engine/shaft coupler alignment? - SailNet Community
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post #1 of 4 Old 04-20-2008 Thread Starter
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engine/shaft coupler alignment?

I searched the forum and found a bunch of posts on this issue, but none that got to my question. So hear it is:

I just installed a new (used) inboard 1GM diesel engine replacing a BMW D7.

I've connected the shaft coupler to the transmission and there is an ever-so-slight wobble as I rotate the shaft by hand. I spent a few hours aligning the engine to the coupler and I just can't seem to eliminate it.

First, I read here that final adjustments should be made on the water with the rig at tension, not on the hard, so I know I'm not done.

Second, do I just need to keep trying to get it perfect, or is this little wobble acceptable? My instinct is "no", but I've never done this before. I keep thinking about how much the engine is going to bounce around when it's running.

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post #2 of 4 Old 04-21-2008
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Is the wobble from the engine mounts, or from looseness in the shaft? Can the mounting nuts be raised or lowered to align the engine itself?

Is the keyway firmly on the shaft side of the coupler? Is there a "grub screw" from the collar of the coupler to the shaft? Is there room to put in a rubber-type "Drive Saver" that might compensate (and eliminate some vibration)?

Do you have a traditional stuffing box and is it slotted into a thrust bearing? Are various hoses and clamps loose?

Do you know if the shaft is in fact straight? Is there play in the cutlass bearing? They wear out and there's a special tool for sliding in replacements.

As you can see, there are a number of variables involved here, including the fairly unpleasant prospect of something loose in the transmission itself, but you didn't mention oil pouring out, so I'll assume that's OK.

Get a knowledgeable owner to have a look. It's unlikely this can be diagnosed over the Internet. All I can do is what I've done here: give you a list of the things I would check if I found shaft play.
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post #3 of 4 Old 04-21-2008
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Even with


If your wobble is more than .003" it is not acceptable..

Even with a Drive Saver or R&D Flex Coupling the alignment needs to be 0.010 inch / 0.25 mm BEFORE you install a flexible coupling! This is NOT a lot of variance and it CAN NOT normally be seen with the naked eye.

Without a Drive Saver, alignment between the propeller shafts coupling and the transmission's coupling can contain a variance NO GREATER than .001in (.25 mm) per inch of the coupling diameter. For example, if your propeller shaft coupling is 3” (76mm) in diameter, the maximum error that can be allowed in the alignment is .003 in (.076mm).

You can NOT do a proper alignment without feeler gauges and you CAN NOT see a .003 variance with your eyes!!

Improper alignment can destroy your transmissions output shaft seal, prematurely wear the engine mounts, damage the cutlass bearing and wear the stuffing box and shaft sooner than necessary. Final alignment should ALWAYS be done after about three to four days in water with the rig properly tensioned...

-Maine Sail / CS-36T

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Last edited by Maine Sail; 04-21-2008 at 10:15 AM.
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post #4 of 4 Old 05-21-2008
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A couple of questions.

Halekai - Are you saying to line it up with the hard coupler, to get it very close. Then unbolt, insert the drivesaver (or R & D) and then rebolt, and check clearance. I have read that one owner found it easy to see the misalignment, with the disk in place, as the rubber is squeezed more on one side, etc.

Before I can line up the engine, I have read that you should center the shaft in the stern- tube. As the stern tube is over a foot away from the strut ( cutlass bearing), there is a fairly wide range of movement possible. There is some wear in the stern tube on one side, as the engine was likely out of alignment at one time or another. I can place the shaft as close to center as possible, and then adjust the engine mounts to allow the trannie to bolt up cleanly. However, it looks to me like the shaft is not bound so tightly in the cutlass bearing, that it cannot be "happy" turning, even if it is not in the center of the tube.
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