Varnished oil can accumulate on the valve stem near the bottom of the valve guides’ contact. This accumulation of varnish is similar to the happenings of valves at high temperature. The buildup of varnish can cause the valve to stick in the guide. Valves that stick in the guides are likely to contact the pistons.
The following items should be examined when a problem with the quality of oil is suspected.
- Quality of oil
- Oil change intervals
- Application of Machine
- Operating conditions
Illustration 12 g01183511
Effects of various qualities of oil
(1) Corrosive Pitting
(2) Bottom of Guides’ Contact
The magnified view shows the surface of the valve stem to be pitted from corrosion. Pitting from corrosion is often confused as adhesive wear. Pitting that occurs in one valve stem usually will display the similar characteristics in all valves.
Illustration 13 g01183516
Surface pitting on valve stem
As the oil condition continues to degrade, consumption will usually increase leaving deposits on the valves’ fillet and the valves’ face. Corrosion may occur in any areas in contact with oil.
Illustration 14 g01183522
Deposits of oil on a valve
(1) Build up of oil coking.
(2) Corrosive pitting
Chordal fractures are the results of cyclic overloading. The chordal fractures usually begin in the area of the valves’ fillet. The overloading is localized to one side of the valve head. The following items are causes for chordal fractures.
- Valve, seat, and/or misalignment of guides
- Bent valve
- Foreign debris on valve face
- High temperatures of combustion
Illustration 15 g01183525