Posted by chriscase on Thursday, May 17, 2007 at 00:49:05 :
In Reply to: Sunken Valves posted by Ken C. on Wednesday, May 16, 2007 at 21:26:10 :
The spring heights differ. But the ends of the stems ought to be the same. Most machinists check the end valves individually. Then they use a straight edge between the end ones to get the middle ones right.
Lifters fill with oil until the valve spring pressure won't allow any more oil in. Valve spring pressure is about 75 pounds. The lifter piston is only about 1/2 inch in diamter, about 2/10 of a square inch area. So, with 35 psi of oil pressure, times .2, that is only seven pounds of pressure against the valve spring. Just barely enough to push all the slack out of the valve train. It is only the check valve in the lifter that won't let the oil out, so the cam can push the valve open. The check valve leaks just a bit, so it won't get too tight with heat expansion. So it is normal to have a couple lifters leak down overnight, and click a bit when first started. Thinner oil will pump them up faster.
Valve spring pressure ought to be checked at valve grind time. As the valve wears and is gorund, and the seat too, the valve will push back into the head. Thus lowering the valve spring pressure. Lower pressure will allow the valves to float- not enough pressure to slam them shut. With the valve train floating around, the lifters can pump up with oil, holding the valves open. Sort of a governor system, limiting revs. High speed engines need hi-pressure springs, which wear the cam, so need roller rockers. Performance ain't cheap. So anyhow, a head that has had a valve grind willusually have copper colored valve spring shims, to bring the seat pressure back into specs.
A truley 'sucked valve' would be caused by pinging. All that detonation makes heat and pressure. The heat softens the parts, and the pressure pushs the soggy vale into it's hole. Not likely to happen uniformly, usually only in the hottest cylinder.
I'd say your engine has either had an amature valve job, or you are suffering from an optical illusion? Check across the valve stems with a straight edge, if all the stems are in line, +/- 1 /32, I'd run it.
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