1 The cylinder bores must be examined for taper, ovality, scoring and scratches. Start by examining the top of the bores; if these are worn, a slight ridge will be found which marks the top of the piston ring travel. If the wear is excessive, the engine will have had a high oil consumption rate accompanied by blue smoke from the exhaust.

2 If available, use an inside dial gauge to measure the bore diameter just below the ridge and compare it with the diameter at the bottom of the bore, which is not subject to wear. If the difference is more than 0.152 mm (0.006 in), the cylinders will normally require reboring with new oversize pistons fitted.

3 Proprietary oil control rings can be obtained for fitting to the existing pistons if it is felt that the degree of wear does not justify a rebore.

However, any improvement brought about by such rings may be short-lived.

4 If new pistons or piston rings are to be fitted to old bores, deglaze the bores with abrasive paper or a “glaze buster” tool. The object is to produce a light cross-hatch pattern to assist the new rings to bed in.

5 If there is a ridge at the top of the bore and new piston rings are being fitted, either the top piston ring must be stepped (“ridge dodger” pattern) or the ridge must be removed with a ridge reamer. If the ridge is left, the piston ring may hit it and break.

6 Thoroughly examine the crankcase and cylinder block for cracks and damage and use a piece of wire to probe all oilways and waterways to ensure that they are unobstructed.