Monday, June 13, 2011

Main bearings Service

Fig 5.22 measuring journal size





With an engine dismantled, the main bearings can be checked visually for wear or damage as outlined below (see later section Analyzing bearing failures’). The crankshaft-to-bearing clearance can be checked by measurement or with Plastigage.
Figure 5.22 shows how the clearance is found by measuring the crankshaft journal with a micrometer and the bearing      with a dial gauge. Readings are taken at four different places on the shall, and four measurements are made of the bearing. The dial gauge reading can he checked with the micrometer to determine the bearing clearance.
Fig 5.22

Using Plastigage
The use of Plastigage to check a bearing clearance is shown in Figure 5.23. The procedure is:
I. Wipe the journal and bearing clean of oil.
2. Place a strip of Plastigage in the centre of the bearing.
3. Replace and tighten the bearing cap and do not turn the crankshaft.                                                                                                 Measuring bearing dia.
4. Remove the cap and, with the scale provided, measure the flattened Plastigage at its widest point this is the bearing clearance.

·           For more information on Plastigage, refer to the section ‘Checking connecting-rod bearings’ in later
posts.
Fig 5.23
  
Conditions during measurement
If the engine is removed from the vehicle and upside down, then the crankshaft will rest in the bearings that are in the crankcase. The clearance will be between the journals and the halves of the bearings in the caps. This is where the Plastigage is used to take the measurements.
However, if the engine is installed in the vehicle, the crankshaft will rest in the bearings in the caps. Therefore, it must be held upwards to obtain the clearance and to prevent incorrect measurement.
One way of supporting the crankshaft is to place a small jack under the web of the crankshaft, next to the bearing being measured. Another method is to remove the bearing from the bearing caps on each side of the bearing that is to be measured and insert soft packing pieces. The cap bolts are then tightened to lift and support the crankshaft.

Checking crankshaft end-play
Fig 5.24
Crankshaft end-play will become excessive if the end- thrust bearings are worn. This can produce engine noise in the form of a noticeable sharp irregular knock.
Check the end-play by forcing the crankshaft endwise as far as it will go with a pry bar and then measuring the clearance at the end-thrust bearing with a feeler gauge (Figure 5.24). Consult the engine manufacturer’s manual for allowable end-play.

Replacing crankshaft rear oil seal
Fig 5.25
An oil seal is fitted at the rear main bearing to prevent oil leakage (Figure 5.25). When main-bearing service is being performed, or whenever leaking is noticed at the rear main bearing, the oil seal must be replaced.
The procedure for replacement varies with different constructions. On some engines using a split oil seal, the crankshaft must be removed and a special oil-seal installer used to insert the new seal in the cylinder block.
On engines with seals of the type shown in Figures 5.25 and 5.26, it is not necessary to remove the crankshaft. However, components have to be removed from the rear of the engine to gain access to the oil seal. These include the transmission, clutch and the flywheel. For an automatic transmission vehicle, the components would be the automatic transmission, the torque converter and its drive plate.
The rear oil seal in Figure 5.26 is a lip-type seal that fits into a housing. The housing is then bolted to the block.


Fig 5.26
·         The illustration shows a drive plate for the torque converter of an automatic transmission

Replacing main bearings
In most cases where main bearings are being replaced, the engine will have been removed from the vehicle and completely dismantled.
Bearings are installed as follows:
1. Wipe the bearing bores and the bearing caps clean.
Fig 5.27
2. Coat the upper halves of the bearing with engine oil and slide each bearing into its correct bore in the crankcase (Figure 5.27).
3. Make sure that each bearing is properly seated in its bore and that the tang of the bearing fits into the notch in the bore.
4. Oil the crankshaft and install it in the bearings in the crankcase.

5. Fit the lower halves of the bearings in their caps, making sure that they seat properly.
6. Install the caps in their original positions. A number will be marked on each cap, or there will be some other identifying mark (Figure 5.28). The front of the cap must also be identified.
7. Tap the caps into place with a soft-faced hammer and tighten the bolts to the correct torque.
8. During tightening, rotate the crankshaft to make sure that it is not binding. The sides of the caps can be tapped lightly with a soft-faced hammer to assist with cap alignment.
Where the thrusts are separate from the bearing (Figure 5.29), the bearing is installed in its bore. The thrusts are then installed in their grooves in the crankcase web beside the bearing.

 
Fig 5.28
·         All main bearings are not the same, some are grooved and one could have thrust faces.

Tightening bearing cap bolts
There are a number of designs of main-bearing caps where the cap bolts have to be tightened in a specified sequence. Figure 5.30 shows a sequence for main bearings that have a beam-type cap. The bearing caps are joined together to provide rigidity. The cap bolts must be tightened progressively in the right sequence as well as to the correct torque.
Figure 5.31 shows another crankshaft and bearing assembly. This has four bolts for each bearing cap and
a bearing cap bridge, or beam, that fits over the bearing caps. The cap bolts secure the bridge as well as the bearing caps. The sequence for tightening the bolts is shown. This starts with the two inner bolts of one bearing. It follows a spiral pattern, with the inner bolts being tightened before the outer ones.

Fig 5.29
·         All main bearings are not the same, some are grooved and one could have thrust faces.



In some engines, the main-bearing inserts can be removed and replaced with the engine in the vehicle. This is done without removing the crankshaft from the engine. This would only be done if the crankshaft journals were not worn or scored.
The method involves slackening off all the main- bearing cap bolts one or two turns so that clearance is obtained between the journals and the upper halves of the bearings.
Starting at one end of the crankshaft, remove the main-bearing cap. The upper bearing insert can then be pushed around the crankshaft journal using a piece of thin, soft metal. The insert
Fig 5.30
is pushed on the opposite side to its locating tang.
A new bearing is installed and the bearing cap is replaced to support the crankshaft before proceeding to the next bearing.
When bearings are replaced in this manner, it is essential that every care be taken in regard to cleanliness. Unfortunately, it is not possible to see how the new bearing is located in the housing once it has been installed, so that every care must be taken during assembly.


·      The preferred method of installing main bearings is with the engine completely dismantled and the crankshaft removed.

Fig 5.31
Bearing oil-leak test
Main bearings and connecting-rod bearings can be checked with a bearing leak tester. The oil pan has to be removed so that the amount of oil passing the bearings can be seen.
The tester is basically a pressure container of oil that is connected by a hose to the oil gallery of the engine. It uses compressed air to supply the stationary engine with oil at normal oil pressure.
A normal bearing will leak between about twenty to 150 drops of oil per minute. A bearing will leak more if it is worn or has excessive clearance. Less than twenty drops could mean a restricted passage or blocked oil-way.
If oil passages in the crankshaft and bearing happen to be in line, then more oil than normal will pass through the bearing. This could give the impression that the bearing has excessive clearance.


·         If this happen; the crankshaft should be turned to move the oil holes away from each
other.

Continued
See Analyzing bearing failures>>>>>>>
 

1 comment:

  1. The best bearings would still be the metal ones. Tested and proven to give the best results.

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    ReplyDelete