Originally Posted by 52Carl
Are we talking about positive valve stem seals? The kind which are affixed to the valve guides? Or the kind of umbrella seals commonly used on older Furd engines

Positive seals clamp to the top of the valve guide with a steel spring, and wipe the valve stem as it moves through the seal. An umbrella seal is a cup-shaped device that grabs the valve stem and rides up and down with it, deflecting the oil away from the top of the guide. Positive seals require a machined outside diameter of the guide and are usually used on race engines because they're very small diameter and allow the use of double or even triple valve springs. They're not really intended to be used on street engines that are expected to run for thousands of miles. I used a lot of positive seals on my round track engines, but a really old roundy-round engine might have 5,000 miles on it! Umbrella seals are a little better mileage-wise, but they still have a habit of getting tired and losing their grip on the valve stem.
Jerry


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