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Coupler Bushing - 231 Alternator


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I was just at my A&P where I heard a story about a problem with another 231 they just dealt with. Since the same thing happened to me once upon a time I thought it was worth putting out so other owners would be aware.  Might apply to other models with a similar alternator connection (geared).  

The alternator on the 231 is connected to the engine via a "coupler." The coupler is just a gear, inside the coupler is a rubber clutch, the purpose is to allow the clutch to slip if for some reason the alternator freezes up, avoiding damage to the engine to which it is directly connected.  The coupler tends to wear out and needs to be replaced, although good ones will last several years.  A prior A&P provided several for me from an aftermarket manufacturer, and they all failed fairly rapidly, leading us to go directly to Mooney for a replacement which lasted, but that is not the point of this post.

The coupler fits over the shaft of the alternator and is held on by a nut, which is locked in place by a very small cotter pin. Internal to the coupler there is a bushing that fits between the alternator shaft and the barrel of the coupler.  The bushing does not go all the way to the outside end of the coupler, so if it is not assembled by the mechanic doing the work, it's absence cannot be detected by the person doing the inspection and sign-off. If the bushing is missing, the coupler vibrates around the shaft, very quickly beating the cotter pin apart, the nut comes off and the coupler falls off. Unfortunately it, and the nut and cotter pin, fall into the operating engine.  If you are lucky (I was), it sits in the bottom of the crankcase and the mechanic can find all the little pieces.  If you are not, the engine ingests it, causing all kinds of random mischief inside the engine.

So if you have a 231 and need a new coupler, best have the work done by a shop that knows Mooneys, and best to make sure the shop knows that the mechanic actually doing the installation must insure that the bushing is installed.

In this instance, it was not my A&P that did the faulty installation, they just got to deal with it.

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