THE SHOP

Performance & Hotrod Business - Mar. '15

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50 n Performance & Hotrod Business n March 2015 HOTROD Even import engines are thriving. "Fifteen years ago we had a hard time finding good rings in metric bore sizes," Meza recalls. "Now there are high-quality rings to be had in even the 'weird metric' sizes." So applications are expanding—and so is demand for all-out custom parts. "The more detailed a restoration or street project becomes," says Urcis, "the greater the chance that a custom piston will be indicated. Shelf parts are always compromised in some way, compared to what an 'ideal' piston would be. Like many other things, a good piston is the result of the least compromise. The closer all the parameters of the piston match the appli- cation, the more power and longevity will be realized." And as McFarland points out: "Custom pistons often lead to shelf-stocked pistons. Our PowerPak line is built around sup- plying the 'custom' features and applica- tions with the consistency, availability and affordability that a shelf-stocked part provides." Urcis outlines some of the special fea- tures available on custom pistons. Such as, all CP-Carrillo pistons come with double pin oilers—that is, two passages diverting pressurized oil from the oil ring groove to the wrist pin. "That just helps the pin last longer," says Urcis. Also standard from CP-Carrillo is an accumulator groove between the top and second rings, which allows more combus- tion gas to, well, accumulate between the rings without back-pressuring to the top ring and causing it to float. The company also double-cuts all its ring grooves. "Aluminum retains residual stress from the forging process," Urcis explains, "so the more you cut it, the more it tries to move on its own." Double-cutting means cutting an undersized ring groove, then finish- machining the piston, then re-cutting the ring grooves as the very last operation, to ensure accuracy. Options offered by CP-Carrillo and others include gas ports, which allow some combustion gas behind the rings to aid sealing. This is particularly desirable with the low-tension rings that are becom- ing increasingly popular (more about this later). Kellering (named for the Keller trac- ing mill) is a low-speed, high-accuracy machining process that can shape a piston dome (or dish) to closely match the inner contours of the head, which is desirable in high-compression, low-clearance applica- tions. And then of course there's under- head milling, which uses a similar process to remove weight. These features "all have a purpose," says Urcis, "so knowing what they are and what they do is very important. Many others are also useful and available." Piston Ringers Some of the most significant advances, at both the racing and OE levels, have involved piston rings. "Smaller ring packages have proven to offer numerous benefits," McFarland says, "and MAHLE has been incorporating them into most every piston we produce. The smaller rings are lighter, create less drag and are more conformable, result- ing in increased sealing and oil control. Additionally, they allow the piston designer to add material—for increased strength— in key high-stress areas. A good example is the cross-section between the valve pocket and the top ring groove, which is a com- mon area for piston failure." The biggest thing in piston rings is the new material technology, adds Keith Jones of Total Seal in Phoenix. Potential leak paths. (Image courtesy Total Seal Inc.) The second leak path is due to a poor surface finish on the axial faces. (Image courtesy Total Seal Inc.) The third leak path is between the face and the cylinder wall. (Image courtesy Total Seal Inc.) The first potential leak path is at the ring end gap. (Image courtesy Total Seal Inc.)

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