LO 8-3 Identify and describe the construction techniques and composition of cylinder blocks and outline the advantages and disadvantages of each design
1. Cylinder Block Materials
1.1. Cast Iron
LO 8-03: Identify and describe the construction techniques and composition of cylinder blocks and outline the advantages and disadvantages of each design. |
Cast Iron
Cast iron is metal made from iron ore with a carbon content of 3–5%. The carbon is scattered throughout the metal in flakes, creating a material with a crystalline-like structure. The metal is poured or cast into molds, commonly made from sand or fiberglass. The advantages of cast iron are that it is inexpensive to produce, is easy to manufacture and machine, it absorbs vibration well, is inherently strong, and is corrosion and heat resistant. The high carbon-graphite content of iron makes it a compatible, or low-friction, wear-resistant surface for moving parts, such as pistons and piston rings. To improve strength, hardness, and corrosion resistance, manufacturers may add nickel, molybdenum, and chromium alloys to the casting. Cast iron is still not as strong as steel, and its crystalline structure allows it to fracture easily rather than bend. This means machined threads in the block for cap screws usually have a coarse thread pitch, which have deeper thread engagement to minimize the problem of pulled or stripped threads. Main bearing caps are often made from harder cast alloys in high horsepower applications. These caps are identified by a different color or appearance of metal from the block casting.
| Technicians Tip | ||
| Cast iron cylinder blocks, heads, and other cast iron components cannot be easily repaired by welding. Broken or cracked parts most often require replacement. Some block porosity problems, causing oil or coolant leaks, can be repaired with special epoxies. |
Iron Casting and Block Seasoning
Sand casting cylinder blocks requires the removal of sand from inner passageways. Core plugs, sometimes called frost plugs (because they are pushed out by frozen coolant), are used to cover holes made in the block to flush sand out of narrow passageways during casting (Figure 13 and Figure 14). After rough casting, blocks tend to warp when rapidly and unevenly cooled. To prevent this, diesel blocks are often stored at high temperatures for a day or two to heat-treat and “season” them. Heating the blocks while slowly and evenly cooling them not only hardens them but also makes them resistant to stress and minimizes future distortion. Subsequent to seasoning, castings are bored, tapped, and machined to finish. Induction hardening may also be performed to increase the hardness and durability of cast iron cylinder blocks. Induction hardening is a heat treatment process that involves passing alternating electrical current through coils of heavy-gauge wire surrounding the material to be hardened. Through magnetic induction, heat is produced in the metal, which is then quenched with water to produce a hard, wear-resistant, metal surface Figure 15.
FIGURE 08-13: Core plugs are installed in a block only after casting sand, used to form the block’s internal passageways, is cleaned out of the block through vibration and pressure washing. |
FIGURE 08-14: Two types of core plugs are domed and dished. |
FIGURE 08-15: The arrow shows an area of discoloration where the upper third of this cylinder has been induction hardened. Induction hardening increases engine life by minimizing cylinder wall wear in the ring turnaround area where there is the least amount of cylinder lubrication. |
| Technicians Tip | ||
| Core plugs, also called expansion or frost plugs , are convex metal discs used to seal round holes in castings left to clean out sand. Occasionally, plugs can leak, corrode, or require removal to install electric block heaters. To remove a plug, never hit it directly or on the edge with a punch or screwdriver. Often, attempts to remove the plug this way will end up sinking it deeper and cause it to become stuck in the block. To remove a core plug, drain the coolant and drill a ¼" (6.35 millimeter [mm]) hole off-center through the plug. Using a ¼" (6.35 mm) drift punch placed in the off-set hole, pry up or down on the plug to cause one edge to protrude enough from the block where it can be grasped with a pair of pliers to complete the removal process. Reinstall a new plug, using a driver, after cleaning and sealing the plug and block edges with bearing sealant. |
Disadvantages of Cast Iron
Cast iron is heavy, and engines made from cast iron add considerable weight to the vehicle in comparison to lighter materials, such as aluminum or CGI. This can negatively affect vehicle handling and braking characteristics. Heavy engines need larger springs and heavier suspension components, and they tend to dive more in the front end when braking. Cast iron does not transfer heat as readily as aluminum and cannot be as easily repaired as steel or other metals. Whenever repairs to damaged blocks are made, welding techniques involve preheating the block and using a welding rod with high nickel content or a powdered iron welding rod. An original equipment manufacturer (OEM) repair recommendation for block porosity, where oil cools and seeps from a block wall, is to apply epoxy sealant over the affected area. This repair technique can only be performed in areas where mechanical stress is not significant.
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