Posted by Alan Bowes on October 19, 1997 at 01:25:15:
In Reply to: Exhaust Valves.....something amiss? posted by Bill Nasby on October 18, 1997 at 19:35:58:
: First, thanks to everyone who's responded to my queries in the past.
: I do have a warped intake valve, and while looking at the remainder,
: I noticed that the exhaust valves were all missing chrome around the stem
: at the top of the valve. I'm going to replace them all, intake and exhaust,
: what would cause the chrome to dissappear....overheating? Could it be that
: confounded water distribution tube's not doing it's job? Thanks folks.
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Hi again,
When you say that the chrome is missing at the top of the valve, I assume you mean in the area swept by exhaust gases. It is possible that there was no plating in that area to begin with. It is also possible that the valve stems aren't actually plated, just polished. Valve manufacturers have used various schemes to reduce friction in the poorly-lubricated valve guides. Different alloys, hardening techniques, polishing, and plating have been tried. Valve stems also tend to get quite shiny as they polish themselves in the valve guide. One of the best approaches is the use of phosphor-bronze valve guides, which, to my knowledge, reduce friction with virtually all kinds of valve materials. They also help improve heat transfer from the valve into the engine block (or cylinder head on OHV engines), where it is taken away by the water cooling jacket.
If a valve is straight, and there is no pitting or eroded areas on the valve stem, the valve-to-guide clearance is within limits, and there is still sufficient metal on the valve to allow it to be refaced, then the valve should be OK to reuse. This said, installing new valves is not a bad idea.
As I mentioned before, you may want to consider new valve guides if the clearance is excessive, and installing extra-hard exhaust valve seats would make valves and seats last longer with unleaded gasoline.
Alan
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