Cylinder bores - examination and renovation 1 The cylinder bores must be examined for taper, ovality, scoring and scratches. Start by carefully examining the top of the cylinder bores. If they are at all worn, a very slight ridge will be found on the thrust side. This marks the top of the piston ring travel. The owner will have a good indication of the bore wear prior to dismantling the engine, or removing the cylinder head. Excessive oil consumption accompanied by blue smoke from the exhaust is a sure sign of worn cylinder bores and piston rings.
2 Measure the bore diameter just under the ridge with a bore gauge (photo) and compare it with the diameter at the bottom of the bore, which is not subject to wear. If the difference between the two measurements is more than 0.006 in, then it will be necessary to fit special piston rings or to have the cylinders rebored and fit oversize pistons and rings. If no micrometer is available remove the rings from a piston and place the piston in each bore in turn about 3/4 in (17 mm) below the top of the bore. If an 0.010 feeler gauge can be slid between the piston and the cylinder wall on the thrust side of the bore then remedial action must be taken. Oversize pistons are available dependent on the bore diameter, and the respective sizes available are given in the Specifications. These are accurately machined to just below these measurements so as to provide correct running clearances in bores bored out to the exact oversize dimensions.
3 If the bores are slightly worn but not so badly worn as to justify reboring, then special oil control rings can be fitted to the existing pistons which will restore compression and stop the engine burning oil. Several different types are available and the manufacturer's instructions concerning their fitting must be followed closely, but the pistons must obviously be in good condition, and they may have to be machined to suit the oversize rings.
Pistons and piston rings - examination and renovation 1 If the old pistons are to be refitted, carefully remove the piston rings and then thoroughly clean them. Take particular care to clean out the piston ring grooves. At the same time do not scratch the aluminium in any way. If new rings are to be fitted to the old pistons then the top ring must be of the stepped type so as to clear the ridge left above the previous top ring. If a normal but oversize new ring is fitted, it will hit the ridge and break, because the new ring will not have worn in the same way as the old, which will have worn in unison with the ridge.
2 Before fitting the rings on the pistons each should be inserted approximately 3 in (76 mm) down the cylinder bore and the gap measured with a feeler gauge. The gap should be as listed in the Specifications. It is essential that the gap should be measured at the bottom of the ring travel. If it is measured at the top of a worn bore and gives a perfect fit, it could easily seize at the bottom. If the ring gap is too small, rub down the ends of the ring with a very fine file until the gap, when fitted, is correct. To keep the rings square in the bore for measurement, line each up in turn by inserting an old piston in the bore upside down, and use the piston to push the ring down about 3 in (76 mm). Remove the piston and measure the piston ring gap.
3 When fitting new pistons and rings to a rebored engine the piston ring gap can be measured at the top of the bore as the bore will not now taper. It is not necessary to measure the side clearance in the piston ring grooves with the rings fitted as the groove dimensions are accurately machined during manufacture. When fitting new oil control rings to old pistons it may be necessary to have the grooves widened by machining to accept the new wider rings. In this instance the manufacturer's representaitve will make this quite clear and will supply the address to which the pistons must be sent for machining.
4 When new pistons are fitted, take great care to fit the exact size best suited to the particular bore of your engine. BL go one stage further than merely specifying one size of piston for all standard bores. Because of very slight differences in cylinder machining during production it is necessary to select just the right piston for the bore. Five different sizes are available for the standard bores as well as four oversize dimensions.
5 Examination of the cylinder block face will show adjacent to each bore a small diamond-shaped box with a number stamped in the metal. Carefull examination of the piston crown will show a matching diamond and number. These are the standard piston sizes and will be the same for all four bores. If standard pistons are to be refitted or standard low compression pistons changed to standard high cornpression pistons, then it is essential that only pistons with the same number in the diamond are used. With larger pistons, the amount of oversize is stamped in an ellipse in the piston crown.
6 On engines with tapered second and third compression rings, the top narrow side of the rings is marked with a T. Always fit this side uppermost and carefully examine all rings for this mark before fitting.
Camshaft and camshaft bearings - examination and renovation 1 Carefully examine the camshaft bearings for wear. Note that on early engines only the front camshaft bearing is renewable. If the bearings are obviously worn or pitted, or the metal underlay is showing through, then they must be renewed. This is an operation for your local BL garage or the local engineering works as it demands the use of specialised equipment. The bearings are removed with a special drift, after which, new bearings are pressed in, care being taken to ensure the oil holes in the bearings line up with those in the block. With a special tool the bearings are then reamed in position.
2 The camshaft itself should show no signs of wear, but if very slight scoring on the cams is noticed, the score marks can be removed by very gentle rubbing down with very fine emery cloth. The greatest care should be taken to keep the cam profiles smooth.