Dec. 16, 2024
Agriculture
Before proceeding with the assembly of my engine heads, I must first select the appropriate valve stem seals. To ensure I make an informed choice, I've researched the various designs and materials available.
Here are some key takeaways regarding valve stem seals:
Primarily, there are two main designs for valve stem seals: Oil Deflectors and Positive Type Seals. Umbrella or O-ring seals fall under the category of oil deflectors, as they move along with the valve stem, while positive seals remain stationary.
Valve stem seals serve as OIL METERING devices. They meticulously control the amount of oil that passes through the seal and into the valve guide. While the requirements can differ based on engine design and operational conditions, the typical oil metering rate is between 0.1 to 1.0 mg per valve per hour. It's important to note that these seals are not installed to completely halt oil flow, as oil is vital for lubrication between the valve stem and valve guide.
These seals function based on the same hydrodynamic lubrication principles as main and rod bearings. When conditions permit, there is no metal-to-metal contact; instead, the seal, valve stem, and valve guide glide on a thin layer of oil, similar to main bearings.
The selection of materials for seals depends on their specific application and design. Common materials include nitrile, polyacrylate, fluoroelastomer (Viton), silicone, nylon, and Teflon.
The parts that constitute a seal are crucial in its effectiveness. To gain a better understanding of these components, you can refer to the links provided below:
xtnbk Product Page
For more technical insights, consider exploring:
For a detailed analysis, check out this very technical article.
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Recognizing that higher RPMs can lead to increased crankcase pressure, I have noticed an uptick in the volume of oil traveling past the tappets, as well as an increase in oil escaping from the cam cover vent.
To mitigate these issues, I've found that using both a block-mounted crankcase vent (the plastic separator) alongside a cam cover vent can substantially reduce oil leakage through the vent hoses. This configuration has proven beneficial for both boosted engines and those with high mileage.
One of the key advantages of a dual vent system is its capacity to eliminate oil leaks. In boosted engines, the quantity of oil directed into the intercoolers has notably decreased.
While I agree that abstaining from a Viton seal on the exhaust could yield some advantages, I prefer not to use stem seals on exhausts. This approach allows oil to reach the exhaust valve stem and guide, contributing to their cooling, albeit minimally. However, every little bit helps, especially when it comes to applying oil splash to the springs.
It seems the primary concern revolves around the size of the crankcase vent system. Here are my suggestions:
My belief is that increasing the crankcase vent system size significantly reduces oil leaks and lowers the volume of oil being expelled through the vent. Ideally, the block vent manages ring blow-by, while the cam cover vent addresses valve stem blow-by.
Given the pressure from increased RPMs or boosted conditions, it appears that the standard crankcase vent system size does not adequately handle the resulting blow-by volumes. Expanding the vent system is crucial for maintaining negative pressure within the crankcase, which I find preferable.
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