1987/08/31 Caterpillar

7271 Marine Transmissions

Description of Change: The original clutch retraction pins used in the ahead and astern clutches in the 7271 Marine Transmission were of a "full floating" design. With this design the head on each retraction pin engages a slot in the piston and the pin moves with the piston when the clutch is engaged.

The full floating pin design experiences fretting and wear where the head of the hardened pin contacts the slot in the cast iron piston. Also, centrifugal force acting on the spring end of the pins causes the pins to cock in the pin bores in the clutch gear and wear the shank of the pin and bores in the gear on diagonally opposite sides.

Wear on the head of the pin and the slots in the piston, combined with wear on the shank of the pin and bores in the gear, can cause the pins to "hang up" or "stick" in the bores of the gear when the clutch is disengaged. When the clutch is then engaged, pressure builds up until the pins and piston break loose and the clutch sometimes engages abruptly with a "bang".

To correct these problems, new improved retraction pins and pistons are available. The improved pins are bolted solidly to the piston so there is no relative motion between the pin and the piston. This eliminates fretting and wear at that point. Also, since the pin is bolted solidly to the piston, the pin cannot cock in the gear bores due to centrifugal force. This eliminates the diagonal wear on the pin shank and gear bore and the pin "hang up" in the gear bores. Refer to the "Procedure To Rework 7271 Marine Transmissions".

Adaptable To: These improvements to the 7271 Marine Transmission are adaptable back to first production. Refer to the chart for identification of the new parts.

Illustration 1. Marine transmission clutch group, "ahead" clutch shown. The "astern" clutch group is similar.

Procedure To Rework 7271 Marine Transmissions

1. If a new 9V7196 Piston is not used, the former 3N6187 Piston must be reworked. The clutch pistons can be reworked by end milling 12 new, flat surfaces for the retraction pins and drilling and tapping 12-5/16"-18 holes for the retraction pin bolts. See Illustrations 2, 3 and 4 for piston rework information.

Illustration 2. Rework of former 3N6187 Piston.

Illustration 3. View A-A of reworked piston.

Illustration 4. Side view of reworked piston.

2. When installing the improved parts, inspect the pin bores in the clutch gears for wear. If the bores are worn, the new pins and O-rings may not seal correctly causing a leak of clutch pressure. If wear exists, either a new gear must be installed and the standard size 9W7195 Pins and 3P647 Seals used or the worn gears must be reworked and the 12 pin bores in the gears must be bored out to 20.62 ± 0.05 mm (.812 ± .002") diameter with a surface texture of 0.80 ?m (32 MU) finish and the .060" oversize 6Y1253 Pins and 618639 O-ring used. A 0.5 mm (.02) × 30° lead in chamfer must also be provided on the piston side of the pin bores to avoid cutting the seals at assembly. See Illustration 5.

Illustration 5. Rework of transfer gear for use with oversize pistons and seals, "ahead" gear shown, "astern" gear is similar.

NOTE: To rework the gears a machine shop capable of boring hardened steel (RKW "C" 59-64) is required. The location for the pin bores can be established from the location of the counterbores for the retraction pin springs. One machine shop on the U.S. gulf coast which has the capability to rework the gears is:

3. When assembling the improved parts, the bolts which fasten the 12 pins to the piston should not be tightened until the clutch piston has been centered in the bore in the clutch gear. This can be done by running a conv. (.007" to .010") feeler gauge around the piston to center the piston in the gear bore. Once the piston is centered, tighten all of the retraction pin bolts (4) and recheck the clutch piston centering. See note in Illustration 1.