PISTON 9T9040 - Caterpillar

The O-ring seal is one of the most common items used in most types of equipment, and has been the subject of many articles. The constant increase in machine efficiency and O-ring applications makes necessary the development of improved materials for specific installations. With the increased number of applications, designs, and materials, the correct selection and installation of O-ring seals for each specific job becomes more important. The purpose of this article is to make correct seal installation easier, and prevent possible problems which can be caused by use of a wrong seal or method of installation. The paragraphs that follow give information about materials currently used in most O-ring seals, plus information about identification, specifications, and other factors that have an effect on seal installation and performance. As a reference, see the Technical Partsgram, "O-Ring Descriptive And Part Number Reference" Form SEBD2423 (April, 1977). Also, see the article "How the O-Ring Does Its Job" on Pages 2 and 3 of the November 25, 1968 Service Magazine, and "Working With O-Ring Seals" on Page 2 of the November 13, 1969 Service Magazine.
Materials
O-ring seals are made from any of several different materials, according to the characteristics needed for the seal application. Different seal materials must be used for high or low temperature, high or low pressures, rods or shafts which slide or turn, and for the different fluids or gases which must be sealed. The materials used for most seals are: Buna-N, Silicone, Urethane, EPDM (Ethylene Propylene Terpolymer), and FKM (Fluoroelastomer).
The Buna-N material is most common, and is used for seals in many applications. It is black in color, and can be used with most petroleum products, in normal temperature ranges, and at medium pressures. The O-rings used in Duo-Cone Seals are made of a soft grade of Buna-N material.
Silicone material is used for seals in high temperature and low pressure applications. It can be used to seal engine or hydraulic oil, but NOT fuels. Also, a silicone oil or grease must not be used as a lubricant on a silicone seal during installation. The color of silicone seals can be yellow, white, off-white, or red.
Urethane material has especially good resistance to abrasion (wear caused by friction). For this reason, urethane seals are used in many applications on rods or shafts which slide or turn, and to keep out dirt. Urethane material also has a black color.
EPDM material is black, and is currently used for cylinder liner seals in addition to some cooling system and air inlet system applications. This material will swell (become larger) in contact with oil or fuel. Because of this characteristic, only liquid soap can be used on the seal during assembly, except in specific cylinder liner applications as shown in the Service Manual.
FKM material can be black, brown, or green in color, and has good resistance to both high temperatures and high pressures. Applications for seals made of FKM material have increased rapidly with the release of new designs.
Identification
To be sure the correct seal