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...


A gearbox that is generating metal would most likely have metal on both the upper and lower chip detector. Scavenge oil from the gearbox also includes scavenge oil from the No. 5 bearing in the turbine and the No. 2 bearing in the compressor. This is important to note because before the gearbox is dismantled, the No. 5 and 2 bearings must be eliminated as possible sources of metal generation. So take your time, use a strong light and a 10-power magnifying glass, and thoroughly inspect both the No. 5 & No. 2 bearing areas.

Once the turbine and the compressor have been eliminated, the search for the source of metal can be focused on the gearbox.

To split a Allison 250 C20 gearbox, the gearbox cover or aft portion is lifted off of the housing or front half. Place the gearbox on a flat, clean work bench with a lot of room. The chip detectors should already have been removed and as much oil as possible should have been drained.

Start by removing all of the nuts and bolts on the split-line. Remove the washers because they sometimes jam in the threads of the studs and create a hang up when splitting the cover from the housing. With all the split-line hardware removed, and the gearbox lying horizontally and with the cover side up, use a soft-faced hammer from below to carefully tap upward the top mount for the turbine. At the same time, use your free hand to lift the gearbox cover from the boss for the bottom chip detector.

Tap the cover a few times at this location and then alternate the tapping on the boss for the chip detector — do this while lifting the cover from the top mount for the turbine and listen for a change in the sound of the gearbox. Once the cover has split from the housing, the sound of the knocking will go hollow.

At this point the cover should come off the rest of the way by hand force alone. If the cover does not separate completely, push down on the power turbine governor gear with the handle portion of a screwdriver. Also, peek through the gap in the split-line around half way up the gearbox on the right side near the oil pump and make sure the first reduction idler gear in the N1 gear train is not lifting off of the oil pump. If it is, carefully use a piece of wood to pry it back down. Be aware that most of the gears come off with the cover, so it will seem a bit heavy.

To dismantle the cover start with the N2 gear train, rest the cover on the gears and take some precautions to protect the gears and bearings. In the middle of the cover, on the aft side, you'll find the torquemeter nut. First, use a punch and a hammer to unlock the torquemeter lock cup by displacing the material out of the notch. With the torquemeter socket Allison P/N 6795597 and a 1/2 inch drive breaker bar, undo the torquemeter nut.

This nut is torqued to 430 inch pounds of torque at assembly. If you have an oil leak from the torquemeter nut, consider undoing the torquemeter nut with a torque wrench. If there is hardly any torque left, there is a good chance that there is considerable wear in the torquemeter bore area of the gear box cover.

Once the torquemeter nut is removed, don't turn the N2 gear train or move the gearbox cover around. A torquemeter thrust washer is located between the torquemeter roller bearing and the gearbox cover. The thrust washer has a key-way notch in it that allows the antirotation pin in the support shaft to pass through. By inadvertently spinning the gears or by moving the cover around, the thrust washer could spin and lock the support shaft into the cover.

If the torquemeter support shaft does not come out with a quick smart blow using a soft-faced hammer, chances are the trust washer has spun and you'll now have to spend some time blind fiddling to get the key-way and the pin lined up again.

Usually the support shaft comes out ant you catch it in your hand. At that point it will feel like the torquemeter gear is free and in your hand as well. Pull your hand out from underneath the cover along with the torquemeter gear and the support shaft.

Now, turn the gear box cover over, and remove the PTO gear. To remove the pinion gear turn the cover again and remove the oil nozzle for the No. 4 and 5 bearings. If there are roller bearings in the 3 & 4 positions already, you'll have to remove the circlip as well.

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