Ring Gaps, etc.
In the ordinary way new piston rings and slotted oil control rings will not be required before the cylinders need reboring and oversize pistons. Inspect the rings for even bearing on the cylinders, and if they are taken off the pistons see that on refitting they are put back the same way up, and into the grooves from which they were taken. The 'top' ring on each piston is a taper faced chromium plated one. The taper is hardly detectable by eye so they are etched with the word 'top' on the face adjacent to the smaller diameter and this face must be fitted nearer to the piston crown.

It is generally not considered satisfactory to fit new rings to partly worn cylinders, as they take a long time to accommodate themselves to bores that may no longer be circular or parallel. The chromium plated rings in particular take very much longer to bed in.

Obviously if a ring is broken it must be renewed so that extreme care should be taken not to damage them in carrying out work to the engine.

New rings supplied as spares are all correctly gapped and as the bores of cylinders that have been in use will have become enlarged, if only very slightly, it is most unlikely that the gaps will have to be increased.

The correct gaps are .007-in. minimum on all rings.

Never, in order to reduce the ring gaps, attempt to use oversize diameter rings into cylinders, even extensively worn ones, if the original bore diameter was less than that for which the new rings have been produced. The bore diameter can be measured just inside the bore mouth at the crankcase (lower) end of the cylinders. This part is not subject to wear by the rings which do not traverse it.

Rebored Cylinders and Oversize Pistons.

The usual method of reconditioning worn cylinders is by reboring them oversize and fitting oversize pistons and rings to suit. Piston assemblies are stocked .020-in. and .040-in. oversize.

Cylinders can be rebored at the factory and fitted with the appropriate oversize pistons.

The bore diameter when new lies between 1.969-in. and 1.970-in. and cylinders are selected into two grades differing by .0005-in. The larger grade are marked ‘+' and the smaller '–‘

It is not possible to state a figure for the degree of wear that is tolerable before reboring is necessary. The necessity for the work depends upon the performance and /or the oil consumption of the engine.

Refitting Pistons and Cylinders.

Enter the gudgeon pin into one side and push it through until it is just proud of the boss inside. Oil the small end bush and place the piston over the end of the rod, locating the protruding end of the pin, inside the piston, in the small end bush. NOTE - Be sure to refit the pistons so that the split in the skirt is uppermost on the right hand side, and underneath on the left hand side piston.

Tap the gudgeon pin through up to the circlip on the opposite side, and fit the other one, being careful that it is seated properly in the groove and quite secure.
Space the piston ring gaps about 120° away from each other round the piston ready for fitting the cylinders. Fit the tappets.

Oil the cylinder bore, fit a new gasket to the base flange, and hold the cylinder horizontal and opposite the opening in the crankcase. Bring the piston up level with the mouth, compress the rings, and enter the top one into the bore. Compress each ring in turn to enable the cylinder to be pushed gently into position over the piston.