LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal
1. Cylinder Component Service
1.6. Piston Ring Installation
LO 7-09: Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal. |
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Piston Ring Installation Care must be exercised when installing rings onto a piston and into the cylinder because rings are fragile. A common cause of ring breakage is overstretching the ring during installation. Ring expanders are tools that stretch a ring just far enough for it to be removed but not too far to cause it to bend or break. To prevent loss of compression and damage to the cylinder wall, ring end gaps must be staggered 120 degrees apart in a three-ring pack configuration. End gaps should not be aligned near the wrist pins Figure 66. If rings are reused, such as after performing warranty repairs on engines having accumulated low travel distances, extra caution should be exercised when removing pistons from cylinders to prevent ring damage. The wear step or cylinder ridge found above the ring travel area in the cylinder should first be removed to prevent damage to the rings and the piston when both the piston and the rings are pushed up over the ridge during removal. Failure to remove the cylinder ridge, by cutting or grinding it away, will score the piston and even break the rings. |
FIGURE 07-66: Piston ring end gaps must be staggered 120 degrees apart to prevent gases from easily leaking past the rings. |
Use a ring compressor to install a piston into the cylinder and prevent damage to the rings Figure 67. A number of designs are available to ease ring installation. Both the cylinder wall and the compressor should be well lubricated during the installation procedure. A soft hammer handle can be used to gently tap a piston into the cylinder bore.
FIGURE 07-67: Various types of piston ring compressors are used to squeeze rings on the piston while inserting the piston into the cylinder. |
FIGURE 07-68: Honing stones used to apply a crosshatch finish to a cylinder wall. |
When new rings are installed into the cylinder, a new crosshatch finish needs to be made on the cylinder wall. As long as taper wear is within specifications, a ball hone or straight-sided honing stone can be used to put crosshatch finish into the cylinder Figure 68. About one stroke per second is required at 60 rpm to establish the correct angle on the crosshatch finish. Cylinders are rarely honed dry; usually diesel fuel or special cutting oil is used to lubricate the stone. Coarse-, medium-, and fine-grit stones are available, depending on whether a new crosshatch is needed to help seat the new rings and retain oil or to remove some taper wear. If excessive scoring or taper wear is present in a cylinder, the cylinder may require boring out to an oversize dimension. The final dimension depends on the availability of oversize pistons and rings. Oversize pistons in sizes 0.01" (0.25 mm), 0.02" (0.5 mm), 0.03" (0.76 mm), and 0.04" (1.0 mm) are typically available for popular engines constructed without cylinder sleeves.
| Technicians Tip | ||
| It is important to note that compression rings and intermediate rings have directional markings to guide the technician during ring installation. Markings are usually located on the top of the ring and orient the ring to the top of the piston. For the intermediate ring, the direction of orientation is critical because an incorrectly oriented intermediate ring can lead to high oil consumption, excessive blowby, and low compression. |
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