Thursday, June 23, 2011

Cont. Connecting rod beaing installation

Fig 7.20
When bearings are to be installed, make sure that hand, workbench, tools and all engine parts are clean. Keep new bearings wrapped until ready to be installed and then handle them carefully. To avoid problems, wipe each bearing with a fresh piece of cleaning rag and install it into a clean bore (Figure 7.20).
The bearings have locating tangs that fit into notches in the connecting rod and cap. Make sure that the tangs enter the notches correctly. See the comments about bearing spread and crush that follow.
Bearing clearance cannot be adjusted. Any attempt to correct bearing clearance by filing the connecting-rod cap will destroy the original relationship between the cap and the rod. This will cause early bearing failure.

Bearing spread and crush

Fig 7.21
Bearing inserts are provided with spread and crush (Figure 7.21). With spread, the ends of the bearing are slightly wider than the bore of the rod or cap into which the bearing fits. When the bearing is being pushed into the cap or into the connecting rod, it will snap into place and the spread of the bearing will keep it there.
Crush is caused because the edge of the bearing insert stands above the parting face of the connecting rod or cap. This additional height is ‘crushed’ when the cap is installed and the bolts tightened. Crushing pushes the inserts into the bore in the connecting rod and cap. This ensures that the backs of the inserts are in snug contact with the bore. Figure 7.22 shows
Fig 7.22
the top half of a bearing correctly installed in a connecting rod.
Crush is a general term as described, but it can also refer to the distance that the edge of the bearing stands above the parting face.

·       New hearings will have the correct crush and must not be
tampered with in any way.

Checking connecting-rod bearings

Automotive engines are fitted with precision-insert bearings that are accurately finished to size and require no adjustment. In fact, adjustment should never be attempted as this will ruin the bearing.
The clearance of a connecting-rod bearing can be found by using a telescopic gauge to measure the bearing and a micrometer to measure the journal. However, the easiest way is with Plastigage.


Fig 7.23
Plastigage is a plastic material that comes in strips and flattens under pressure. It is used to check the clearance between bearings and shafts.
When used to check the clearance of a connecting- rod bearing, a strip of the material is placed in the bearing in the cap. The cap is installed and the nuts are tightened to the correct torque. The bearing cap is then removed and the flattened Plastigage is measured
(Figure 7.23). If the Plastigage is flattened only a little, then there is a large-clearance, and vice versa.
A printed scale is supplied with the Plastigage. The scale has bands of different widths which are marked to show the bearing clearance
for example, 0.02 mm, 0.03 mm etc. The scale is placed against the flattened Plastigage to check its width as shown in the illustration.
The scale measures the width of the flattened Plastigage, but shows this as its thickness, which is the clearance between the bearing and the shaft.

Using Plastigage

1. Wipe the crank-pin and bearing clean of oil.
2. Position the crank-pin of the bearing to be checked about 30° before BDC, because this is where the bearing clearance is likely to be greatest.
3. Place a small strip of Plastigage in the centre of the bearing and install the bearing cap on the connecting rod.
4. Remove the bearing cap and check the thickness of the flattened Plastigage against the scale.

·   Do not move the crankshaft while the cap nuts are tight,- this would disturb the Plastigage and give an inaccurate measurement.

Connecting-rod alignment
Fig 7.24
The connecting rod must be in alignment — that is, there should be no twist or bend in the rod. Any misalignment will cause side thrusts on the piston and irregular loading of the bearing.
Figure 7.24 shows the effects of a misaligned connecting rod. The heavy loading at points A and B on the bearing would cause bearing failure at these points.
The heavy-pressure spots C and D on the piston will cause heavy wear and possibly scoring of the piston and cylinder wall. A basic inspection check is to look for uneven wear or shiny spots on the pistons, which will indicate misalignment. If this condition is found, the connecting rod should be checked and as necessary, straightened or replaced.

See connecting rod aligner>>>>>>>>>>

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