Reconditioning or renewing parts
The visual checks made during dismantling and the measurement taken during inspection, will determine which parts are serviceable and which parts are service able and which parts are unserviceable. Unserviceable parts will have to be renewed, reconditioned or repaired. In many cases, particularly with smaller parts and components, a new (replacement) part will be obtained and fitted.
- Machining operations
A complete engine overhaul can include valve grinding, valve seat grinding or inserting, cylinder reconditioning, cylinder head and cylinder block prefacing, crankshaft grinding, camshaft grinding, connecting rod bore resizing and flywheel prefacing. These are all machining operations and with the exception of valve work, are usually beyond the capacity of a general workshop. Jobs this nature is carried out in a specialist workshop equipped with special engine reconditioning equipment.
- Exchange components
To save delay, exchange components are also used, with the worn component being exchanged for a reconditioned one. Reconditioned engines are also available. Short motors (engines) consist of the cylinder block, piston and connecting rod assemblies, and the crankshaft.
- Parts list
Keep list of all the parts needed. This should be done when the engine is first dismantled and the parts have been inspected. List the new parts and the exchange parts required, and also the parts to be reconditioned. The list should include all gaskets and seals, which should always be renewed whenever an engine is dismantled.
Reassembling of the engine components
An engine is reassembled in the reverse sequence to dismantling, but particular attention must be paid to detail. Cleanliness is most important. Any dirt or abrasive particles left behind after grinding valves or honing cylinder could cause damage. They will also accelerate engine wear. Some points to be observed during reassembly are as follows:
- All moving parts should be well oiled as they are being reassembled to prevent corrosion and to provide initial lubrication.
- New gaskets and seals should be carefully fitted so that they are correctly located and not damage.
- Bolts and nuts should be tightened to the specified torque. Threads should be clean, and thread sealer or locking compound should be applied where recommended.
- Bearing bores should be perfectly clean before installing the bearings. The bearing inserts should have sufficient spread to snap into position when being fitted. They should stand slightly above the bore, indicating that they have crush. Tightening of the bearing cap should be completed with a torque wrench.
- The rear main bearing oil seals is not readily accessible once the engine is assembled, so extra care should be taken to avoid leaks.
- The connecting rods should have the pistons correctly installed. Both the pistons and the connecting rods are identified so that they can be reinstalled in their original locations.
- The piston rings should be installed on the piston with their gaps staggered and away from the thrust sides of the engine.
- The pistons, rings and cylinder bore should be liberally coated with oil to facilitate installation when using the ring compressor. The oil also provides initial lubrication.
- The cylinder heads bolts should be tightened to the specified torque, gradually and evenly, working to the recommended tightening sequence. Generally, this is from the center towards the ends. Over tightening or under tightening can lead to thread damage and cylinder head or cylinder bore distortion.
- When the camshaft is installed, timing marks on the gears or sprockets must be correctly aligned for correct valve timing.
- Manifolds should be uniformly tightened to the cylinder head so that neither air nor exhaust gas occur
- Installation of the cylinder head.
Cylinder heads installation will vary from one manufacturer to another, so whenever refitting a head refer to the service to the service manual for the recommended procedure. In the absence of a service manual. Installing a cylinder head is, generally, the reverse procedure to removal. Following are related points:
- Before installing the cylinder head, the surfaces of both the cylinder head and cylinder block must be clean. The bolts and threads in the cylinder block must also be clean.
- Anew cylinder head gasket must be used. Sealer should only be applied if recommended by the manufacturer. Aligning studs can be installed in two diagonally apposite holes to keep the gasket in place. Most gaskets will only fit one way, but some have the top identified as shown in fig 3.10.
- Cylinder head bolts must be tightened in the correct sequence and to the correct torque as shown in fig 3.11. Incorrect tightening can cause head or block distortion, gasket leakage or bolt failure. Generally, a spiral sequence is used, starting with the bolts in the center of the head. Refer to the particular service manual. Cast iron cylinder heads can be retightened when hot, and aluminum heads can be retightened when cold. Some gaskets do not need the head to be retightened.
- Manifold bolts should be retightened gradually, in the correct sequence and to the correct torque as shown in fig 3.12.
- With OHC engines, the camshaft is installed on top of the head. The valve timing is set to the timing marks with No. 1 piston on TDC. Adjust the chain or belt tensioned to provide the correct chain or belt tension.
- When the rocker gear is being installed, screw the bolts down progressively until the shaft is seated on its mountings. Then tighten the bolts to the correct torque. Screw adjustment can be checked off to relieve the load of the valve springs.
- Adjust the valve clearances. This is done with the engine cold. In some cases, the clearances can be rechecked when the engine is hot
- Assembling the crankshaft
Prior to the reassembling the crankshaft back into the block it must be thoroughly cleaned. All plugs must be removed if this has not already been done, and the oil passages cleaned with the aid of a nylon brush and suitable cleaning fluid, as shown in fig 9.34. After the oil passages have been cleaned, install new plugs and circlips if applicable. The block and its oil galleries should also be cleaned prior to assembly.
Before fitting the upper bearing to the block, make sure the bearing bores and the back of all bearings shells is clean and dry. Never put anything between the back of the bearing shell and the bearing housing. After the upper bearing shells are installed, check for oil hole alignment between the bearing and the block. Lubricate all the bearings, and then place the crankshaft gently and squarely on to the main bearings. The main caps with their bearings shells fitted can now be installed in their correct locations. Clean all retaining bolts and lubricate the treads and up under the head of each bolt so as to reduce friction during tightening, thereby obtaining accurate clamp loads. Do not oil internal threads because the hole may partially fill with oil and hydraulic locking will prevent proper tightening of the cap bolts. Tightening of the main bearing cap bolts should be done in stages of one-third torque, two-thirds torque and full torque, or as per the service manual specifications.
An alternative method of tensioning bolts is torque turn tightening, which requires bolts to be retightened to an initial torque followed by a further turning of the bolt head to a specified angle, as shown in fig 9.35.
- Camshaft installation
After the camshaft and its bearings have been inspected, the camshaft can be fitted to the engine. Prior to installing the camshaft, turn the crankshaft and camshaft timing gears until the timing marks on both gear teeth align. At this point, install the camshaft into the block and mesh the two gears together, taking note that the timing is correctly aligned. Secure the camshaft thrust plate in position then check the camshaft gear backlash as shown previously. Depending on the type of cam followers used, they may have to be installed before or after the camshaft is installed.
- 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 (fig 7.12). The ring can be turned in its groove if it is correctly installed.
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 (fig 7.13). This supports the rings 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 7.13. 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 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 7.14 is an example of how the gaps are arranged.
- Installing piston and connecting rod assembly
To install the piston and connecting rods in its cylinder, turn that the crankshaft journal to BDC. Place the piston and connecting rod into its cylinder. While guiding the rod bolts over the crankpin with one hand, tap the piston into the engine with a WOODEN HAMMER HANDLE.
A soft wooden hammer handle will not mar or dent the head of the piston. Hold the ring compressor squarely against the cylinder block face. Keep tapping until the connecting rod bearing bottoms around the crankshaft journal. If piston rings pop out of the compressor, do not try to force the piston down into the cylinder. This would break or damage the piston rings or piston. Instead, loosen the ring compressor and begin again.
- Timing gear installation
The marks on the timing gears must be aligned to provide correct valve timing. When this correct, the timing gear cover can be fitted and the vibration damper installed on the front of the crankshaft. This should be in good condition to prevent engine vibration.