ENGINE OVERHAULING Check each cylinder bore carefully with the cylinder bore gauge or internal micrometer, noting in each bore the maximum amount of wear, which should be at the top of the ring travel. The amount of wear is the difference between the maximum diameter at this point and the smallest diameter at the bottom of the bore, which will still be approximately the original size. If the maximum wear is less than 0.007" and the bores are not scored or patchy, reconditioning of the bores should not be necessary. If the oil is passing up the bore it will probably be due to worn rings and the trouble should be overcome by a new set of rings which should restore oil consumption and compression to satisfactory figures for another 5-10,000 miles. If the wear exceeds 0.007", or if deep scores or patches are present, reboring or resleeving is definitely necessary. These figures apply to engines with bore sizes less than 3.5". With larger engines and particularly diesel engines, cylinder reconditioning is not usually necessary before the maximum wear reaches 0.020". However, it is difficult to lay down a hard and fast rule and each case must be treated on its own merits. Scored cylinders must always be reconditioned. If badly scored it may be necessary to fit sleeves. Unpolished dark or brown patches on the cylinder walls should be looked for. This indicates either that distortion of the bore has occurred when tightening down the cylinder head or alternatively, that the water jackets may be clogged and a local hot spot is present. Pistons should now be removed and in engines where they must be withdrawn through the top of the bore, care is necessary, first to remove all traces of carbon desposit from the top of the bore, and then to push the rings carefully past the wear ridge at the top of the ring travel. Lack of care can mean cracked ring lands. It may be necessary to remove the ridge before the pistons are removed. A bore ridge reamer may be used. Dismantle each piston from the conrods and then remove the rings. Ring wear can be checked by fitting them into the unworn top portion of the bore (above the wear ridge) and measuring the gap. If the gaps exceed 0.0 10 for every inch of bore diameter (i.e. 030" in a 3" bore engine), fit new rings. Remember to clean the carbon from the ends of the rings before checking the gap. However, if the engine has covered any appreciable mileage, it is advisable to renew all rings, as their efficiency is bound to have deteriorated through wear and loss of tension. The pistons should be thoroughly cleaned in some good cleaning solution. Any carbon left, either on top or inside the piston crown, should be removed. All ring grooves should be cleaned out carefully with the end of a broken ring, this being of correct width to fit the groove. A wire brush can be used to clean the crown and the inside of the piston, but should never be used to clean ring lands and grooves as it will scratch the upper and lower surfaces of the grooves. Similarly it must not be used to clean piston skirts. (It must be remembered that ring lands should be treated as carefully as a valve face, or if damaged, leakage takes place around the back of the ring as easily as it passes the face of a ring not fitting the bore.) The drain holes in and below the scraper ring grooves must be cleaned out, using care not to damage the ring lands. The safest way to clean these holes is to attach a small tap wrench to the shank of a twist drill and turn it by hand. Never use a drilling machine as the lands can be easily be ruined by contact with the drill. When a thorough job of cleaning has been done, the piston can be examined for cracks in the crown skirt. Check the upper and lower faces of each ring groove for chipping, distortion or uneven wear, as these faults will not allow the new rings to seat correctly. Check for ring groove wear by fitting a new ring into each groove. If a 0.006 feeler can be inserted to a depth of 1/16" between the top of the ring and the groove of a petrol engine piston - the piston should be renewed. Pistons should also be checked for skirt collapse which is most likely to occur after prolonged running at overload, every metal being subject to fatigue if overstressed for a long period. If the skirt clearance at the bottom of the stroke exceeds twice the recommended clearance, pistons should be renewed. When there is too much clearance a piston rocks in the cylinder and thus keeps the rings from riding squarely against the cylinder wall. This condition will rapidly become worse because the ring faces will soon wear barrel shaped and lose their ability to scrape oil from the bore or maintain effective seal. Finally the gudgeon pin should be inspected for wear, cracking or pitting. Try the fit in the piston and if there is excessive clearance, replace with a correctly fitting pin. If circlip fitting, the circlips should always be renewed and their grooves examined for wear or burrs which will prevent correct seating. If soft end pads are fitted, make sure that these are not loose or cracked. Examine the main and big end bearings for pitting, cracks or flaking. If such bearings have failed they should be renewed since it is impossible to control excessive oil consumption if they are left in an engine. Carefully test each main and big end bearing for clearance and do not reassemble if it is found to exceed the maximum specified by the manufacturer. Where the correct clearance in unknown, the following table will give a general guide. Type of Bearing Maximum Diametral Clearance Engine Material Main Big End Petrol White Metal 0.002" 0.0015" Diesel White Metal 0.003" 0.003" Diesel Lead Bronze 0.008" 0.005" To check these clearances use a piece of shim stock of the correct thickness. 1" wide and 1" long. If this is not available, a piece of paper of the correct thickness may be used. This shim should be placed in the bearing cap, the cap installed in place and the screws tightened. Test the clearance by attempting to rock the bearing by hand, about 1 in either direction - not more, to avoid damaging the bearing. If the bearing moves freely without the shim in place and locks when in place, the clearance is standard and will not cause excessive oil consumption. If it is possible to rock the bearing with the shim in place, the bearing should be adjusted or renewed. However, first examine the crankpins and main bearing journals for scoring. It is essential also to measure them for wear, checking particularly for ovality or taper. It is no use taking up bearings if the journal is worn oval or taper as there will always be a constant source of oil leakage. When the journals are worn oval or taper in excess of 002 for petrol engines, or 003 for diesel engines, the crankshaft should be reground and new undersize bearings fitted. The older type of engine will need remetalling. The importance of maintaining correctly all bearing clearances cannot be overemphasised as these control the quantity of oil being flung up the cylinder bores. It has been shown that when the big end clearances are doubled, five times the normal amount of oil is thrown on the cylinder walls. When there is four times as much clearance as there should be, twenty-five times the normal amount of oil is released from the bearings. Examination of the small end bearing should also be made. On no account should a bushing be loose in the connecting rod. Make certain that the pin is a good fit and that no bell mouthing or ovality has occcurred. If this examination has been made thoroughly the cause or causes of excessive oil consumption should be abundantly clear and a full estimate of what repairs and replacements are necessary can now be made. Naturally, whilst the engine is dismantled, opportunity will be made to examine all working parts for wear and tear, including oil pump, camshaft, timing gear, etc. If attention is given to such items whilst the opportunity offers itself it can save another expensive overhaul within a short time.