We are often told to think big if we want to achieve success, but today I want you to think small. In fact, I want you to think about clearances in a racing engine that are smaller than one thousandth of an inch. These minuscule measurements can mean the difference between long life and sudden death for a racing engine. 

     Every year we see hundreds of motors come through Reher-Morrison Racing Engines. Some are built by professionals, others are assembled by racers in home workshops. Almost without exception, the problems that we find are the result of insufficient clearances. I've come to the conclusion that a little extra clearance is infinitely better than not enough clearance. There are hundreds of parts in a racing engine. Most of them rub, slide or rotate against another component. In a perfect world, there is no metal-to-metal contact between parts; instead, chains of oil molecules separate the two like atomic roller bearings. (The petroleum experts may disagree with my analogy, but that's my layman's explanation.) 

     A hydrodynamic wedge of oil can withstand thousands of pounds of pressure while producing very little friction. But if this lubricating film breaks down due to inadequate clearance, friction and pressure create heat that microwelds the two surfaces together. As the parts continue to move against each other, pieces of metal are ripped from their surfaces, further accelerating the destructive process. Eventually the two parts seize, producing the blackened and burned appearance that we see in spun bearings, scuffed pistons and galled lifter bodies.

      So how can engine builders keep the hundreds of parts in a motor from reaching this dire condition? By providing enough clearance for the lubricating molecules to do their job. 

     The cardinal rule of engine building is to measure every clearance. The one dimension that you assume is correct is the one that will destroy your engine. At our shop we use build sheets to record every vital dimension - main bearing bore diameters, bearing thicknesses, crankshaft journal diameters, connecting rod bores, and on and on. Whether you build one engine or 50 engines in a season, filling out a build sheet reminds you to check every piece that should be checked. If you don't want to design your own assembly records, the Chevrolet Power manual has sample sheets that you can photocopy and use for your engine building projects. 

     I encourage you to purchase a good set of micrometers, a dial indicator and calipers. The price of precision tools has dropped dramatically, and good tools are an investment that lasts a lifetime. If you prevent just one catastrophic failure by finding an out-of-tolerance clearance, you've paid for the cost of your tools. 

     All manufactured components are built to tolerances. By checking parts before assembly, we learn exactly what those tolerances are. For example, your machinist may finish your block's main bearing housings on the small side of the specified plus-or-minus dimension. The crankshaft you bought from a friend may be on the high side of the range of outside diameters. Both parts are within their respective tolerances, but in combination they can produce