THE SHOP

Performance & Hotrod Business May '14

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May 2014 n Performance & Hotrod Business n 55 the bearings, and may require something in the 0.0030- to 0.0034-inch range (requiring running a heavier-viscosity oil). Without getting into the details of specific engine platforms, all we're doing here is providing ballpark clearances. It's best to check with the bearing maker for forced induction recommendations. Pistons Depending on the amount of boost and resulting cylinder pressure, OE cast or hypereutectic pistons may not be able to handle the increased temperatures and cylinder pressure. The use of quality forged or billet pistons is highly recommended. Pistons designed for forced induction will typically feature a thicker piston deck area. Piston skirt-to-wall clearance for most V-8 engines will generally be in the 0.0045- to 0.005-inch range and depend- ing on the piston, this can be as much as 0.007 inches. With forced induction, stay on the max side in terms of skirt clearance. In very general terms, for forced induc- tion, adding about 0.001-inch of clearance per inch of bore diameter is a theoretically accepted practice. But this always depends on the specific alloy and density of the pis- ton material. (Since performance/racing piston mak- ers use different/proprietary alloy formu- las, always follow the wall clearance speci- fied by the piston maker.) Wall clearances will be listed as a minimum to maximum range. As a very, very generalized statement, typical small- block engines will use a wall clearance of about 0.004 inches and typical big-block engines will call for about a 0.005-inch clearance. (Again, this is a very broad recommendation). Note: When skirt clearance is increased, there may be a tendency to experience a slight piston "slap" when the engine hasn't warmed up to full operating temperature. This can be addressed by having the piston skirts moly-coated, which provides addi- tional lubricity to protect the skirts. Most performance piston makers offer this coat- ing already installed. Piston Rings Since a forced induction system (or the use of nitrous injection) generates added cylinder pressure and heat, a "standard" specification ring gap may be too tight, since the pistons grow in diameter as heat increases (more so with higher cylinder pressure). If the gap is too tight, the rings may end up butting together, which can place undue stress at the piston crown, possibly resulting in piston failure. A general rule of thumb is to slightly increase the top ring gap to the tune of about 0.006 inches per inch of bore diameter. For example, if the bore is 4.125 inches, top ring gap may need to be in the 0.025- The use of MLS (multi-layer steel) cylinder head gaskets can provide superior sealing for forced induction, as opposed to com- posite-type gaskets. PHBMAY.indd 55 4/2/14 11:55 AM

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