Piston, Connecting Rod, Cylinder Bores
Pistons, connecting rod, cylinder bores
Removal of pistons
This method presupposes that the cylinder head has been removed, see.
1 Remove any burrs or carbon deposits from the tops of the cylinders.
2 Fit lifting beam 83 94 850141 to keep the engine raised.
3 Remove the sub frame and oil sump.
4 Wipe up any dripping oil.
5 Remove the big-end caps and fit protective collars 75 19 53 on the studs. Push the pistons and connecting rods out of the cylinder bores. Note the markings on the pistons and connecting rods so that they can be refitted in their original positions. Loosely fit the bearing shells and caps on the connecting rods so that each set is kept separate and the parts are not mixed.
6 Remove the gudgeon pin circlips and drive out the gudgeon pins, using a plastic mallet and drift.
Matching of pistons
IMPORTANT: Pistons of different makes must not be used in the same engine. The name of the manufacturer is cast inside the piston.
Spare pistons are stocked in both standard and oversize diameters. Where the latter are used, the cylinder bore must be honed or rebored to obtained the correct piston clearance.
When matching pistons to the cylinder bores, use a 1/2" wide feeler gauge. First, lightly oil the wall of the cylinder in which the piston will be working and then insert the piston, without rings. Attach the feeler gauge to a spring balance and insert it between the piston and cylinder wall at right angles to the axis of the gudgeon pin. At a tractive force of 8 - 12 N (1.8 - 2.7 lbs.), the mean clearance equals the thickness of the feeler gauge. Repeat the test with the piston at several different depths. For piston clearances, see.
Example cylinder classification A piston type A.
When the piston clearances are measured in the engine by means of a 0.05 mm feeler gauge, no measurable force will be recorded on the spring balance when withdrawing the feeler gauge from the No. 2 and No. 3 cylinders. This means that the piston clearance in these cylinders is greater than 0.05 mm and the maximum bore in them is therefore judged to be 90.020 mm. Since the pistons may have bedded-in to a certain extent, giving a further +0.003 mm, it may be assumed that the bore is 90.023 mm. Assuming a standard class B piston, the theoretical piston clearance will be 0.034 - 0.052 mm and this lies within the clearance limits which for class A piston/class A cylinder is 0.031 - 0.069 mm, and for class B piston/ class B cylinder is also 0.031-0.069 mm. In No. 1 and No. 4 cylinders, the pistons cannot be fitted into the cylinder bores with a 0.05 mm or 0.04 mm feeler gauge inserted. With a 0.03 mm feeler gauge inserted the force necessary to withdraw it will be 20 N (4.6 lbs.) and with a 0.02 mm feeler gauge it will be 6 N (1.4 lbs.). We can therefore assume a piston clearance of 0.024 mm in the No. 1 and No. 4 cylinders and the pistons these cylinders will therefore not need to be replaced.
Piston rings
Fitting in a new or rebored cylinder
1 Push the piston rings down into the cylinder one by one, using a piston to position them correctly.
2 Measure the ring gap with a feeler gauge as shown. If necessary, widen the gap by means of a special file. For correct gap sizes, see.
Fitting in a worn cylinder
Piston rings for a worn cylinder should be matched in the same way as for a new or rebored cylinder, but they must be tried at the lower limit of piston travel because the bore is narrowest at this point.
In connection with piston ring replacement, the cylinder bore should be honed before the rings are fitted.
a Use a piston ring clamp to fit the rings on the piston.
b The lower compression ring should be fitted with the side marked 'lop" facing up.
c Check the fit of the piston rings in their grooves by rolling them round the grooves. Also measure the clearance at several points.
d Oil the piston and rings before assembly. Turn the compression rings so that their gaps are roughly 180° to each other and position each of them above one of the gudgeon pin holes.
e Make sure that the gaps in the top and bottom rings of the three-part scraper ring are staggered round the piston and not in line with each other.
Fitting pistons (in situ)
1 Assemble piston and connecting rod by driving in the gudgeon pin, using a plastic mallet and drift. Make sure that the mark c the piston crown faces the timing cover and that the numbers on the connecting rods face the exhaust side.
2 Fit the gudgeon pin lock ring.
WARNING: It is extremely important to fit the lock ring with its opening upwards.
3 Place the bearing halves in place in the connecting rods.
4 Fit protective collars 75 19 531 over the connecting rod studs and oil the piston rings, bearings and cylinder.
5 Fit the piston, using piston fitting tool 78 62 287.
IMPORTANT: Pistons of different makes must not be used in the same engine. The name of the manufacturer is cast inside the piston.
6 Fit the big-end bearing caps with bearing halves (connecting rod numbers facing in the same direction).
Tightening torque 20 Nm + 70° (15 ft. lbs. + 70°).
7 Fit the subframe and oil sump, see.
8 Place a wedge 83 95 239 between the sub frame and oil sump and remove the lifting beam.