PC is a brand name, it stands for Perfect Circle, which is a division of Dana Corporation. PC is a manufacturer of valve stem seals for OE as well as the aftermatket. All types of seals are available from PC as O-ring type, Shedder (umbrella) and positive seals. Many materials are uses including Nitrile, Silicone, Nylon, Polyacrylic, Flourelastomer and Teflon. For most replacement seals, you are limited to O-ring or umbrella type with stock valve guides/castings.
Heads can be machined for positive seals and this is by far the best way to go for high performance engines. PC positive seals are available in three materials: Flourelastomer, Teflon/rubber and all Teflon. I personally use teflon seals for all performance and racing engines that I build. The teflon seals are smaller diameter and will fit with most dual and triple spring set-ups.
There is some misinformation out there, that teflon seals will dry up the guides/stems and cause galling. This is the case ONLY if the stem/guide clearance is too tight. If you look at a PC Teflon seal, the stem hole is very slightly chamfered, which draws an minute amount of oil each time the valve opens. Some competitive Teflon seals have a very shape edge on the stem hole, therefore they do not meter oil as the PC seals do. All PC TEFLON seals are white in color!!
As far as clearances, I always ream the valve guides to the middle of the recommended spec range for bronze and manganese-bronze guides and the high end of the spec for cast iron guides. In order for these clearances to be done correctly, it involves much more machinist's time and the investment in much more tooling (reamers). Also, I use a different reamer for bronze type guides than for cast iron.
64 Chevelle Malibu Sport Coupe (original owner) car now has 427" BB, Jerico 5-spd. trans., Dana 60 rear end.
SAE Member (25+ years)
ASE Master Certified Engine Machinist
AERA Certified Professional Engine Machinist