Originally Posted by blazinlow
I just bought a set of redone Sportsman heads. The were complete with valves, but no springs. They have bronze guides and valve seals on the intake only. Do I put seals on the exhaust or not. I read conflicting reports , many saying "do not install seals on the exhaust valves with bronze guides. they need the lubrication". Anyone know for sure
Typically the issue of excessive lubrication in the guide/stem clearance isn't a problem with the exhaust because it sees exhaust pressure which would like to blow the lubrication out, where-as the intake sees a vacuum on the guide/stem interface which wants to pull oil thru the interface into the incoming mixture.
So it's more important to insure positive oil control on the intake. The exhaust can get by with umbrella seals, which aren't so much of "seal" as they are a means of directing excess oil off the stem.
With the proper stem to guide clearance, the umbrella's will do you fine if the springs have space for them. When you get into multiple spring configurations, there can be insufficient space so a positive seal is used since it will fit the available space. They can starve the exhaust guide especially where hard chrome stems and cast iron guides are used. Bronze is a bit more tolerant of running dryer even with a hard chrome stem as the bronze stays wetter with the oil than cast iron. Chrome stems don't need so much oil as plain steel stems, but chrome doesn't wet out and can get too dry on the thrust sides. By "wet out" I mean that oil on hard and shiny chrome tends to puddle leaving dry places where on plain unpolished steel the oil will flow out and form a film over the entire surface. Oil also flows out and penetrates into the pores of bronze and too a lesser extent cast iron.
In the end the seal choice is different between the intake and exhaust because of suction present at the intake and pressure at the exhaust. And it needs to be consistent with the materials of the valve stem and those of the guide. Chrome operates with less lubrication but at the same time will become too dry in loaded places, plain steel wets out but also needs better lubrication. On the guide side, cast iron wets out but not as much as bronze.
Temperature is also a consideration, the exhaust runs hotter, especially in a working or racing engine and needs both a looser clearance and more lubrication or better materials that will function with less lubrication as one of the problems with the exhaust valve is the stem can get hot enough to cook off the lighter molecules of the oil and leave a sticky residue that can cause the valve to stick in the guide.