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World Guide To Blueprinting

You're Probably As Tired Of Wondering What "Balanced And Blueprinted" Actually Means As We Are Of Hearing It, So We Bring You

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Piston Compression HeightThis may be a bit esoteric, but useful information for those of you considering doing a stroker engine package. All engines incorporate what is called block deck height. This is the distance from the crank centerline to the top of the block deck where the head bolts on. There are four basic variables that go into determining the stack-up of components for this spec: the stroke, connecting-rod length, block deck height, and piston compression height. These are all self-explanatory with the exception of piston compression height. This is defined as the distance from the centerline of the wristpin to the flat portion of the piston that is parallel with the deck. This does not count any dome or recessed portions of the piston. The formula goes like this: piston compression height + rod length + half the stroke + the piston deck height should equal the block deck height

Let's take an easy example. Let's say we wanted to build a stroker small-block Chevy, with a block deck height of 9.025 inches. The stroke for a 383 is 3.75 inches, and let's say we wanted to run 6.00-inch rods. What's left is to figure out what compression-height piston to use. Half of the 3.75-inch stroke is 1.875, and adding 6.00 for rod length equals 7.875 inches. Assuming a 9.025-inch deck height, subtracting 7.875 from 9.025 equals 1.15 inches. This can be the piston compression height, but that doesn't leave any room for milling the block and assumes a zero piston deck height. In this case, we would look for a piston with a slightly shorter deck height to accommodate these needs. Luckily, the piston manufacturers have done most of this homework for you, but it still is worth knowing how it all fits together.

Rod Bolt StretchThe most highly stressed fasteners in any internal combustion engine are the connecting-rod bolts. While the troglodytes will continue to use a torque wrench and the classic "I've been doing it this way for 20 years!" excuse, the point is that there is only one correct way to torque rod bolts and that's by measuring rod-bolt stretch. All fasteners are designed to stretch when torqued. The problem is creating the proper torque to establish the ideal bolt stretch. Variables of lubrication and uncontrolled friction account for most of the torque applied to a bolt, the rest actually stretches and preloads the bolt to create a clamp load.

So the bolt manufacturers created a tool that only measures stretch. The beauty of a rod bolt is that both ends of the fastener are accessible to measure stretch, unlike a main cap or head bolt. In the case of ARP fasteners for a typical 31/48-inch rod bolt, the stretch figure is 0.0064 inch. That means you continue to tighten the fastener until the body of the bolt has stretched to that figure. That creates the maximum clamp load while still leaving approximately 25 percent of additional stretch to accommodate operating loads. Remember that as rpm increase, load increases geometrically. That alone should motivate you to never assemble another engine without measuring rod-bolt stretch.

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