LO 8-4 Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 8-4 Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:44 PM |
1. Crankcase Construction
LO 8-04: Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block. |
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Crankcase Construction High cylinder pressures, combined with tension and compression forces from rapidly accelerating and decelerating cylinder components, generate strong torsional vibrations. These forces bend and twist the crankshaft, and cylinder pressures work to separate the crankshaft from the block, pushing the crank out through the bottom of the block, along with the main bearing caps holding the crank in place. Blown engines at tractor pulling contests frequently provide clear evidence of the enormous combustion forces acting to split a cylinder block from the crankshaft. Stronger and more rigid block designs that are naturally heavier are needed to prevent destruction of the engine and to minimize transmission of forces into the block. Opposing this is the need for lightweight, durable designs necessary for engines to perform well under all operating conditions. The best solution for achieving quiet, smooth engine operation of a highly stressed block is a strongly reinforced crankcase Figure 24. Increasing the block strength and rigidity in the crankcase area allows the block to be as compact as possible while reducing its overall weight and size. Unnecessary weight is also the enemy of hauling heavier revenue-generating payloads, good vehicle braking, and precise handling characteristics. Several cylinder block designs, examined in the next sections, take different approaches to reinforcing the crankcase to rigidly retain the crankshaft in its place. |
FIGURE 08-24: An example of a conventional cast iron block design. The block walls extend only to the main bearing caps to reduce the height of the block. The oil pan forms most of the crankcase boundaries and reinforces the block strength. |
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1.1. Deep-Skirt Blocks
LO 8-04: Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block. |
Deep-Skirt Blocks
A deep-skirt block design refers to a block configuration in which the bottom edge of the block extends to well below the crankshaft’s centerline Figure 25. The sides of the crankcase are separated from the main bearing caps to minimize the transmission of combustion noise that has been converted to mechanical vibrations from the main bearing caps. Bearing caps are held in place with two or four vertically placed bolts. Lengthening the block walls past the main bearing caps enhances structural rigidity, operating smoothness, and even allows for a larger mating surface with the transmission. A reinforcing bedplate may also be attached to the oil pan rails and main bearing caps to enhance the rigidity and strength of the block’s crankcase Figure 26. High horsepower, deep-skirted engines will often tie the oil pan rails and bearing caps together into a single rigid structure to prevent vibration and distortion of the crankshaft bearing bores. Main bearing caps are held in place not only by bolts but also by using an interference machine fit. This means the cap is slightly wider than the machined ledge it sits in and some force is required to both remove and position the cap correctly in the bearing bore.
Deep-skirted blocks will also use what is called a serpentine block wall in the crankcase area. A serpentine or wavy shape with curves minimizes flexing compared to a flat surface and adds rigidity, much like a curve imparts this property to a body panel or shovel blade. The block wall can be made thinner without sacrificing strength, and the serpentine shape reduces noise transmission from the crankcase Figure 27. Adding ribs and webbing to the crankcase area provides strength to areas that cannot be easily curved.
FIGURE 08-25: Deep-skirt blocks extend the block walls below the main bearing caps. |
FIGURE 08-26: A bedplate attached to the oil pan rails and the main bearing caps of this high-torque output, deep-skirted, International DT-466 engine block adds extra rigidity and strength to the block. |
FIGURE 08-27: A serpentine block wall used by this DD15 engine adds rigidity to the crankcase and reduces noise transmission while allowing for use of a thinner block wall without sacrificing strength. |
©2026 - CDX |
1.2. Cross-Bolted Blocks
LO 8-04: Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block. |
Cross-Bolted Blocks
A variation of the deep-skirt block is a cross-bolted block. Also known as tie-bolted or bolster-bolted blocks , this design uses additional, horizontally placed bolts to connect crankcase walls of the block to the main bearing caps. Cross-bolted blocks permit the use of more than the traditional two or four vertical bolts to hold the main bearing caps Figure 28. By tying together the block walls and main bearing caps, cross-bolting limits crankshaft flexing, stiffens the engine’s structure, and reduces overall vibration Figure 29. This type of block reinforcement technique permits further weight reductions than deep-skirted blocks because the crankcase walls do not extend far past the main bearing caps.
FIGURE 08-28: A cross-bolted main bearing cap in an aluminum block with cast iron liners. Cross-bolting is used to enhance engine block strength and rigidity while reducing noise and lowering engine weight. |
FIGURE 08-29: This older Mack E7 block is cross-bolted to stiffen the main bearing bore. |
©2026 - CDX |
1.3. Ladder-Frame Blocks
LO 8-04: Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block. |
Ladder-Frame Blocks
Some newer engine designs are incorporating ladder-frame crankcase reinforcement as a construction feature. Like other crankcase reinforcement techniques, which can strengthen a block while reducing weight, noise, and vibration, ladder frames provide an additional structure. In ladder-frame blocks, the sides of the block extend exactly to the centerline of the crankshaft bearing bore. A separate, additional ladderlike section of the block structure, called a ladder frame , attaches to the crankcase oil pan rails on one side and to the oil pan on the other. The main bearing caps are integrated into this single piece, lower section of the block (Figure 30 and Figure 31). Removing one main bearing cap requires removing the entire ladder frame because it is formed with all the main bearing caps. The ladder frame may also contain balance shaft housings and the oil pump. Ladder frames considerably improve block rigidity and strength, but they also can add to the cost and complexity of an engine. Another drawback, depending on construction technique, is the potential for leakage in the gasket joint created by the device. Engine crankshaft bearings cannot be serviced in frame or in chassis. Removal and replacement of a crankshaft bearing requires complete removal of the engine from the vehicle Figure 32.
FIGURE 08-30: The ladder-frame block incorporates the main bearing caps into a single structural component resembling the rungs of a ladder. The frame supports the crankshaft and adds structural rigidity and strength to the block while reducing noise and vibration. |
FIGURE 08-31: This 6.0 L (366 in 3 ) ladder-frame block uses a two-piece crankcase. The upper section incorporates the main bearing caps into the crankcase structure. |
FIGURE 08-32: The exterior view of a ladder frame forming the crankcase. |
©2026 - CDX |
1.4. Tunnel-Bore Blocks
LO 8-04: Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block. |
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Tunnel-Bore Blocks Tunnel-bore blocks are one of the smoothest-operating and strongest block designs. Integrating the main bearing bores into a single, solid block structure with no bearing caps makes this block the most resistant to distortion in the main bearing bore, as well as the most rigid, to reduce the transmission of noise and vibration. The crankshaft looks like most other crankshafts, but the main bearing journals are each enclosed with a bearing support that wraps around the main bearing and journal. Parting halves of the bearing supports are bolted together to form a circular enclosure that fits snuggly into circular block bores forming the cylinder block’s crankshaft tunnel. The entire crank and the main bearing enclosures are assembled and then lowered into the block’s tunnel during assembly. Locating bolts in the block align oil passageways through the bearing enclosures to the crankshaft and lock the main bearing supports into position Figure 33. Trucks and buses do not use these types of blocks, but Kubota is one manufacturer using these cylinder blocks for its smooth-running, in-line, four-cylinder engines in compact, off-road equipment, such as skid-steer loaders.
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FIGURE 08-33: This tunnel-bore engine block has the main bearing bores and block formed into one solid structure for maximum block rigidity and strength. |
©2026 - CDX |