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Old 01-30-2013
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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

hello Jeff, I have done many many engine alignments, and I'm going to suggest to you by observation of the pictures that either your output shaft coupling or the shaft coupling or both were hit many times by a hammer that is a definite no-no. This is probably what initially started the problem. That is why I suggest that you have both couplings put in a lath at the machine shop,and the mating surfaces machined. Whenever I replace one coupling I always do this unless they are both in really good condition. The seal rides on the output shaft coupling which is more than likely damaged you will find the seal in the output shaft bearing housing I can plainly see it in the picture. There is no sense in making a repair that could cause more damage. Also to answer your other question about vibration, when you're engine is in neutral and you throttle it up about 1000 RPM you'll notice it really gets smooth. It should act no different when it is in forward or reverse gear if everything is in the proper alignment. You may have to ajust or shim the motor mounts to attain alignment to the couplings with a feeler gauge. I usually use a dial indicator but a feeler gauge will work fine I recommend 25.000 most of the time you need to use a hammer but never hit on the flange of either coupling. The shaft coupling can be hit on the body that surrounds the shaft and hold it in place. The transmission output coupling needs no hammer just the nut to install. When I use a hammer I always use a sacrificial brass rod or a block of wood. After looking at the pictures if you follow this practice you'll probably never have another problem with it. As said earlier make the correct tool out of an old socket using a hand grinder, or do it at the machine shop preferably half inch drive and torque the nut to specifications also using some Loctite if you can get some.
looking at the shaft coupling itself you'll notice a ridge on the inside as a mating surface, that ridge fits inside the output shaft coupling and if it is damaged in any way which I can clearly see it is, it could've been the cause of the problem. The correct assembly and alignment will not only solve the problem but it will also solve the cause as long as your output shaft bearing is in good condition. CaptG
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