Engine Valve System part14

Finding Top Dead Center (TDC)

Timing a camshaft has long been considered a simple matter of aligning the markings on the cam and crank sprockets. However, when setting up a high performance engine, which involves camshaft replacement, this method permits the crankshaft­ to-camshaft relationship to be several degrees off, either advanced or retarded. Therefore it is impor­ tant that a new camshaft be “degreed in” to insure maximum performance. To do this it is first neces­ sary to make an accurate check for top dead center.

There are several methods of doing this, and the two most common involve the use of either a positive piston stop or a dial indicator and a degree wheel. The first of these is probably the most popular and is carried out as follows:

1. The crankshaft, pistons, camshaft, cam gear­ ing and lifter should be in place. Rotate the crank until No. 1 piston appears to be at TDC. The lifters for No. 1 cylinder should be on the base circle of the cam.

2. Mount a degree wheel ( 360° protractor) on the nose of the crankshaft and fasten a suitable pointer to the block. Align the de­ gree wheel so that the pointer indicates TDC and tighten the degree wheel to the crank­ shaft.

3. Rotate the crank opposite to its normal direc­tion to lower No. 1 piston enough to allow for the installation of a piston stop. (See Fig.176).

4. Install the stop on the cylinder block and slowly rotate the crank in its normal direction until the piston is against the stop. Record the reading on the degree wheel.

5. Rotate the crank opposite its normal direc­ tion until the piston again touches the stop. Record the reading on the degree wheel.

Note that reversing the crankshaft rotation takes up any clearance in the connecting rod bearings which could otherwise affect the second reading of the degree wheel.

6. If the degree readings on either side of TDC are the same, the pointer is reading the cor­ rect position of TDC, and no correction is needed. If the readings are not identical, it

will be necessary to rem ove the piston stop and rotate the crank until the pointer is aligned exactly half way between the two degree readings. This point is TDC. Make the necessary correction on the damper by making a scribe mark to indicate exact TDC. This will insure that future checks with a timing light will be properly oriented. With­ out disturbing the crank, carefully loosen the degree wheel and adjust it so the pointer aligns with TDC. Then tighten the degree wheel in place.

Timing the Camshaft

With TDC established and the degree wheel showing the correct location of TDC, the camshaft is now ready to be “degreed in”.

The following procedure for accurately timing the camshaft involves the determination of the cam lobe center line or point of maximum lift in relation to TDC and degrees of crankshaft rota­ tion.

1. Install the camshaft, sprocket and chain.

With the camshaft in place, insert the tappet for the intake valve of No. 1cylinder.

2. Place a dial indicator on the tappet with the cam at maximum lift. Make certain that you are working with the intake cam lobe and tappet for No. 1 cylinder. The degree wheel with the correct location of TDC should be installed on the crank.

3. The next step involves use of the timing card received with the camshaft from its manu­ facturer.

Let us assume the card reads as follows:

Using this information, it is possible to compute the cam centerline or point of maxim um lift. The cam centerline comes at half of total intake dura­ tion or 165°. To find when maximum lift occurs in relation to TDC, subtract the point of intake opening from the 165° or 165° – 60° = 105°. In this example, the point of maximum lift occurs 105° after top dead center.

Effect of Valve Lash on Cam Timing

A final point should be covere d before leaving the subject of camshafts. Total valve duration and timing can be materially altered by changes in valve lash setting. Increased valve lash will shorten duration and make the valve open later and close earlier, since more crankshaft rotation is required to close the gap between the rocker arm and valve stem and open the valve. Tightening the lash will effectively lengthen duration and may increase engine output at higher speeds. Generally speaking, .010″ of valve lash difference will be about one degree of cam advance or retard. Vary­ ing the valve lash could be a way to increase power, but it will affect valve timing and can be extremely hard on the valve train.

Review Questions

23. What condition would be indicated by a low compression reading between two adjacent cylinders? …………………. · · · · · ·· · · ·· · · · · · · · · ·

24. Describe the procedure for positioning the valve lifter correctly prior to a cold valve adjustment. …………………… ···· · · · · · · · · ···· ··

25. What is the purpose of the adjusting screws on some engines with hydraulic lifters? …..

26. Why is it inadvisable to remove a cylinder head when the engine is hot? …………… .

27. Why is it important to use a torque wrench when tightening cylinder heads? ……… .

28. Explain the meaning of the term “pounds:feet.” …………………………….. .

29. Explain the purpose of valve stem seals on overhead valve engines …. …………… . .

30. How is excessive valve stem clearance eliminated on engines without replaceable valve guides? ………………… · · · · · ·· · · ··· · ··· · · · ··· · · · · ·

31. How does a valve spring tester measure spring tension? ……… . .. . ………… .. .

32. When using a valve spring tester, what precaution should be taken to avoid weakening the spring? ……………………………………………………… ..

33. When grinding valves, why is it important to have the correct valve seat width? ….. .. .

34. Explain the difference between valve face overlap and rim margin . . . . . . . . . . . . . . . . . . .

35. Aside from shortened valve life, what other effect would insufficien t rim margin have on engine performance? …………………………………….. . . .. … . · . …. ··

36. In valve work, what is meant by topping? throating? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37. The angle between the valve face and the seat is called the interference angle. What advantage is gained by the use of this angle? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

38. After the valves have been ground, describe two methods of determining how tight the valves fit into the seats? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39. How is dry ice (carbon dioxide) used in valve work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40. Why is it a mistake to assume that the oil in a hydraulic valve lifter gives a cushioning effect? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41. When repairing valve lifters, what precaution must be taken to maintain the original “selective” fit of the lifter components? . . . . . . . . . . . . . . . . . . . . . . . . . . .

42. What are the most frequent causes of hydraulic lifter troubles? . . . . . . . . . . . . . . . . . . . . . .

43. What effect would too much oil in the crankcase have on hydraulic valve lifter operation?

44. How is a compression test made on a unit-type hydraulic valve lifter? . . . . . . . . . . . . . . . .

45. Describe the two methods commonly used to mark valve timing on chain drive valve gear.

46. What is meant by the taut side of a timing chain? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47. What types of camshaft drives are currently being used on most American passenger car engines? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48. Why is it practically impossible to have valve timing a mere two, three or five degrees early or late? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49. What is the purpose of a camshaft thrust plate? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50. On engines using timing gears, what is the purpose of not using a steel gear at the cam-shaft? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51. What type of valve lifter can be removed without removing the camshaft from the engine?

52. How does the oil pump or distributor drive gear interfere with camshaft removal? . . . . . .

53. Why do camshaft bearings so seldom require replacement? . . . . . . . . . . . . . . . . . . . . . . . . . .

54. What is the function of a welch or expansion plug? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .