1981/03/09 Caterpillar

7155 Truck Transmission

Description of Change: Three new poppet valves and a new piston are available as replacement parts for the 7155 Truck Transmission.

Two of the new poppet valves are used together in the air pressure regulator for the output clutch. The other poppet valve and the piston are used together in the transmission control group and in the transmission ratio selector control group.

Also, the Starrett, No. 18-B Automatic Center Punch can be used to install the new piston on the new poppet valve.

Adaptable As: The new parts, used together, are direct replacements for the former parts as shown in the chart. Illustrations 3 and 4 show the exact locations in which the poppet valves and pistons are used.

Poppet Valve Designs

Fifty-six 9N4115 Valve - 9N4116 Piston units are used in the transmission. Fifty-three are used in the 9N4117 Plate and Valve Assembly and three are used in the transmission ratio selector control group. See Illustrations 3 and 4.

Also in the 9N4117 Plate and Valve Assembly, a 9N4120 Valve and a 9N4118 Valve are used together in the air pressure regulator for the outout clutch.

The original poppet valve-piston unit in the 7155 Truck Transmission was made up of a 9N2684 Valve and a 9N1567 Piston. See "A" in Illustration 1. The piston was press fit on the stem of the poppet valve. It was possible for the piston to move on the stem, so the same parts were then press fit and staked (fastened by distortion of the metal with a punch).

Next, the 9N4098 Valve and the 9N4100 Piston were released. The stem of the poppet has a smaller diameter end and a shoulder. The piston fit on the stem, against the shoulder. The end of the stem was peened (bent) to fasten the piston to the poppet valve. See "B" in Illustration 1.

The new 9N4115 Valve has a larger diameter end on the stem, but it still has a shoulder. The end of the stem has an opening, similar to a tube-type rivet. The new 9N4116 Piston is installed on the stem, against the shoulder. A punch is used in the opening in the stem for expansion of the stem to fasten the piston to the poppet valve. The result is a stronger poppet valve - piston connection. See "C" in Illustration 1.

Illustration 1. The three poppet valve-piston units.

The pressure regulator originally had a 9N2688 Valve on the end of the stem on a 9N2684 Valve. See "A" in Illustration 2. It was some times possible for the 9N2688 Valve to come off the stem.

When the 9N4098 Valve was released, the 9N4113 Valve was fastened to its stem with a 1L5316 Pin. See "B" in Illustration 2.

The new 9N4120 Valve has a longer stem than the earlier 9N4098 Valve, and its stem does not have a shoulder. The new 9N4118 Valve is pressed on the stem and fastened with a 1L5316 Pin. See "C" in Illustration 2. The result, again, is a stronger poppet valve to poppet valve connection.

Illustration 2. The three double poppet valve units.

Illustration 3. Location of the valves in the 9N4117 Plate and Valve Assembly.

Installation Of The Poppet Valves And Pistons

Use of the Starrett No. 18-B Automatic Center Punch permits easy installation of the 9N4116 Piston on the 9N4115 Valve. See Illustration 5. Correct use of the spring-operated punch gives even expansion of the stem without damage to the piston. See Illustration 6.

Illustration 4. Location of the valves in the 9N1803 Transmission Ratio Selector Control Group.

Illustration 5. The Starrett No. 18-B Automatic Center Punch.

Illustration 6. Installation of a piston on a poppet valve.

A replacement point, Part No. 2948, also is available. For the name of the nearest dealer and/or repair center, write or call:

To prevent damage to the poppet valve, it is Caterpillar's recommendation that an FT1480 Support be used to hold the poppet valve while the piston is installed. Also, the FT1484 Support is available to hold the 9N4117 Plate and Valve Assembly during installation of the poppet valves. Drawings for the FT1480 and FT1484 Supports are available from your Division Service Manager.

Special Instruction, Form No. SEHS7764-01 has the complete procedure for installation of the poppet valves and pistons.