Question 1

What steps should be taken before refitting the piston/connecting rod assemblies on Volvo V40?

Accepted Answer

Before refitting the piston/connecting rod assemblies make sure that the cylinder bores are clean, the top edge of each cylinder is chamfered and that the crankshaft and intermediate section is in place. Remove big end bearing cap from No 1 cylinder connecting rod and extract original bearing shells, wiping bearing recesses with lint free cloth, keeping them spotless. Have new big end bearing cap retaining bolts on hand. Clean the back of the new upper bearing shell and fit it to No 1 connecting rod and the other shell in big end bearing cap and make sure it is placed in the same position to that of the old shells. Place the piston ring gaps in proper position, and lubricate the piston and rings with clean engine oil, and place a piston ring compressor, and leave the piston crown protruding slightly to guide the piston into the cylinder bore. Turn the crankshaft around until No 1 big-end journal is at Bottom Dead Centre and add engine oil to the cylinder walls. Set the No 1 piston/connecting rod assembly in line with the arrow on the piston crown pointing to the timing belt end, ease it into the cylinder bore, and position the ring compressor on the engine block. Tap the top edge of the ring compressor to make sure you have made contact with the block, and then gently tap the piston with a wooden hammer handle and guide the big end of the connecting rod onto the crankpin with a long screwdriver or long-nosed pliers. Maintain pressure on the ring compressor to keep the piston rings from popping out if there is resistance felt. Clean out the bearing surfaces and put a uniform coating of clean engine oil on both, perhaps pushing the piston back a little to expose the bearing surface of the shell in the connecting rod. Slide the connecting rod back onto the big end journal, refit the big end bearing cap, lubricate the bolt threads, fit bolts/nuts and tighten them in stages to correct torque. Repeat this process for the other piston/connecting rod assemblies, being careful not to turn the crankshaft around until the first pair of big end bearing caps has been tightened to their final torque values to prevent dislodging the bearing shells. Key points to remember include, that it is important to keep the backs of the bearing shells and recesses clean, the correct piston/rod assembly for each cylinder and to lubricate cylinder bores and bearing surfaces before fitting the big-end bearing caps. After you have all of the assemblies installed, rotate the crankshaft by hand to see if there is any binding.

Question 2

How should the piston rings be checked and fitted to ensure proper assembly on Volvo V70?

Accepted Answer

At this stage, it is assumed, however, that the pistons have been assembled correctly to their respective connecting rods, and that the piston ring-to-groove clearances have been checked. Before the rings are fitted to the pistons, the end gaps should be checked with the rings fitted in the cylinder bores. Lay out the piston assemblies and the new ring sets, keeping the components together during and after end gap checking, with the cylinder block on its side so that it can be accessed on the top and bottom of the bores. Take the No. 1 piston top ring and insert it in the top of the first cylinder pushing it down the bore using the top of the piston to make sure it is square with the cylinder walls so that it is near the bottom of the cylinder bore at the lower limit of ring travel. The top and second compression rings are of a different type, the second ring being identified by the step on the lower surface. Measure the ring gap with feeler blades, and repeat the process with the ring at the top of its travel, and compare the measurements with the specifications. If new rings are being fitted, it is unlikely that the end gaps will be too small, however, if a measurement is found to be undersize, then it must be corrected to avoid the risk of the end of the rings contacting each other during the engine operation, which could lead to engine damage. Ideally new piston rings with the correct end gap should be fitted, but as a last resort the end gaps can be increased by carefully filing the ends of the rings with a fine file. Mount the ring in a vice using soft jaws, slip the ring over the file, moving the ring slowly to remove material from the ends, be careful because piston rings are sharp and easily broken. It is also unlikely that the gap at the end will be too large, if the gaps are too big, check that the right rings for the size of engine and cylinder bore are being used. Repeat the checking procedure for every ring in the first cylinder, and then for rings in the other cylinders, making sure that rings, pistons, and cylinders are matched up. Once the ring end gaps have been checked and corrected if necessary, the rings can be fitted to the pistons using the same technique as for removal, starting with the bottom scraper ring and working up. For petrol engines the lower oil scraper ring is a 3-part ring, so you must install the spring-like expander ring first, then the two plain rings either side. Observe the text markings on one side of the top and bottom rings which must face upwards when fitted, but the middle ring, which is bevelled, must have the bevel facing downwards. Do not expand the compression rings too far or it would break and always follow any instructions supplied with the new piston ring sets as different manufacturers may use different procedures. And avoid mixing top and second compression rings as they differ in their cross-section. When all the rings are in position, the ring gaps should be 120o apart except the 3-part oil scraper ring in which the two plain rings should be 90o apart from each other.

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