Classic/Antique Car Repair: valve adjustments, valve lifters, pontiac engine

Question
QUESTION: the valves on my pontiac 400 have started to chatter a bit, it might be because I lost some oil pressure, down to 25psi, from 75psi.  Are they adjustable? if so how it is done.  kevin

ANSWER: What year car or engine please, and is it the old Pontiac engine or the newer Chevy engine?

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QUESTION: The engine in question is the older pontiac gto 400

ANSWER: THERE IS NO ADJUSTMENT ON THESE.

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QUESTION: I was afraid of that.  I have asked around  and some of the response I get is that I could have a collapsed valve(s).  What is a collapsed valve and is there any way to fix it?, without replacing the valves.   this is my first hydraulic valve car.

Answer
The valve lifters are the things that run on the cam and the push rods set in the cup on the top of the lifter. If they are defective and need to be replaced the intake manifold and the valley pan need to come off as well as the valve covers and rocker arms. You do not need to remove the cylinder heads or the valves themselves. The lifters are a hydraulic unit that has a piston inside the outer shell. Oil is supplied by the oil pump and when the cam pushes up on the lifter a disc valve in the lifter closes and the unit stays pumped up and opens the valve. When oil pressure drops or there is a hole worn in the bottom of the lifter there is not enough oil pressure to open the valve and the clatter comes in. This is one possibility.