elcaminodragster said:
i need to change my valve stem seals but i dont want the o ring style i want the umbrella style. i would also like to upgrade my valve springs. they are 350 heads from a 69 camaro. i believe they are single springs with the damper spring.
i have a 268h cam but i never upgraded the spring as it is recommended so i would like to now since i have to replace the stem seals.
i want the umbrella seals because ive heard lots are read lots more that they are the best seal. but i dont know which ones to get. some say machine work required others say its not needed.
as far as valve springs. should i buy single or dual springs. what about a damper? spring rate? bind hight? etc. im not sure what to look for.
what other information do i need to list to help you tell me what i should look for?
If the heads have reached a point where stem seals (which-ever configuration used) need replacing, then it's also time to look at stem to guide clearance as the guides may be worn as well and there's little point in new seals when the guide is so loose that excess oil gets into the clearance regardless of attempts to keep it out. Then your also at a valve job as restoring the guides also changes how the valve closes which will be from rolling around on the seat because of the loose stem to guide clearance to a specific narrow edge seal between the valve and seat as it had when new.
O rings and umbrellas aren't really seals in that they don't make an oil tight fit that wipes excess oil off the moving parts, they are more of a shield that directs some amount of oil away from the guide clearance in the hope of reducing the amount of oil present that can be pulled into the clearance. A true seal would be like the old metal banded Teflon Sealed Power seal that attaches to a surface machined into the top of the guide. These days these things are sold by everybody in the cam and gasket business in Teflon or rubber. These actually scrub oil off the stem, not just redirect from the stem like O rings and umbrellas. So general rules:
1) Intake valve stem and guide is exposed to manifold vacuum which always wants to pull oil thru the clearance. Positive seals work best here, but they are better with bronze alloy guides which don't need so much lubrication as cast iron where O rings and umbrellas may be better.
2) Exhaust valve stem and guide is exposed to pressure which tends to blow oil out the clearance, they can benefit from a bit more clearance and O rings or umbrellas to simply reduce the amount of oil but not nearly so eliminate it. They also need better guide and stem quality as this is a major source of the exhaust valve's cooling.
3) Whether the intake or the exhaust, oil pulled onto the backside of the valve forms a layer of coked carbon, this increases valve weight making it harder for the spring to control, causes turbulence and obstructs the port both of which reduce flow past the valve, insulates the back side of the valve head reducing heat transfer and increasing valve temperature reducing its life.
.050 inch is a good magic number for clearance between the bottom of the spring retainer and the top of the guide or the top of the seal on the guide. This number is also good for spring coil to coil clearance of the compressed spring.
Dampers are intended to function like a friction shock absorber on the spring, they aren't there to add stiffness, they are there to modify the springs natural harmonic frequency, which if left undamped could allow the spring to go into an undamped response resulting in spring surge which places very high loads on the valve train components and or looses control of the valve resulting in event's occurring whether or not commanded by the cam. The out of control valve spring behaves something like a car that has bad shocks, in that after the car hits a bump, the suspension springs keep on bouncing. Unlike the random nature of road bumps, while the valve spring is bouncing another bump is coming in musical time, if the spring isn't under control by then the next bump can result in broken parts. Consider this, the factory is always looking for ways to cut cost, if they didn't think dampers were necessary, you can bet your last dollar they wouldn't be there. Use the springs the camshaft manufacturer recommends unless you have the test facilities, engineering expertise, and financial where-with-all to pick up the costs of experimentation. Not doing this might save you a hundred bucks if you get lucky, but may cost you several thousand if you aren't so lucky.
Bogie
