Common maintenance events on the Allison 250 C20

Common Maintenance Events on the Allison 250 C20 By Jim Taylor February 1998 his article discusses some of the most common maintenance events on each of the three major components that make up an Allison 250 C20. The compressor, the turbine...


Before flushing out the main oil gallery in the gearbox, remove the torquemeter support shaft, the two oil jets, and the return gravity check valve which is located under the filter housing. With the gearbox totally dismantled review the condition of the gearbox cover and housing to determine if you should send them out for rework, or purchase an exchange unit. Things to consider on the cover are:

• Worn 3 & 4 bearing cage, or upgrading to 3 & 4 roller bearings.
• Worn turbine mounting surface known as "surface G."
• Loose or worn bearing cages.
• Incorporation of CEB-A-1226 (through-bolt retention of the idler gears).
• Helicoils® required for the turbine oil scavenge ports.
• Leaking or worn torquemeter insert.
• Bearing bore position suspected due to poor wear pattern on gear teeth.

Items to consider on the gearbox housing are:

• Loose compressor mounting inserts. Note: more than two loose inserts requires the housing be sent to an AMC to have all of the compressor mount inserts checked for being square to the compressor axis.
• Incorporation or CEB-A-1226 (through-bolt retention of the idler gears).
• Worn compressor pilot diameter, causes misalignment of the compressor, resulting in possible high vibrations and an oil leak from the compressor packing.
• Installation of helicoils® in the oil fitting ports.
•Bearing bore position suspected due to poor wear pattern on gear teeth.

The above items should be discussed with your AMC to help you decide what is the best method of accomplishing the repairs.

Assembly tips for the gearbox housing — Keep in mind that these tips are not step-by-step instructions on how to put a gearbox together — you must still refer to the Allison maintenance manual for complete instructions.

A good maintenance practice which can give you piece of mind and help you sleep at night is to save all of the removed packings. Then take all of the empty packages for the new packings installed and do a comparison of the packings removed versus the ones installed. If they don't match, investigate the problem.

Install and lockwire the oil jets in the gearbox housing immediately while there's adequate room in the housing. Once the oil pump and filter housing are installed, it's a good idea to arrange for oil to be supplied to the oil inlet, and then turn the pump over by hand.

Listen for the return gravity check valve to pop. Then turn the housing back over and make sure there are no oil leaks from the pressure oil system.

Once the housing is fully reassembled, install the following items to aid in the mating of the gearbox housing, with the gearbox cover:

• Place the muff spline on top of the oil pump (mates with the fuel control gear).
• Place the first stage reduction N1 reduction gear on top of the oil pump.
• Install the three oil transfer tubes beside the oil pump, and the short oil transfer tube into the gearbox scavenge oil transfer tube at the bottom of the gearbox.
•Install the N2 tach & PTG drive gear into the housing.

Finally apply petrolatum to the rollers of the PTO roller bearing. This is to keep the rollers from falling inwards when installing the cover on the housing.

Assembly tips for the gearbox cover —The gearbox cover is best reassembled if you first reinstall all of the N1 gear train. Remember to install the garlock seal for the air/oil separator gear and the circlip that goes in between the seal and the air/oil separator gear.

The fuel control gearshaft is best installed with the cover lying horizontally. Use the specially designed pliers P/N 6796966. As the gear is lowered into position, bend down to the bench level and try to peek in from side to see when the circlip is at the circlip groove. To test the circlip for proper engagement, move one of the tangs from left to right. If the circlip is properly installed the other tang will follow the movement.

The pinion gear is to be the first N2 gear to be installed, followed by the PTO gear. Have the cover sit as level and as steady as possible. Then place the torquemeter thrust washer and bearing thrust plate so that the cut-outs line up with the keyway in the torquemeter insert. Use petrolatum to help keep the pieces in place.

Carefully maneuver the torquemeter gear into position while holding the torquemeter roller bearing in the gear. The idea is to place gear and the bearing into position without moving or disturbing the torquemeter thrust plate and thrust washer.

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