If you're going to leave your stock heads stock, you'll likely replace them with OEM seals. Typically they come included in a valve grind gasket kit, and most people throw them out.
To use a higher quality seal, take your heads to a machine shop. The machine shop will reduce the OD of the valve guide for the replacement seal, and size the seal according to the OD of the valve stem, and valve guide.
You have your choice of all teflon, or rubber w/teflon inserts. I've always favored the insert style (Perfect Circle used to make them), as they allowed a little more
oil on the valve than all teflon, but virtually never wore out. Some folks prefer the all teflon seals. They offer more
oil control to the valve, and in some cases too much
oil control. Depends on what kind of valve guides you are using, valve stem clearance, type of head, and engine use.
Ask your machinist for his recommendation, and let him machine and install.
Valve Stem Seal Dia.
Linear Mode
