MGB 4 synchro overdrive gearbox servicing guide

Initial tasks

1.                 Drain oil

2.                Stand box on end to allow oil to drain forward

3.                Drain rest of oil

4.                Undo 6 bolts retaining gearlever extension tower

5.                Remove tower (note it is located on two dowels)

6.                Undo eight nuts retaining OD unit (the unit will have to be eased away from the adapter housing to release some of the nuts

7.                Remove the OD unit


Gearbox disassembly

8.                Remove (and carefully store) the OD pump drive cam and locating ball from the mainshaft

9.                Remove the circlip from the mainshaft (store with the cam & ball)

10.            Remove the selector interlock mechanism from the adapter housing

11.             Undo the bolts and nuts holding the adapter housing to the main case

12.            Remove the adapter housing

13.            Undo the nut retaining the clutch arm pivot pin and remove pin

14.            Remove the clutch release arm

15.            Undo the front plate nuts

16.            Remove the front plate, retaining any shims fitted behind the plate (the condition of these shims will tell you if the input bearing has been rotating in its housing)

17.            Undo the three bolts on the top of the main case and extract the springs and detent plungers

18.            Undo the bolts holding the side plate and remove the side plate

19.            Undo the locking bolts on the selector rods and remove the rods and selectors

20.           Undo the reverse gear locking bolt (may be tablocked or spring washer)

21.            Withdraw reverse gear shaft from the rear of the case and remove gear

22.           Measure and note the layshaft end float

23.           Withdraw layshaft from the front of the case

24.           Recover layshaft thrust plates

25.           Drop laygear into bottom of case and withdraw input shaft and bearing

26.           Recover mainshaft needle roller bearing and third/fourth synchro hub and rings

27.           Withdraw mainshaft & bearing from rear of case (hold up by rear of mainshaft and tap case gently downwards assistant required)

28.           Remove laygear and its needle roller bearings from main case


Gearbox - inspection

29.           Inspect layshaft and needle roller bearings if pitted or worn replace

30.           Inspect mainshaft nose and needle roller bearing if pitted or worn the mainshaft will have to be disassembled and the mainshaft re-tipped

31.            Measure the thrust washer clearances if over spec the mainshaft will have to be disassembled and parts replaced

32.           Inspect the synchro ring to hub gap if less than 1/16 inch the mainshaft will have to be disassembled and rings replaced

33.           Inspect the gear teeth  - if they are chipped, blackened, missing or worn the mainshaft will have to be disassembled and parts replaced

34.           Inspect the synchro dog teeth if badly rounded or worn the mainshaft will have to be disassembled and parts replaced

35.           Slide the first / second hub back and forth if the action is weak then the mainshaft and hubs will have to be disassembled and springs replaced. Ditto the loose third / fourth hub but be careful or springs and balls will fly!

36.           Inspect the main bearing for roughness (this double row bearing rarely fails)

37.           Inspect the rear spacer if this is loose the lock tab can be released and the nut tightened before re securing the locktab

38.           Inspect laygear teeth if worn, chipped, etc replace laygear

39.           Inspect input shaft for wear

40.           Inspect main case for wear and cracks


Mainshaft disassembly

41.            Release lockwasher behind front nut

42.           Undo front nut

43.           Slide off the gears, etc. keeping them in order

44.           The first gear and main bearing are pressed on from the rear of the shaft and need not be disturbed unless they need replacing


Mainshaft reassembly

45.           Clean all parts thoroughly and lightly oil

46.           Reassemble in reverse order, replacing worn parts

47.           Fit new lockwasher and nut

48.           Stake lockwasher into nut recesses


Input shaft bearing replacement

49.           Knock back the lockwasher

50.           Undo retaining nut (left hand thread!)

51.            Press off bearing

52.           Press on new bearing (circlip towards the front)

53.           Do up nut

54.           Secure lockwasher


Gearbox reassembly

55.           Clean all parts thoroughly lightly oil shafts / bearings

56.           Place the roller bearings inside the laygear and place in the bottom of the case

57.           Insert the input shaft tap the bearing into place

58.           Stand case on end and place mainshaft needle roller bearing in the input shaft recess

59.           Place fourth / third synchro hub and rings in place on the input shaft cone

60.           Insert mainshaft from rear (top) and line up dowel in case with cut-out in bearing housing

61.            Tap main bearing housing into case, guiding the mainshaft into the roller bearing at the same time

62.           Ensure that the main bearing and input bearings are fully seated in their housings and that there is not excessive end float on the fourth / third synchro hub

63.           Insert the laygear end thrust washers and align the laygear (a thin rod helps here)

64.           Insert the layshaft from the front and align the cut-out in the front end with the recess in the front plate

65.           Recheck the laygear end float

66.           Replace the front cover and measure the gap between the cover and the case subtract 12 thou from this figure and note the result

67.           Remove end cover and fit new gasket

68.           Fit shims to the thickness of the result of 66 above to plus 1 or minus 0 thou

69.           Fit front cover

70.           Fit the clutch fork and pivot bolt

71.            Replace the reverse gear in the case and insert the shaft from the rear

72.           Align the locking hole (slot in the end helps turn the shaft) and replace locking bolt & tab / lockwasher

73.           Place the selectors on the synchro hubs and reverse gear

74.           Insert the selector rods from the rear of the case and replace locking bolts

75.           Replace detent plungers, springs and retaining bolts

76.           Fit side plate with new gasket

77.           Fit oil drain plug

78.           Fit the adapter housing

79.           Fit the selector interlock plate


Overdrive Unit disassembly

80.            Remove filter cover and filter screen

81.            Undo relief valve plug and remove relief valve

82.           Undo pump plug and remove pump (don't lose the ball bearing)

83.           Undo the bolts holding the ID plate and remove solenoid (don't lose the ball bearing)

84.           Undo nuts holding the case together

85.           Separate the halves

86.           Undo the four nuts holding the piston operating bars

87.           Withdraw the sliding member refit bars and nuts to retain springs and spacers

88.           Using thin nosed pliers, pull out the operating pistons

89.           Remove the speedo drive bearing and pinion

90.           Clamp the drive flange and undo the retaining nut

91.            Withdraw the flange and rear oil seal


Overdrive unit inspection

92.           Examine the teeth inside the annulus for wear

93.           Examine the one way clutch for correct operation

94.           Examine the planet gear teeth for wear and the bearings for smooth operation

95.           Examine the sun gear teeth for wear

96.           Examine the sliding member bearing for smooth operation

97.           Examine the cone clutch for burning, loose rivets and wear

98.           Examine the bronze bushing in the front casing (or thrust washer) for wear or missing parts (if in need of repair, send away to ORS)

99.           Examine the relief valve and pump for wear or heavy scoring

100.        Examine ball valve seats for correct seating

101.         Examine the front casing for loose circlips

102.        Place the pump one way valve spring in its recess in the pump retaining screw and ensure that it stands 1/16 to 1/8 inch proud of the surface if not then gently stretch the spring by running a thin screwdriver up the coils sideways

103.        Ensure there are no sharp edges on the pump retaining screw that will catch on the spring lightly chamfer using a drill if necessary


Overdrive unit reassembly

104.        Clean all parts thoroughly lightly oil bearings and gear assemblies

105.        Renew O rings on operating pistons, lightly oil and fit to front case

106.        Refit sliding member (cut-outs on bars face forward)

107.        Fit new rear oil seal, lightly oil seal surface

108.        Fit drive flange and torque nut to 55 / 60 ft lbs

109.        Align the marks on the planet gear and insert into annulus gear

110.         Apply a thin coating of Loctite (red) to the mating surfaces and loosely assemble the front and rear cases

111.          Align the internal splines and slide the unit temporarily onto the gearbox mainshaft to ensure correct alignment

112.         Tighten nuts clamping the two halves and remove unit from mainshaft take care not to rotate drive flange from now on

113.         Renew O rings on the relief valve assembly and fit to case

114.         Renew O rings on the pump assembly and fit to case note that the flattened side of the pump operating shaft faces rearwards against the case

115.         Fit the new filter screen and filter cover

116.         Renew O rings on the solenoid assembly and fit to the case

117.         Renew the O ring on the speedo drive bearing

118.         Fit speedo pinion and bearing into case


Final assembly

119.         Place new gasket on gearbox adapter housing

120.        Fit the circlip, locating ball and pump drive cam to the gearbox mainshaft

121.         Rotate the mainshaft so that the cam lobe is uppermost

122.        Slide the OD unit onto the mainshaft

123.        If it fails to reach the studs the internal splines are not aligned remove OD and using two spanners to lift the operating bars, use a long screwdriver to rotate the cone clutch until the splines are aligned

124.        If it fails to mate within 3/8 inch then the pump operating roller is caught on the cam, use a thin long screwdriver to depress the pump plunger

125.        Fit the retaining nuts, note that some must be fitted with the OD unit slightly raised from the adapter housing

126.        Fit a new gasket to the remote tower housing and fit the housing, locating the plastic bush into its hole in the selector arm

127.        Finally  - check the gearlever anti rattle spring & plunger in the tower for presence and free operation