THE GRINDING IN MYTH.

Evening Post, Volume XCII, Issue 76, 27 September 1916, Page 10

THE GRINDING IN MYTH.

"Grind it in" was at/"'one time the advice regularly given to the motorist who was troubled with a flooding carburetter or loss of compression (says a writer in an English motor journal). It was the needle in the carburetter and the valves in the engine that had to be ground in. The 'advice is now comparatively rarely published, but belief in the value of the grinding-in process still persists. Grinding-in is, in point of fact, mechanical madness. It really consists in grinding out*. In the case of tho carburetter it upsets the petrol level, but the main trouble is that it wears out the seating. It is about a hundred to one that, if the amateur does the work, the seating will be so distorted that the needle will never seat properly again, and that tho flooding will be worse than ever. With the valves the trouble is still greater. It is only rarely that the seating gets pitted. It is nearly always the valve that suffers from mechanical smallpox. But grinding-in means that both seating and valve are attacked by the treatment, with the result that, as •the valve is invariably much harder than the seating, it is always the seating that gets the grinding-out, the pits in the valve being only slightly affected. So, by repeated grindings, - the seating becomes a dug-out, and the" inlet or exhaust gases thus get pocketed and stilled, while the cylinder casting is mined. So far as the valve itself is concerned, I doubt if any amateur ever yet succeeded in restoring a considerably pitted surface to its pristine condition of even and continuous brightness by grindingin, and it is such weary, thankless, blistering work. All said and done (and there is generally a great deal said and precious little done), the compression is usually no better than before when the valve is replaced. Never grind in, therefore: havo the valves trued.in a lathe or with a special tool when the work ' is , really needed (though see Myth No. II below), and then, when replacing them, give just a few turns to the valve on its seating with a smear of abrasive mixture between the two. The same advice applies to the carburetter needle : have the main work done outside the carburetter, not inside.