Overhaul of 4G18 Engine
1)Main Inspection Items and Standards
（1）Cylinder head inspection:the surface levelness of cylinder head is below 0.03mm,and the grinding extreme value is 0.2 mm.
（2）Cylinder block inspection:the surface levelness of cylinder block is below 0.03mm,and the extreme value is below 0.1 mm.
（3）Bearing shell clearance:The standard value of connecting rod bearing shell is 0.02–0.04mm and the extreme value is 0.1 mm.The main bearing shell clearance is 0.02-0.04mm.The standard value of crankshaft axial clearance is 0.05-0.18 mm.
（4）Gas cylinder inspection:Bore diameter of gas cylinder is 75.5 mm,cylindricity of gas cylinder is below 0.01 mm and the standard value of clearance between gas cylinder and piston is 0.02–0.04 mm.
2)The Selective Matching of Bearing Shell
Grade No.of main bearing shell=Grade No.of main journal+Grade No.of spindle hole
Grade No.of connecting rod shell=Grade No.of connecting rod journal+Serial No.of connecting rod.Compare it with the base number 4,and then convert it to Grade 1,2 and 3.The specific conversion method is shown in Table4.21.
Table4.21 The Specific Conversion Method
The position marking of various grades of 4G18 Engine bearing shell selective matching is shown in Fig.4.35.
Fig.4.35 The Position Marking of Various Grades of Bearing Shell Selective Matching
3)Torque Requirements of Main Bolts
The torque requirements of main bolts of 4G18 Engine are shown in Table4.22.
Table4.22 The Torque Requirements of Main Bolts of 4G18 Engine
Among them,cylinder head bolt,connecting rod bolt and bearing cap bolt adopt the torque angle protractor to fasten the bolt and the torque angle protractor is shown in Fing.4.36.
Fig.4.36 Torque Angle Protractor
4)Main Points of Timing Gear Assembly
(1)The front side of camshaft,the inserted part(internal and external)of VVT timing gear,the VVT timing gear bolt thread and the surface of bolt cap and the VVT timing gear bolt side should be painted less motor oil,as shown in Fig.4.37.
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Fig.4.37 Parts Needed to Paint Less Motor Oil
(2)Insert VVT timing gear into the inside and make sure that the alignment pin is inserted into the spacing hole.Then use the wrench to fix the hexagon part of camshaft and ensure that there is no rotation of VVT timing gear.Lastly,screw the VVT timing gear to(55±5) N·m.
(3)When installing the crankshaft timing gear,make sure that the timing mark on the crankshaft timing gear is aligned with the timing mark on the front shell,as shown in Fig.4.38.
Fig.4.38 Installation Position of Crankshaft Timing Gear
(4)When installing the camshaft,make sure that the VVT timing gear mark and camshaft timing gear mark are aligned with the marks on the rocker chamber cover respectively,as shown in Fig.4.39.
Fig.4.39 Installation of Camshaft Timing Gear
(5)Timing pulley Installation.Install the timing pulley on the crankshaft timing gear firstly,and then on the idle gear,camshaft timing gear,VVT timing gear in sequence and on the tension gear lastly.
(6)Install the automatic tensioner and remove the metal needle.
Note:When dismantling,use the metal needle to fix the ejector pin of the automatic tensioner to avoid being popped out.
(7)Rotate the crankshaft two rounds clockwise and check whether the extrusion length of automatic tensioner ejector pin is in the standard range of 4-5 mm or not.If yes,it means the tensile force is appropriate.
5)Selective Matching of Tappet
Rotate the crankshaft clockwise to make the piston arrive at the compression upper dead point of one cylinder and then use the thickness gauge to measure the valve clearance marked by the arrowhead in the figure.Then rotate the crankshaft one round clockwise to make the piston arrive at the compression upper dead point of four cylinders and then use the thickness gauge to measure the clearance between the rest valves.
Fig.4.40 Valve Inspection
If the valve clearance doesn’t comply with the requirement,then we use the below formula to calculate the thickness of valve tappet which is selected for matching.
Intake valve:A＝B＋(C-0.22 mm)
Exhaust valve:A＝B＋(C-0.30 mm)
Among them,A represents the new selected valve tappet,B stands for the thickness of the original tappet and C is the measured valve clearance.
Note:the thickness of tappet is between 2.70-3.30 mm,and the thickness difference between the adjacent two models is 0.02 mm and there are 31 models.
Fig.4.41 Tappet Models(302 stands for the tappet with thickness of 3.02mm and 300 stands for the tappet with thickness of 3.00mm)