THE SHOP

Performance & Hotrod Business - January '15

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January 2015 n PRECISION ENGINE n 11 Granted, only the more severe cases will be a cause of concern on a street engine. However, if the goal is to produce an optimized racing engine, every factor that can affect dynamic ring shape must be considered. Sophisticated Equipment Engineering and development labs use highly sophisticated equipment, such as a PAT gauge, in order to accurately "map" a cylinder bore in order to obtain a clear dimensional picture of how a bore is shaped from top to bottom. This is an inclinometer that features a shaft and probe. The shaft is affixed to the deck or torque plate and runs verti- cally through the bore centerline. The probe runs along the shaft vertically and monitors the bore walls radially. This provides a dimensional perspective view of the entire bore, relative to the theoretical bore centerline. Don't expect to run out and buy one of these, as they cost around $175,000 and are primarily used by piston ring and hon- ing equipment manufacturers for analysis applications. This allows plotting the actual shape of the bore in addition to bore diameter. The readings can be displayed radially (viewing the bore from overhead) to show where the bore shifts from the centerline; and in an isometric view (side angle perspective in a variety of view angles) that allows you to see the entire bore in a dimensional manner. Three views of a "typical" dynamic bore distortion scenario. (Photo courtesy MAHLE Clevite) This dimensional "isometric" view further illustrates static dimen- sional shift that can occur from cylinder head installation. (Photo courtesy MAHLE Clevite) This stress graph illustrates how a cylinder bore may be distorted from round to out-of-round as a result of head fastener loads. (Photo courtesy MAHLE Clevite) This graph illustrates how cylinder concentricity can be affected by head clamping, operation with regard to temperature and operation with regard to wall temperature and gas pressure. (Photo courtesy MAHLE Clevite) In order to obtain bore diameter readings in your shop without the use of lab equipment, the cylinder walls can be measured with a bore gauge at four different levels at four clock positions (12, 3, 6 and 9 o'clock). Once these numbers have been recorded, place a honing plate (one on each deck) on the block, torque the plates to specification, and re-measure the same points to reveal differences that have occurred. Bear in mind that this will not reveal concentric bore diameter shifts relative to the centerline. Different Situations Another variable relating to bore distortion is the cylinder head itself. After measuring the bores (accessing from the bottom of the bores) on a relaxed block (no heads or torque plate), install and torque a cylinder head, and read the bores again from the bottom of the bore to note the changes that have taken place. Next, remove that head and install a different head of the same type and measure the bores once again. If you find a different distortion level/pattern, don't be surprised. Depending on the makeup of the cylinder head, the clamp- ing forces may reveal a different situation due to the material and molecular structure of the head, especially on cast heads, due to differences in the hard/soft internal makeup of the casting core. If the head pulls down harder or softer in various areas, this These isometric views show bore shape changes relative to temperature shifts. (Photo courtesy MAHLE Clevite)

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