Pistons and connecting rods - removal and inspection

Engine removal and overhaul procedures / Pistons and connecting rods - removal and inspection

Removal
1 Refer to Part A of this Chapter and remove the cylinder head, flywheel, sump and baffle plate, oil pump and pickup.

2 Inspect the tops of the cylinder bores; any wear ridges at the point where the pistons reach top dead centre must be removed; otherwise the pistons may be damaged when they are pushed out of their bores. This can be achieved with a scraper or ridge reamer.

3 Scribe the number of each piston on its crown, to allow identification later; note that No 1 is at the timing belt end of the engine.

4 Using a set of feeler blades, measure the bigend to crankpin web thrust clearance at each connecting rod and record the measurements for later reference (see illustration).

5.4 Measure the big-end to crankpin web thrust clearance at each connecting rod

5 Rotate the crankshaft until pistons No 1 and 4 are at bottom dead centre. Unless they are already identified, mark the big end bearing caps and connecting rods with their respective piston numbers, using a centre punch or a scribe (see illustration). Note the orientation of the bearing caps in relation to the connecting rod; it may be difficult to see the manufacturers markings at this stage, so scribe alignment arrows on them both to ensure correct reassembly. Unbolt the bearing cap bolts/nuts, half a turn at a time, until they can be removed by hand. Recover the bottom shell bearing and tape it to the cap for safe keeping. Note that if the shell bearings are to be re-used, they must be refitted to the same connecting rod.

5.5 Mark the big end bearing caps and connecting rods with their respective piston numbers

6 On certain engines, the bearing cap bolts will remain in the connecting rod; in this case the threads of the bolts should be padded with insulating tape, to prevent them from scratching the crankpins when the pistons are removed from their bores (see illustration).

5.6 Pad the threads of the big end bolts with tape

7 Drive the pistons out of the top of their bores by pushing on the underside of the piston crown with a piece of dowel or a hammer handle. As the piston and connecting rod emerge, recover the top shell bearing and tape it to the connecting rod for safekeeping.

8 Turn the crankshaft through half a turn and working as described above, remove No 2 and 3 pistons and connecting rods (see illustration) Remember to maintain the components in their cylinder groups, whilst they are in a dismantled state.

5.8 Removing a piston from the cylinder block

9 Insert a small flat bladed screwdriver into the removal slot and prise the gudgeon pin circlips from each piston. Push out the gudgeon pin and separate the piston and connecting rod.

Discard the circlips as new items must be fitted on reassembly. If the pin proves difficult to remove, heat the piston to 60°C with hot water - the resulting expansion will then allow the two components to be separated.

Inspection
10 Before an inspection of the pistons can be carried out, the existing piston rings must be removed, using a removal/installation tool, or an old feeler blade if such a tool is not available. Always remove the upper piston rings first, expanding them to clear the piston crown. The rings are very brittle and will snap if they are stretched too much - sharp edges are produced when this happens, so protect your eyes and hands. Discard the rings on removal, as new items must be fitted when the engine is reassembled.

11 Use a section of old piston ring to scrape the carbon deposits out of the ring grooves, taking care not to score or gouge the edges of the groove.

12 Carefully scrape away all traces of carbon from the top of the piston. A hand-held wire brush (or a piece of fine emery cloth) can be used, once the majority of the deposits have been scraped away. Be careful not to remove any metal from the piston, as it is relatively soft. Note: Take care to preserve the piston number markings that were made during removal.

13 Once the deposits have been removed, clean the pistons and connecting rods with paraffin or a suitable solvent, and dry thoroughly. Make sure that the oil return holes in the ring grooves are clear.

14 Examine the piston for signs of terminal wear or damage. Some normal wear will be apparent, in the form of a vertical ‘grain’ on the piston thrust surfaces and a slight looseness of the top compression ring in its groove. Abnormal wear should be carefully examined, to assess whether the component is still serviceable and what the cause of the wear might be.

15 Scuffing or scoring of the piston skirt may indicate that the engine has been overheating, through inadequate cooling, lubrication or abnormal combustion temperatures. Scorch marks on the skirt indicate that blow-by has occurred, perhaps caused by worn bores or piston rings. Burnt areas on the piston crown are usually an indication of pre-ignition, pinking or detonation. In extreme cases, the piston crown may be melted by operating under these conditions. Corrosion pit marks in the piston crown indicate that coolant has seeped into the combustion chamber and/or the crankcase. The faults causing these symptoms must be corrected before the engine is brought back into service, or the same damage will recur.

16 Check the pistons, connecting rods, gudgeon pins and bearing caps for cracks.

Lay the connecting rods on a flat surface and look along thelength to see if it appears bent or twisted. If you have doubts about their condition, get them measured at an engineering workshop. Inspect the small end bush bearing for signs of wear or cracking.

17 Using a micrometer, measure the diameter of all four pistons at a point 10 mm from the bottom of the skirt, at right angles to the gudgeon pin axis. Compare the measurements with those listed in the Specifications.

If the piston diameter is out of the tolerance band listed for its particular size, then it must be renewed. Note: If the cylinder block was re-bored during a previous overhaul, oversize pistons may have been fitted. Record the measurements and use them to check the piston clearances when the cylinder bores are measured, later in this Chapter.

18 Hold a new piston ring in the appropriate groove and measure the side clearance using a feeler blade (see illustration). Note that the rings are of different widths, so use the correct ring for the groove. Compare the measurements with those listed; if the clearances are outside of the tolerance band, then the piston must be renewed. Confirm this by checking the width of the piston ring with a micrometer.

5.18 Checking the piston ring side clearance

19 Using internal/external vernier callipers, measure the connecting rod small end internal diameter and the gudgeon pin external diameter. Subtract the gudgeon pin diameter from the small end diameter to obtain the clearance. If this measurement is outside its specification, then the piston and connecting rod bush will have to be resized and a new gudgeon pin installed. An engineering workshop will have the equipment needed to undertake a job of this nature.

20 The orientation of the piston with respect to the connecting rod must be correct when the two components are reassembled. The piston crown is marked with an arrow (which can easily be obscured by carbon deposits); this must point towards the timing belt end of the engine when the piston is installed in the bore.

The connecting rod and its corresponding bearing cap both have recesses machined into them close to their mating surfaces - these recesses must both face in the same direction as the arrow on the piston crown (ie towards the timing belt end of the engine) when correctly installed (see illustrations). Reassemble the two components to satisfy this requirement.

5.20a Big end bearing cap and connecting rod markings

A identification (cylinder number) markings B Orientation recesses

5.20b Orientation arrow on piston crown (highlighted)

21 Lubricate the gudgeon pin and small end bush with clean engine oil. Slide the pin into the piston, engaging the connecting rod small end. Fit two new circlips to the piston at either end of the gudgeon pin, so that their open ends are facing 180° away from the removal slot in the piston. Repeat this operation for the remaining pistons.