a passion for engineering and a global network of research facilities toiling on solutions for problems faced by both racers and the mainstream OEMs.mms-pistons_text_image_full

Finish operation features are the key minutia we plan to have a look at here. These small machining operations more often than not, go unnoticed when you hold a new piston in your hand. Some of the features require an analytical explanation, least they be misinterpreted as a liability rather than benefit. We sat down with Trey McFarland of MAHLE Motorsports to get a better grip on the theory and practice behind what they are doing for certain specialty applications.

Pin Bore Profiling

Mahle2Performance and racing engines are stressed beyond the original intent of their design – especially when using an OEM architecture. Higher combustion temperatures, cylinder pressures, exhaust gas temperatures (EGTs), and accelerating piston speeds can all aggregate to make for one seriously hostile operating environment inside your engine.

Acting as the front line for mechanical work in the physics sense, are pistons. As in any system of parts that work in unison, you can never get rid of a load or force, you can only pass it on to the next link in the chain.

Linking the pistons to connecting rods are the deceptively simple wristpins and pin bores. This parts interface is far more dynamic than most visualize under hard running. The pin bore surface is easily disregarded as a bearing surface – one just like a rod or main bearing. Maintaining an oil film and proper clearance is a challenge when parts are flapping in the breeze at ballistic piston speeds.

Pin Bore Profile Trace

A graphical representation of the barrel-shaped pin bore is necessary to show the profile.

“When it comes to pin bores, they’re a pretty simple looking feature. In most cases the upper level teams and builders are honing pin bores to work with what they’re doing. With the equipment we have, we are able to shape it – it’s not just a round hole,” McFarland simplified.

Holding solid steel and aluminum parts in your hands, you don’t think of them as floppy, but you have to consider the forces they experience inside an engine are orders of magnitude more intense than any force you encounter acting on your person.

When you get into a fast piston speed application there’s quite a bit more pin loading. An application with a long stroke needs this feature. -Trey McFarland, MAHLE Motorsports

Imagine holding a piece of cardstock paper and waving it in front of your face, if you move it casually the paper remains flat and holds its integrity; wave it a little bit faster and the cardstock nearly folds back on itself. Piston speeds act in the same way inside an engine, wrist pins and pistons can drastically change dimensions under stresses.

“If you hold a piston and pin in your hand, it’s a solid piece of material. Most people’s perception is that these parts are not moving in relation to each other – all of this stuff is moving. The ring grooves are moving around, the piston skirts are moving, the pin bores are moving, the pins are flexing, the small end of the rod is distorting and twisting – it’s a living, breathing system with all those components moving together,” McFarland illustrated.

“People often times think about piston speed as RPM-related. That is part of the equation, but people ask us how they put F1 pistons at 20,000 rpm and we remind them that they have 2-inch strokes. In reality, a big-block Chevy running at 10,000 rpm is probably running a faster piston speed. When you get into a fast piston speed application there’s quite a bit more pin loading. An application with a long stroke needs this feature.”


Wristpins on the MAHLE manufacturing line. A thick wall, heavy wristpin isn’t always the right solution to flex.

Just what does pin bore profiling look like? Well instead of a perfect cylindrical through-bore with straight walls, Mahle has developed a specialized barrel-shaped bore. “There are two common designs we’re working with. We can do a profile from inboard to outboard, which has a barrel shape to it so the diameter of the pin bore on the inboard where the gap for the rod is located, it’s trumpet shaped. We also have (to a lesser degree) a trumpet shape on the other end of the pin bore. What that enables the pin to do is bend and flex.” McFarland described.

“The profiled pin bore enables the pin to flex and do what it’s supposed to do without hard-loading the piston. It keeps that whole system of married-together components working together. The stronger the profile or trumpet shape is on the inside edge of the pin bore, you also have to account for the fact that if you allow that pin to move more, the larger distance being traveled is out at the edge of the pin. It can dig into the pin bore and possibly stop the pin from spinning but also introducing stress to the edge of the pin as well as to the edge of the piston. Anytime we can alleviate stress loads you get a stronger part. And we can run a little tighter clearances because the parts are working better together – that’s a very popular benefit.”

Ovality addresses a different axis of distortion in the wristpin/pin bore relationship. While the flexing may require inboard clearance in the pin bore – hence the barrel profiling – the actual pin end profile can distort and flatten from a perfect circle to an oval. This changes the major and minor axis dimensions of the pin.

“What the ovality does is (again) help for high piston speed applications with a combination of RPM and stroke. The wrist pin distorts, and the way it distorts is into these areas where we’re able to have some ovality – instead of it distorting to the point where it takes out all the clearance and then grabs the piston and tries to snap it over on its side. This rockover can be something as simple as disrupting the rings and losing your ring seal or it can be as severe as putting some heavy abuse on the piston skirts and top land,” McFarland explained.


Piston side view Oil Slot

Visible at the 10 and 2 o’clock position are pin-oiler relief grooves.

Despite incorporating elliptical pin bore profiling and ovality into their designs, the typical bearing clearance for wrist pin oiling remains so tight that extreme situations can still result in pins biting the bore and halting the dynamic floating action.

Traditional pin-oilers commonly manifest as through-bored gallies from the oil scraper ring into the pin bore. This design presents little relief from the problem it is meant to address for a glaring reason – there’s nowhere for the oil to go.

“Most performance pistons have a pin-oiler that’s drilled from the oil ring groove down to the pin bore. The phrase ‘forced pin oiling’ got coined by one of the piston manufacturers years ago and it’s a mis-application of the vocabulary – there’s nothing forced about it, it’s still gravity fed. The oil is coming from the backside of the oil ring, it’s coming down this port and it’s being confronted with the pin and a .001-inch clearance to squeak out and oil that pin,” McFarland deduced.

Pin Bore Oil Slots VS Oil Drillings

Traditional pin-oiler bores run from the oil control ring to the pin bore.

Adding potential injury to insult, the location of some pin-oiler bores drill right through a high-stress area of the piston. Cracks and failures can ensue as a result of introducing a stress-rise like a hole in a vital structural area.

“With pin-oilers you are drilling through a key high-stress area of the piston, it’s a feature we’ve all but eliminated in most of our upper level applications because of strength. People pushing parts to the ragged edge were having fracture issues, but we took out the drillings and the problem goes away,” McFarland explained.

MAHLE Trapizoidal Pin Bore Side Reliefs

This CAD cutaway of a pin bore shows two of the four side relief pin-oiling solutions used by MAHLE.

“We’ve gone to these side reliefs and they’re as effective or more so in most cases and they don’t drill through a high-stress area of the piston giving us a stronger part. Some of the more simple ones are broached and typically they are around the 10 and 2 o’clock position straight on to the side of the piston. They also can aide with distortion of the pin,” he continued.

“Some companies do an annular groove that connects the two together, but still, to get outside of that groove it’s got to squeak out that .001-inch clearance. With these side reliefs, if you run them all the way to the inboard and outboard section of the piston, they offer four entries into the pin bore that are much greater for oil to get into. It’s not ported down to it from the oil ring scraping oil, but you have so much oil splashing around, and in newer engines with oil-squirters, you have more oil in that area.”Oil Slot close up

Top Land and Rings

One piston feature that has been making a steady advance toward the efficiency needs of OEMs is top-ring land thickness. MAHLE has recognized this trend and responded with aftermarket support to keep the goals of builders within reach.

Hard annodized top ring

Anodized ring grooves, and side skirt coatings are becoming OEM standard features. MAHLE makes sure not to leave features like these on the table.

“They keep moving the rings higher on the piston for emissions reasons, they are trying to reduce that crevice volume above the top ring but between the piston and the bore. That area gets uncontrolled combustion under light loads and it’s dirty in terms of emissions. The problem is, you used to be able to buy a stock engine and throw a supercharger on it and get 100 more hp, now if you take a stock Hemi, Coyote or LT engine and do that you’re running the chance of lifting the top land off the piston,” McFarland cautioned.

“Typically that fire land area above the ring has carbon buildup, and carbon is hard, it will wear the bore more readily. Especially in a drag racing application – where the components never reach full operating temperature. Typically those engines are run under load with a little more clearance anyway,” McFarland pointed out.

Ring Microwelding

Micro welding on the top compression ring can become a problem with uncontrolled combustion. MAHLE hard anodizes the top ring groove for added protection.

MAHLE introduced their 1-1-2 (millimeter) ring package, and the OEMs came with their checkbooks to do research to make manufacturing a thin steel ring more affordable. This kind of cross-pollination between motorsports and OEM development is something that would have looked like a detractor in the past to “hardcore racers,” now it’s a necessity to stay competitive.
With the mass-spread application of direct injection, an aftermarket forged piston isn’t an instant upgrade anymore. McFarland emphasized the importance of making sure you’re actually moving forward with your piston swap and not leaving standard features on the table.

Research, Development, and Manufacturing

produkt_04_contentThese precision features are not the result of engineer “dead-reckoning” and “eye-balling.” When you are creating new fundamental designs for maintaining and lubricating clearances smaller than most machine shops can measure with calipers, you need infrastructure to do valid research and development.

By dabbling in so many different fields of automotive science, MAHLE has research centers around the world. While there may be multiple project teams working under one roof, their fields of expertise may not overlap, and they may not even know what the other is doing right under their noses. The motorsports division of MAHLE has the distinct advantage of having a finger on the pulse of all the different branches. Combining research serving the needs of OEMs, the diesel and industrial market, and the demands of racers from Formula 1 to DTM, drag racing, to tractor pulling, MAHLE has a toolbox of engineers and facilities no other piston manufacturer can boast.


BandAid Fixes

“The way other manufacturers (who don’t have the MAHLE equipment) will do it is to open up the clearance to allow for pin flexing and distortion. When we are able to run a tighter clearance the whole piston is more stable and consistent in its operation. Wear and noise is brought down and loads are decreased,” McFarland explained.

“It has taken a lot of testing to get to know how to use the equipment. MAHLE is up to about 15 tech centers around the world and we glean the information from all of them. Some of the things we learn are so application specific they don’t have much of a place outside of their application. But we get to earmark that information and when we are confronted with a similar situation we can go back and look at it and come up with a solution much quicker,” McFarland recalled.

In order to be able to offer such specialized finish operations the MAHLE engineers found themselves in the machinery business – again by necessity.

“All of those operations are actually done on MAHLE-manufactured equipment – we make our own equipment. The pin bore machine is a pin bore machine. It is a CNC machine, but it can’t do anything but pin bores. We have a special machine to do the ring grooves and then we have a MAHLE machine that does the ovailty and profile on the piston. The machines are able to do things that MAHLE engineers came up with years ago but you couldn’t buy equipment to do it,” McFarland explained.

“15 Years ago if you told somebody that you were involved as a performance entity and you did a lot of work for OEMs, they’d think you’re not really performance. Now, you look at the pistons that come out of a new car and gone are the days that you can just take a piston out, put a forged one in and know that you had a better part. These pistons have a lot of tech in them, but they also have a lot of short comings.”Failure


So if you’re building a late model performance engine, especially one that has a long stroke and high piston speeds, or is often run at high loads below operating temperature where clearances are out of spec, keep a careful eye on your piston buying decisions. The OEM pistons in all these contemporary powerplants are engineered for very specialized application-specific environments.

Improving upon the OEM is getting more and more complicated than swapping in just another forged piston. If you believe your build is appropriate to benefit from some of the new finish features pioneered by MAHLE Motorsports give them a call and let them work out where the feature make sense and where they are not necessary.

“Among our shelf stock parts, we decide whether or not an application needs these features. If someone comes in and wants a custom set of pistons and he’s learned about some of these features, we’ll go ahead and talk about what he’s doing and tell him whether it’s necessary or not.” As McFarland concluded, it’s no good to throw features at a part when they don’t match the application, for this reason a case-by-case analysis is important to maximize your value.

“They are small features, they’re not going to equate into a whole bunch of horsepower. A lot of the finish details in our parts are about keeping the part working at its optimum performance level over a longer life cycle,” McFarland concluded.

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About the author

Trevor Anderson

Trevor Anderson comes from an eclectic background of technical and creative disciplines. His first racing love can be found in the deserts of Baja California. In 2012 he won the SCORE Baja 1000 driving solo from Ensenada to La Paz in an aircooled VW. Trevor is engaged with hands-on skill sets such as fabrication and engine building, but also the theoretical discussion of design and technology. Trevor has a private pilot's license and is pursuing an MFA in fine art - specifically researching the aesthetics of machines, high performance materials and their social importance to enthusiast culture.
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