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Common maintenance events on the Allison 250 C20
To remove that hard-to-get circlip for the No. 1 bearing housing, your best bet is to use a good set of needle nose or duck bill pliers. A word of caution: if you're going into the No. 1 bearing area because of a suspected bearing failure, take steps to ensure bearing debris does not fall down inside the compressor assembly when you take off the bearing housing.
You should find a spring and a cup washer sitting on top of the No. 1 bearing. The 9/16 inch nut sitting on top of the No. 1 bearing is a 100 percent replacement item; it should be used only once and then discarded. The dust cap on this nut prevents oil from getting inside the compressor rotor. When removing the No. 1 bearing nut, test the depth of your socket and make sure your socket can't touch the brass ball retainer of the bearing. Technicians in the past, have accidentally damaged the retainer when removing the nut, then reinstalled the damaged bearing, only to have it fail in a few short hours.
Pressure check of the compressor front support
This is an example of compressor wheel rub. The result is heat distortion and heat transfer to the inside of the wheelTo remove the bearing, first establish if you have a puller groove on the forward side of inner race. If you do, you should use puller P/N 23005023 as per CEB 1171. Use the older style puller P/N 6796925 if there isn't a puller groove on the bearing itself. With the old style puller you lift off the bearing with the seal follower. Again, if you think you've damaged the bearing in this process, replace the bearing — Only P/N 23054538 No. 1 bearing nuts are available, and they are to be used with the new style bearings that have the integral puller groove on the forward end. So if your compressor has an old style bearing without the puller groove, you'll need to order a new bearing to go with the new nut. If you just want to change the bearing, you should also change both the follower and the carbon seal assembly.
You can save a bit of money if you buy the kit to replace the carbon element in the seal housing. You can also save a bit of money by purchasing an overhauled seal follower. The No. 1 bearing is seated by torquing the No. 1 bearing nut.
The No. 1 bearing is an one-way thrust bearing that accepts a light forward thrust load from the compressor rotor. Other than analyzing the bearing and figuring out where the thrust shoulders are, the easiest way to install this bearing correctly is to make sure the puller groove is up or on the forward side during installation.
Again, when torquing the nut, be careful not to touch the brass ball retainer with your socket. If you think you've damaged it, replace the bearing. The mating of the bearing housing with the circlip on the outside of the carbon seal is a two-person job. One person should hold the carbon seal up with the aid of two screwdrivers lightly leveraged under the seal and against the balance rim on the first stage wheel. The second person, with one hand, lowers the housing onto the seal and bearing assemblies. The person also makes sure that the spring and cup washer are in place. With the other hand the person squeezes the circlip on the carbon seal with a good pair of needle nose or duckbill pliers.
With one person pushing down on the housing and the other resisting that force with the two screwdrivers, there's a fair amount of force involved. However, with a little bit of finagling, the snap ring will pop into place. Check to see that the circlip is fully seated in its groove by trying to move one of the tangs of the circlip. If it moves relatively easily, and the other tang follows along, the circlip is in the right place. It is common for the circlip to hang up in one place. If this is the case, a concentration of some leverage from a screwdriver, lightly pried up against the bottom of the seal, will seat the ring.
