It is often assumed that when an engine is new it gives its maximum power, but in actuality this is not the case. At the outset the bearings are tight and the bearing surfaces comparatively rough, and it is not until the engine has run some months that the highest power is developed. It is not to be assumed that up to this period the engine gradually increases in power, but that a higher power can be obtained from an engine after being well run in than when it is new.



In actual practice the engine gradually gives less and less power unless it receives the necessary small amount of attention to keep it up to its best pitch. The cause of this is a gradual loss of compression, chiefly owing to the burning away or pitting of the valves and their seats, the remedy for which is regrinding. Valve-grinding is a somewhat laborious process, but it is one which generally effects considerable improvement in the engine's running, and it should be made a periodic practice, and not be left till the engine is running very badly, as the necessary grinding will then be somewhat difficult.

If a valve is examined it will be found that it has a flat angular surface on the underside of the head, and this surface rests upon a similar conical surface, called the valve seat, formed in the cylinder head. These two surfaces must be absolutely true and smooth so as to prevent any gas leakages when the valve is closed by the valve spring.

When the exhaust valve is opened the exhaust gas, which is an intensely hot flame, passes between these two conical surfaces, making them practically red hot, which is their normal condition during running of the engine. It is hardly surprising, therefore, that in course of time the surfaces deteriorate, and the necessity for regrinding can be ascertained from an inspection of the valve and its seat. If smooth and bright all round, the valve is in good order. If dirty and rough, it requires regrinding. When loss of power is experienced, to ascertain if it is due to loss of compression, the engine should be turned round slowly, with the switch off, and if the resistance on the compression stroke is not what it was when the engine was new, it is clear that there is a leakage somewhere, and the exhaust valve should then be examined as mentioned. Possibly only one or two of the valves are at fault, so the compression in each cylinder in turn should be tested in this way.

Before a valve is ground in it is, of course, necessary to remove it, and this, with its replacement, forms probably the most difficult part of valve grinding, on account of the stiffness of the valve spring, coupled with the fact that the parts are generally difficult of access, on account of the presence of the piping, etc.