Mercruiser Alpha MR - Rattle

mlackore

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Jul 11, 2009
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I have a pretty good hunch that this is a classic u-joint rattle but I wanted to bounce it off of the pros here on this forum. A little, condensed version of the back-story. The boat is a 1984 Sylvan, 120 (2.5L) Chevrolet 4 cylinder Mercruiser with an Alpha MR out-drive. Gimbal bearing ( Greaseable type) has 2 hours of run time and was installed at certified Mercruiser service center last month. Water in the bellows killed the last gimbal bearing that they installed after only 2 years and they replaced it under warranty since I have had repeated water intrusion issues since they worked on it. The video, linked below, was taken on the shakedown cruise after the latest gimbal bearing install 2 hours in. Engine is idling this video and rattle occurs when turning in both directions 3/4 to full lock in either direction. I also have a vibration during acceleration. Rattle begins at the 0.19 second mark and in this video, I am turning only to port. Thoughts.

Thanks in advance.
 

alldodge

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Could be U joints, gimbal, alignment, worn coupler, outdrive carrier bearing

Given the history, I would suspect U joints

Your gimbal was just replaced, and its probably not the issue, but for me its still in the mix until proven it is not.
 

achris

More fish than mountain goat
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5 minute job to pull the drive... Easy to check unis...

Chris...
 

mlackore

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Well, I'm sure hoping this isn't a worn coupler or another bad gimbal bearing. Why would a worn coupler cause the noise only when turning? I've popped the outdrive off now and removed the carrier bearing set. Gimbal bearing feels tight and smooth from through bell housing. Bellows are dry inside save for a bit of grease. I can't feel any slop in the u-joints but they seem a little loose to me. The shop pumped them full of grease to flush the water out of them though. I'm going to replace them for insurance and re-seal the bearing set as well. I don't have an alignment bar but one is in the mail. The carrier bearing set feels smooth too; I see no wear. Am I on the right track here? What are the other symptoms of a bum coupler?
 

alldodge

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Just stating things that may cause the issue. If the coupler is worn, were the teeth are worn down from an incorrect alignment, then maybe

If the joints had water in them, then they would need to be replaced
 

mlackore

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The joints have been flushed a couple of times now after water has gotten into them and the shop has done it each time. The last gimbal bearing was roaring like crazy when I started it up for the first time after winter storage so I suspect that there was water in there when it went into storage. It sat and rusted in there over the winter. I am thinking the same thing happened to the u-joints. I cannot see any damage to the splines on the shaft that I removed with the outdrive. I can post a pic if you would like and if you think it would help....maybe you guys will see something I didn't? I appreciate the ideas and feedback immensely, thank you.
 

mlackore

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Well,...I thought I would give an update regarding this rattle. I resealed the upper bearing set, replaced the u-joints and re-applied the proper torque to everything on the outdrive. When I removed that spanner nut, I did find it very loose, maybe slightly more than finger tight but definitely not the 200 ft-lbs spec'd in the book. I went to check alignment before installing the drive and it was WAY off. The bar wouldn't go into the coupler at all. I spent the next 2 hours working on getting it aligned and it the best we could get was about 1/2" of engagement with the coupler. I checked all of my stringers and mounts, all seemed good. Reaching through the gimbal bearing and gauging the space between the oil seal and the inside of the bearing, we discovered that the gimbal bearing was installed crooked ( the gap was very uneven around the perimeter). Reaching through a little further, through the oil seal, that too wasn't fully seated in the transom assembly. Not having the tools to pull the gimbal bearing or install a new one, it's now back at the Mercruiser service center receiving it's third, new gimbal bearing and grease seal in three years.
 
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