Piston and connecting rod assemblies - refitting and big-end bearing clearance check

Engine removal and overhaul procedures / Piston and connecting rod assemblies - refitting and big-end bearing clearance check

Big-end running clearance Note: At this point, it is assumed that the crankshaft has been fitted to the engine, as described in Section 10.

1 As with the main bearings (Section 10), a running clearance must exist between the bigend crankpin and its bearing shells to allow oil to circulate. There are two methods of checking the size of the running clearance, as described in the following paragraphs.

2 Place the cylinder block on a clean, level work surface, with the crankcase facing upwards. Position the crankshaft so that crankpins No 1 and 4 are at BDC.

3 The first method is less accurate and involves bolting bearing caps to the big-ends, away from the crankshaft, with the bearing shells in place. Note: Correct orientation of the bearing caps is critical; refer to the notes in Section 5. The internal diameter formed by the assembled big-end is then measured using internal vernier callipers. The diameter of the respective crankpin is then subtracted from this measurement and the result is the running clearance.

4 The second method of carrying out this check involves the use of Plastigage, in the same manner as the main bearing running clearance check (see Section 10) and is much more accurate than the previous method.

Clean all four crankpins with a clean rag. With crankpins No 1 and 4 at BDC initially, place a strand of Plastigage on each crankpin journal.

5 Fit the upper big-end bearing shells to the connecting rods, ensuring that the locating lugs and recesses engage correctly.

Temporarily refit the piston/connecting rod assemblies to the crankshaft; refit the big-end bearing caps, using the manufacturers markings to ensure that they are fitted the correct way around - refer to Final Refitting in the following sub-Section for details.

6 Tighten the bearing cap nuts/bolts as described below. Take care not to disturb the Plastigage or rotate the connecting rod during the tightening process.

7 Dismantle the assemblies without rotating the connecting rods. Use the scale printed on the Plastigage envelope to determine the bigend bearing running clearance and compare it with the figures listed in Specifications.

8 If the clearance is significantly different from that expected, the bearing shells may be the wrong size (or excessively worn, if the original shells are being re-used). Make sure that no dirt or oil was trapped between the bearing shells and the caps or connecting rods when the clearance was measured. Recheck the diameters of the crankpins. Note that if the Plastigage was wider at one end than at the other, the crankpins may be tapered. When the problem is identified, fit new bearing shells or have the crankpins reground to a listed undersize, as appropriate.

9 Upon completion, carefully scrape away all traces of the Plastigage material from the crankshaft and bearing shells. Use a plastic or wooden scraper, which will be soft enough to prevent scoring of the bearing surfaces.

Piston and connecting rod assemblies - final refitting 10 Note that the following procedure assumes that the crankshaft main bearing caps are in place (see Section 10).

11 Ensure that the bearing shells are correctly fitted, as described at the beginning of this Section. If new shells are being fitted, ensure that all traces of the protective grease are cleaned off using paraffin. Wipe dry the shells and connecting rods with a lint-free cloth.

12 Lubricate the cylinder bores, the pistons, and piston rings with clean engine oil. Lay out each piston/connecting rod assembly in order on a worksurface. On engines where the big end bolts are captive in the connecting rods, fit short sections of rubber hose or tape over the bolt threads, to protect the cylinder bores during reassembly.

13 Start with piston/connecting rod assembly No 1. Make sure that the piston rings are still spaced as described in Section 12, then clamp them in position with a piston ring compressor.

14 Insert the piston/connecting rod assembly into the top of cylinder No 1. Lower the bigend in first, guiding it to protect the big-end bolts and the cylinder bores.

15 Ensure that the orientation of the piston in its cylinder is correct - the piston crown, connecting rods and big-end bearing caps are equipped with markings, which must point towards the timing belt end of the engine when the piston is installed in the bore - refer to Section 5 for details.

16 Using a block of wood or hammer handle against the piston crown, tap the assembly into the cylinder until the piston crown is flush with the top of the cylinder (see illustration).

12.16 Using a piston ring compressor to install the piston assemblies

17 Ensure that the bearing shell is still correctly installed (see illustration). Liberally lubricate the crankpin and both bearing shells with clean engine oil. Taking care not to mark the cylinder bores, tap the piston/connecting rod assembly down the bore and onto the crankpin. Refit the big-end bearing cap, tightening its retaining nuts/bolts finger-tight at first. Note that the orientation of the bearing cap with respect to the connecting rod must be correct when the two components are reassembled. 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 - refer to the illustrations in Section 5 for details.

12.17 Big end bearing shell lug and locating recess (arrowed)

19 Working progressively around each bearing cap, tighten the retaining nuts half a turn at a time to the specified torque (see illustration).

12.19 Tightening the big-end bearing caps

20 Refit the remaining three piston/ connecting rod assemblies in the same way.

21 Rotate the crankshaft by hand. Check that it turns freely; some stiffness is to be expected if new components have been fitted, but there should be no indication of binding or tight spots.