The PROBLEM of VALVE-STEM and VALVE-HEAD Deposits 400161
DEPOSITS on valve stems today are causing about 50% of the valve trouble in the automotive field, and slightly more than 50% in the aircraft field, Mr. Colwell reveals. He contends that, although much work has been done on varnish and lacquer deposits, most of it has applied to pistons and rings, with valves receiving only passing interest.
Valve deposits are divided into six classifications: those on the valve head; the hard, flint-like deposit from operation; varnish on the stem; deposits under the head and on the stem at the valve-head end of the guide formed by shuttle driving; deposits on intake valves; and deposits on valve seats.
The fact that any oil will oxidize or decompose at some temperature found along the valve stem makes a complete solution of the build-up problem most difficult, Mr. Colwell points out. It has been definitely proved, he continues, that oils with good oxidation resistance at high temperatures cause the least trouble. The design problem, therefore, is to attempt to get a valve-stem temperature at the top of the guide which will not cause oil decomposition, or to meter the oil so that the deposit formed is not thick enough to cause trouble between overhauls; or to remove the deposit in some way as it is formed.
Comparing automotive and aircraft remedies, he brings out that the automotive status is more confusing because operating conditions vary far more. He then presents and discusses a number of aircraft and automotive remedies for preventing valve deposits.