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Performance & Hotrod Business - January '15

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January 2015 n PRECISION ENGINE n 15 Not only are there thermal dimensional changes, but these changes are far from uniform throughout the bore. The major effect is the gross dimensional change in bore diameter, which can change 0.0005- 0.001 inches, per inch of bore diameter, at temperatures between 65 and 210 degrees. For a common 4-inch bore, this equals 0.002 to 0.004 inches of growth. The rest of these thermal variations within the bore are largely non-uniform and can create critical concerns. These dis- tortions include the local diametric varia- tions in the walls, the barrel or hourglass effect, bores departing from circular and becoming non-round and the bowing or arcing of the bore from a true cylinder. A normal bore gauge can detect all but the last of these distortions. Since the cylinder walls are not of uniform thickness and the cylinder is captured at each end, thermal expansion of the metal can and does cause distor- tion of the cylinder walls. A normal cylinder wall will assume a slight barrel shape as the temperature increases, with high and low spots that vary from cylin- der to cylinder, causing a non-circular, non-uniform bore as the engine heats. Temperature is not the only effect on a bore. Under normal conditions, the engine cooling system operates at 10-14 psi. Normal engine pressure forces the cyl- inder into an hourglass shape. While 10-14 psi may not seem like much, this pressure can compress the cyl- inder walls in the center by approximately 0.0005 inches. The normal pressure effect from the cooling system tends to partially offset the barrel effect of this thermal dis- tortion. However, increased coolant pres- sures below the head gasket exist (as high as 35-45 psi) and could cause even greater bore distortions. Because of these offsetting effects, hot honing can be considered with the block pressurized to achieve the best possible true bore. While a few attempts at creating a "hot honing" environment have been seen in previous years, an excellent example of a viable system is KW Products' Hot Hone 2000. This is a completely self-contained system that features a stainless steel car- tridge pump, brass tank and armored safety hoses with quick-connect couplings. The system includes a fully adjustable electronic thermostat, pressure and tem- perature gauge, automotive-style filler, soft plug and block adapters for various blocks. According to KW, the unit is adaptable to all honing machines and uses standard torque plates. Note: The intent of this article is to pro- vide a basic understanding of how cylinder bore distortion can occur and various meth- ods of minimizing this issue. For the average high-performance engine, the use of deck plates during final honing, and adherence to torque values and tightening sequence is generally adequate. Mike Mavrigian has written thousands of technical articles over the past 30 years for a variety of automotive publica- tions, in addition to writing nine automotive technical books for four different publishers. Mike also owns and operates Birchwood Automotive in Creston, Ohio, where he builds custom engines, street rods and performs vehicle restorations. Mike can be reached at 330-435-6347 or birchwdag@ frontier.com. Birchwood's website is www.birch- woodautomotive.com. Always use the same fasteners, or at least the same brand and type, for main caps and heads that you plan to use during final assembly, prior to block machining. Prior to honing, the pis- tons that will be used must be measured for skirt diameter, pay- ing close attention to the measuring point specified by the piston maker. All blocks should be honed with a deck plate on each deck. This helps to more closely simulate the stresses that the block will see as-assembled. Granted, even with the use of a deck plate, although a statically-round bore may be achieved, cylinder wall profile may change when subjected to opera- tional heat and pressure, but the use of deck plates greatly minimizes bore distortion. The deck plate must be torqued to the same value that will be used when installing the heads. During the honing process, a dial bore gauge is inserted to check bore diameter from top to bot- tom, in at least two clock positions 90-degrees apart. The hot honing unit is plumbed to the block on the honing machine. The theory is to attempt to simulate engine operat- ing temperature (at least in terms of coolant temp) during honing. (Photo courtesy Sunnen)

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