Loss of compression is also due to wear set up between the piston and cylinder. In engines that have done a considerable amount of running the cylinders may have worn slightly oval, and they should be carefully examined to see if this is the case, and if so, should be rebored. The trouble is usually, however, confined to the piston rings, and new rings are generally required on a water-cooled engine after 10,000 to 20,000 miles.

In good engines carefully looked after perhaps they will not be necessary until 30,000 miles have been run. To fit new rings is, though not a difficult matter, one that requires a certain amount of care. It is very easy to put a ring into the top slot, but to get it into one of the middle ones is not so easy. To do so the adjacent grooves should be temporarily filled up with some packing or, what is better, some strips of sheet metal or whalebone should be employed and the ring slid over these until the proper groove is reached. These strips will prevent the ring from fouling the adjacent groove.

Each ring after being in use must be bright the whole way round ; if this is not the case it shows that contact is not made throughout the whole length of the ring, and a new ring should be fitted. Piston rings should fit comparatively tightly in the grooves, and when compressed so as to lie nearly flush with the piston the slots should be closed.

The weak point in piston rings, as regards holding compression, is the slot, as a slight leakage is bound to take place along it. Consequently when two or more rings are fitted the slots should be arranged alternately on opposite sides of the piston, so that as long a path as possible is made for the escaping gas and leakage reduced. Thus when the cylinders are removed care should be taken to turn the piston rings into this position. In some cases the piston rings are pegged to prevent their taking up wrong positions.

After a water-cooled engine has run a considerable distance, say anything from 5,000 to 10,000 miles, it may be found that power is falling off, in spite of the compression being good, that when going up a hill with the throttle wide open the engine knocks and does not seem to run so well as when new, or that the engine preignites or continues to run when switched off. These are definite signs that the inside of the cylinders and the piston heads require cleaning.