As automatic transmissions have evolved, so have their components. The demands on these transmissions in relation to efficiency, load capability, and technology have increased significantly. They’re running at higher temperatures and cycling more often than ever. These technological advances require that the sealing components withstand the rigors of today’s and tomorrow’s transmissions.
The three basic sealing components that we’ll be focusing on are:
- Rubber seals, which includes O-rings, D-rings, and lip seals
- Gaskets, including those for oil pans, pump, extension housing, and valve bodies
- Sealing Rings for supports, shafts, drums, and pistons
Most components found in automatic transmission overhaul repair kits fit into these three categories. We’ll look at how these components originated and have evolved to meet the needs of the ever-changing transmission aftermarket repair world.
Rubber compounds have made great advances over the years. In the early days, the predominant rubber compound was Buna, or NBR. The rubber had a very high wear index but a low temperature limit. As the complexity of transmissions increased, we needed a higher-level rubber compound, so it had to evolve.
The chart (figure 1) shows all rubber compounds, past and present, used in automatic transmissions. From this chart, you can see that the most popular are currently Polyacrylate, Vamac, and Viton.
One compound that’s currently gaining popularity is HNBR (Hydrogenated Nitrile Butadiene Rubber). It’s based on the original NBR compound and is becoming more popular due to its high wear index. HNBR is formulated for compatibility with the latest transmission fluids, including GM Dexron and Ford Mercon, as well as ZF and Honda fluids.
Similar to Poly and Vamac, there are varying recipes for HNBR. This makes it very versatile. It can be used in various static, rotating, and reciprocating applications.
As we see it, due to their durability and temperature range, Viton and HNBR will be the predominant rubber compounds for the next five to ten years.
Surface preparation is something that’s become more and more important. Mechanics and techs have seen the need to prep the surfaces before rebuilding. To prolong the life of all rubber seals, we recommend polishing the bore surface with Scotchbrite® (a registered trademark of 3M). This will leave a crosshatch pattern to allow lubrication to reach the seal’s contact area.
Much like rubber, gaskets have undergone major changes over the years. The use of traditional paper gaskets made from wood is dwindling. A popular paper gasket from the ’60s and ’70s was CV-405, which was used for pump, case, and extension housing gaskets (figure 2).
Then came NV-519, which evolved from CV-405 to advance the reliability of the product. NV-519 is a paper gasket that’s still in use today. It’s very popular for pump, case, extension housing, covers, and valve body gaskets. For certain applications, paper gaskets are still the best way to go.
The main downside with NV-519 material is that, if you leave it in humid conditions for a long time, it may swell and become oversized.
Synthetic gaskets are surpassing paper in popularity in certain applications because of their durability. The most popular are MP-2N, MP-15 (Figure 3), VB-72, CMP-4200 (Figure 4), D7201-C, and HFL-971C.
These gaskets will continue to be used over the next several years due to their density, tensile strength, and longevity.
Sealing rings were originally made from all metal, typically cast iron (figure 5) and sometimes aluminum.
The metal on the sealing rings was so hard that the original sealing rings were cutting into other aluminum parts, causing manufacturers to turn to a new metal composite called nickel-tin, which was more compatible with the rest of the transmission. This improved their sealing abilities.
In an effort to provide better gas mileage in the late ’70s, manufacturers began updating the housing drums and supports to make cars lighter. That’s when Teflon or PTFE rings started dominating the industry. The rings were made out of various materials and combinations, such as graphite, glass-filled, or carbon, bronze, and graphite all together.
Moving forward, we’ve seen that PTFEs don’t always withstand the rigors of today’s transmission, which spawned the use of plastic rings.
The first plastic ring to show up in a transmission was the second clutch hub ring in a General Motors TH440T4. This was in 1987 and the material was Peek® (a product of Victrex; figure 6).
Peek has had its ups and downs as it doesn’t always air check very well upon initial installation. There are specific instructions found in Precision overhaul kits on how to install Peek® rings because of this. While it’s still used in some applications, it’s very limited in today’s transmission world. The more predominant plastics are Vespel® (a product of DuPont) and Torlon® (made by Solvay Plastics).
Both Vespel and Torlon exhibit similar durability to Peek, but are much more forgiving on imperfect surfaces and provide crisper applies. They hold pressure better over the long haul.
Vespel seems to be the material of choice for many Jatco and Aisin transmissions, while Torlon shows up in many ZF and Mercedes. This trend is poised to continue into the future… as long as there are shafts and hubs in transmissions!
This article was provided by Precision International Technical Department