Renewable Valve Seats.
Page 19
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The sender of the following communication has been awarded the Ws. prize this week.
[1232] "S.C." (Scotstoun) writes :—" There have been, from time to time, letters on your `. ll. and M.' page, which have dealt with the problem of valves and valve Beatings, chiefly, of course, those of the poppet type. It is rather surprising to me that more makers of petrol engines do not fit renewable seats. The extra expense would be small compared with the saving which would occur later on in the life of the engine. When a valve seat gets very wide, owing to the valves' being repeatedly ground in, I should think that a new seat of gunmetal could be fitted; the old passage would be suitably bored out to receive the new seat " Most of these renewals are, in standard practice. of course, made a tight fit and forced into position. If enough metal be allowed, a renewable seat may be repeatedly machined to its original shape. The whole area above the opening should be flat, and no ridges should be presented which could retard the gases.
"As regards grinding in poppet valves in the ordinary way, the usual screwdriver slot provided for this purpose on the top of the valve is, I think, rather unsatisfactory. The fault of the screwdriver slot is that it does not enable one to lift the valve off the seat occasionally while grinding in. It is necessary that this should be done if a good surface is to be procured. Some mechanics, when doing this operation, put a light spring under the valve, so that it will lift of its own accord when pressure is taken off. The more satisfactory method, I think, is that used by a friend of mine in his own workshop. The stem of the valve used with this tool was fairly thick at the top, and a in. hole could be drilled and tapped without weakening the metal to any dangerous extent. I should say that most of the poppet valves on commercial motors will admit of treatment of this kind. The same dodge could be applied to other parts of the chassis, for instance, the smaller minor shafts, for convenience of handling. There are other parts, of course, such as the crankshaft, in which it is obviously bad practice to tamper with the original centres put in for machining purposes."
[The trouble with renewable valve seats has teen found in practice to lie in the fact that it is impossible io make these fittings remain perfectly tight under actual working conditions.— En.]