Piston/Valve Clearance

[Adger Smith]

Robert,
The thing to remember when you have real tight (close) PTV clearances is they usually never hit when the engine is under load. The valve dynamics change when the engine is in a free wind (no load) situation. That is when the valve gear gets unstable and close clearances can come in contact with one another. Overwinding in the burnout and streching the RPM out at the top of a gear are your enemy. You might want to keep a constant watch on valve spring pressures and conditions. Keep then good and strong, esp the open pressure. Seat pressure can drop a few lbs, but the open pressure falling can put an ex valve in contact with a piston. I like to take a look at the contact of the rocker on the valve stem tip, esp when I'm adjusting valves. Signs of Multiple contacts of the rocker tip and the valve is a sure sign of valve float or loss of spring control. The spring seat or shim being eaten or beat up is also a bad sign of valve control. You can get away with close clearances if you are careful, esp with smaller head dameter, stem valves. Valve gear weight is a big consideration in just how close you can go.
Good Luck...