LO 8-5 Identify and describe the configuration and construction of common cylinder block cylinder arrangements

LO 8-05: Identify and describe the configuration and construction of common cylinder block cylinder arrangements.

Parent Bore Cylinder Blocks

A parent bore cylinder block, also commonly called a no-sleeve block , has holes cast and bored into the block, and the pistons are inserted directly into these holes. No provision is made to add a cylinder sleeve to this type of block. The cylinder walls form a part of the block structure, giving it rigidity and strength Figure 34.

The most notable advantage of the parent bore block is that it costs less to construct because machining and fitting of cylinder sleeves is not required. Unlike sleeved blocks, the cylinder walls are structural members of the casting, adding strength and rigidity to the block while requiring less block material. Thin cylinder walls, with minimal material thickness between cylinders, permit compact, lightweight construction. These blocks are best used in applications in which chassis life and expected engine life are approximately equal. Part of the reason for this shorter service life is the relative lack of hardness of cylinder block materials forming the cylinder walls. Cylinder wall surfaces are not as easily hardened as sleeved engines. Without costly alloys and hardening techniques, the cylinder walls will wear at a faster rate than hardened, replaceable cylinder sleeves. Because cylinder walls are irregularly thick, heat transfer differentials create uneven cylinder dimensions and accelerate cylinder wear. Induction hardening of cylinder walls and hardened steel piston rings is one approach used to increase engine durability in parent bore blocks.

FIGURE 08-34: This 6.7 L (409 in³) Cummins is an example of a parent bore block. A parent bore block forms the cylinders and block from a single casting. Water jackets and other internal passageways are cast into the block.

FIGURE 08-35: Reboring increases the diameter of a cylinder, which requires a new, oversize piston and ring set to fit the new cylinder dimensions.

Unlike sleeved engines, wear in a parent bore is offset by the use of coolant jackets formed inside the block that permit cooling of cylinder walls all the way to the top of the block. This feature reduces carbon formation on pistons and other problems related to excess heat near the top of the cylinder.

Another major disadvantage of parent bore blocks is what repair or rebuild methods are available when an engine is worn out. If an engine is rebuilt or repaired at the end of its service life, a worn cylinder must be rebored to an oversize dimension or heavily honed. Reboring requires special machining equipment to cut away the worn parts of the cylinder. The cylinder wall is machine cut to remove its worn dimensions, which gives the walls a straightened, uniform diameter from top to bottom Figure 35. After reboring, the cylinder is honed to give it a proper crosshatch finish. An oversize OEM or aftermarket piston must be available to fit into the larger-diameter cylinder. Often only a 0.040" (1.016 mm) oversize is available for a single major rebuilding. An alternative to boring is to use a straight-sided honing stone to grind in a uniform cylinder dimension. This operation is performed only if the cylinder is not excessively worn and a new piston with the correct oversize dimension of 0.010" (0.254 mm) is available. Both honing and reboring are typically done at a metal machine shop. Usually, the engine must be removed from the chassis and completely disassembled to perform the machining processes. The number of times the engine can be rebuilt is limited to available piston oversizes and the thickness of the cylinder walls. What this all means is parent bore blocks are not as durable or rebuildable as sleeved engines. Note, however, that parent bore CGI blocks do not wear as fast as cast iron parent bore blocks.

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