Wednesday, June 22, 2011

Checking rings on a piston

Fig 7.12
Rings are checked in their grooves to make sure that they are free and also that the groove is the right depth. The grooves must be clean, with all carbon removed. A groove-cleaning tool or a broken ring is used for this purpose.
Place the face of the new ring in the groove, as shown in Figure 7.12, and roll it around the piston. it should be free in all parts of the groove. If the ring is tight in any spot, the groove should be cleaned and the ring tried again.
Check the ring-groove depth. With the face of the ring held into its groove, place a steel rule across the lands.
There should be clearance between the rule and the ring. Average specifications are around 0.3 mm. Some replacement rings might require deeper grooves, and the instructions supplied by the manufacturer should be closely followed.

Fig 7.13
Ring side clearance
The side clearance of the ring in the groove is checked with a feeler gauge (Figure 7.13). This can be done by holding the ring in its groove, or with the ring installed on the piston. Average specifications for side clearance are 0.035 mm for petrol engines and 0.05 mm for diesel engines.

Installing rings on a piston
The steel rails and separators of segmental rings are carefully wound into the groove in the piston, one piece at a time, making sure that they are correctly seated (Figure 7.14). The ring can be turned in its groove if it is correctly installed.
Fig 7.14

Cast iron compression rings can be installed in a similar way, but care must be taken that the

rings are not distorted, The best method for compression rings is with a ring-expanding tool (Figure 7.15). This supports the ring while it is being spread. The ring should be expanded just enough to allow it to pass over the piston.

Compression rings
A compression ring with a tapered face or counter-bore must be installed with the correct side of the ring to the top, as shown in Figure 7.15. If a ring is fitted upside down, its action will be reversed. A scraper ring, for example, would carry oil up the cylinder wall into the
Fig 7.15
combustion chamber. This would cause high oil consumption and associated problems.

Ring gaps
When the rings are installed in their grooves, the ring gaps should be staggered so that all the gaps are not in line. The gaps are kept away from the thrust sides of the piston. Figure
7.16 shows a set of rings and how the gaps are arranged.

Installing pistons in cyLinders

Fig 7.16
A piston-ring compressor is used to install the piston in the cylinder (Figure 7. 17). The compressor clamps around the piston rings and compresses them into their grooves so that they will enter the cylinder bore without being damaged.
Following are related points:
1. Oil the piston and rings and stagger the ring gaps.

2. Fit a piece of plastic tube over the connecting-rod bolts so that they will not scratch the crank-pin journal (Figure 7.18).

3. Fit the ring compressor over the piston and tighten it firmly so that the rings are compressed.
4. Place the piston and connecting rod in the cylinder, with the front mark of the piston towards the front of the engine.
5. Insert the piston using light even taps with a hammer handle on the head of the piston. Only light taps will be needed if a good compressor is used and operated correctly. Heavy

Fig 7.17
blows will damage the piston and break the rings.
6. Hold the compressor firmly against the surface of the cylinder block so that there is no gap for the rings to escape.

·       The piston rings must be compressed to enter the cylinder. A ring that is not held by the compressor is likely to be broken.

Installing connecting-rod bearings
New precision connecting-rod bearings are required if the old ones are defective or have worn so much that clearances are excessive. New bearings will also be required if the crankpin journals have worn out-of- round or tapered to the extent that
Fig 7.18
the crankshaft has to be reground. In this case, the journals will be finished undersize and undersize bearings would be used. Undersize bearings would have some form of identification to show that they were not standard bearings (Figure 7.19).
Crank-pin journals should be checked with a micrometer for taper or eccentricity. If the journals are out-of-round or tapered more than 0.04 mm (example) the crankshaft must be replaced or the crank-pins reground.

·       For bearing faults, refer to the section ‘Analyzing bearing failures’ in previous posts
Fig 7.19
showing typical engine bearing problems.

See Installing connecting rod bearings>>>>>>>>>>

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