Jeff Smith Senior Technical Editor
May 1, 1998
Photos By: emap usa Archives

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This is a typical Ford small-block crankshaft made up of a series of main journals that support the crank in the block and a series of rod journals offset from the main centers. The amount of offset from the main journal is called stroke. Ford small- and big-block crankshafts have five main journals and four rod journals.
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Numerous crankshaft configurations are determined mainly by the engine’s design. For example, there are inline fours, V-4s, inline sixes, 60-degree and 90-degree V-6s, inline eights, 90-degree and 60-degree V-8s, V-10s, and V-12s. Each engine calls for a different type of crankshaft to accommodate the cylinder layout.
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Here, we’re illustrating a couple of different crankshaft types. Flat, 180-degree crankshafts are rare and are used mostly in racing. The 120-degree crank arrangement is used in the 60-degree V-6. The 90-degree crank is typical of most V-8s. The degree reference refers to the included angle of the cylinders in the block, not the number of degrees between cylinder firings. On a 60-degree V-6 engine, for example, the 120-degree firing angle makes the 60-degree V-6 a true even-fire engine. The 2.6L, 2.8L, and 3.0L Vulcan V-6s are 60-degree designs from Ford of Germany. The 3.8L and 4.2L Essex V-6s are 90-degree V-6s.
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Stroke is defined as the rod journal’s total vertical travel in a 180-degree rotation of the crankshaft. The easiest way to measure stroke is to check the total travel of the piston from its bore from bottom dead center (BDC) to top dead center (TDC). A stroker crank lengthens the amount of stroke. One example of that would be dropping a 400M crank into a 351W block. The stock 351W has a 3.50-inch stroke. The 400M crank brings that stroke to 4.00 inches, adding 0.50 inch to the stroke. Another way to add stroke is to offset-grind the rod journal as shown in the illustration. This increases the stroke by a small amount, usually around 0.010 inch. Another alternative is to weld material to the journal and offset-grind, but this is an expensive process that can lead to stress fractures. Manufacturers such as Crower, Lunati, Ford Motorsport SVO, and SCAT offer stroker crankshafts for most Ford V-8 engines.
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Many engine builders and crankshaft specialty shops radius the oil holes in the journals, which improves the flow of oil to the bearings. A slight radius of the oil-feed holes (1) helps relieve stress cracks in the journal. Arrow 2 indicates the radius or fillet area that transitions the rod or main journal to the cheek of the crankshaft.
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Internally balanced engines utilize concentric or zero-balanced harmonic balancers, while externally balanced engines require balancers and flywheel/flexplates that are offset-balanced. The offset weight is used to balance the rotating mass of the engine. This is the case with 221, 260, 289, 302, and 351W engines, which are externally balanced. Small-block Fords such as the 289 and 302 have a 20-ounce offset balance. Newer ’85-and-later 5.0L engines have a 50-ounce offset. Do not mix these two types.
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Crankshaft lubrication is generally handled by pressure-feeding the main bearings through passages in the main bearing web in the block. The oil feeds the main bearings, then travels through drilled passages in the crank between the main and rod bearings, as indicated in this photo. The oil then runs back to the pan to be filtered and pressure-fed back into the system. Cross-drilling is the process of drilling the main-journal oil-feed hole to increase oil flow to the mains.
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A crankshaft assembly must also be dynamic-balanced to prevent vibration. If you add aftermarket pistons, rods, or a different crank, the entire assembly should be balanced. Spin, also called dynamic balancing (shown) is the best procedure and involves spinning the crankshaft with bob weights attached to the journals to simulate the actual weight of the rods, bearings, pistons, rings, and even oil.
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Lightening holes (arrow) are often drilled to get the weight consistent.
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The three most common types of crankshafts are cast/nodular iron, steel alloy, and custom-ground billet steel. Cast-iron cranks can be identified by their thin cast parting lines (1). Forged-steel cranks can be identified by the wide parting line (2). Often, custom-forged steel cranks don’t show a forging line, even though they’re not true billet. Billet cranks (3) are machined out of a single chunk of forged steel—typically 4340 alloy.
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The best way to measure bearing clearance is to measure the journal’s outside diameter (o.d.) with a micrometer as shown. Then the bearing (in this case a main) is placed in the bearing saddle, torqued, and measured for its inside diameter (i.d.). Subtract the main journal o.d. from the bearing i.d. to establish the actual main bearing clearance. Typical bearing clearances are 0.0015 to 0.0030 inch.
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All crankshafts must employ a means to limited fore and aft movement within the block. This is accomplished with a thrust main bearing, which is normally at the No. 3 main bearing position (arrow) on Ford V-8s. The thrust bearing is different from the rest because it employs lips that give the crank thrust surfaces something against which to ride. Extreme clutch pressure-plate loads on manual-transmission vehicles can place excessive pressure on the thrust bearing, wearing both the crankshaft and the bearing surfaces.

Your engine's crankshaft is likely your Ford's hardest working component. Its sole job is to convert the reciprocating motion of the piston and rod into rotary motion that's transferred to the transmission, rear axle, and drive wheels. For a low-rpm grocery-getter, that's a fairly simple task. Add the complexity of high rpm and the enormous power created by today's high-performance engines, and the task becomes monumental.

Follow along as we learn all about the basic properties of the crankshaft and details you should be aware of when it comes time to have your crankshaft machined. High-performance crankshafts come in a bewildering variety of alloy steel materials. So we're providing information about alloys that go into a high-performance crank. This will help you understand how a 1053 steel crank compares with a 4340 steel cousin. We won't make you an expert on crankshafts, but at least you'll be able to pick out a stocker from the strokers.