2GM20F Shaft Alignment... - SailNet Community
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post #1 of 6 Old 04-11-2013 Thread Starter
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2GM20F Shaft Alignment...

Brief History...

There has always been some vibration in the boat while motoring. I know this is partly due (well at least I think) to the engine which is a 2GM20F due to the compression ratio and its normal vibration. Last night I went to Autozone and bought a feeler gauge to confirm that I have proper alignment from the shaft to the transmission. I was not able to get even the smallest gauge between the flanges. There was some corrosion so I separated them and cleaned them up with 400 grit sandpaper then wiped them down with acetone and put some oil on them to set a little better and help prevent corrosion.

Even after cleaning them up and putting them back together I was not able to get the feeler gauge between the flanges.

Now for the question...

Am I doing this correctly? Is it possible that bolted together they line up perfectly? There is no torque specification that comes with this engine/transmission that I found, so once together I rotated and tightened each screw a turn at a time until snug.

Anyone have any experience with doing this?

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Portsmouth, RI
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post #2 of 6 Old 04-11-2013
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Re: 2GM20F Shaft Alignment...

It sounds like you are measuring *after* the tightening the flange bolts. You dont want to do that.

Here is how I do it:

1 - loosen to the top bolts on all 4 motor mounts.
2 - remove the flange bolts and slide the shaft out (away from the transmission)
3 - verify that the shaft is centered in the stern tube. I do this just a sanity check. you can feel for the center by moving/wiggling it up down back forth. But if there is much play you may need to block it in place. if course if there is too much play you may have another problem (worn cutlass bearing).

-> The prop shaft is the fixed point in this system. The goal is to move the engine around to line up with it in X, Y and Z.

4 - Align the centerline of the prop shaft and the transmission output. This is a coarse adjustment. To do this you will be adjusting the bottom nut on the motor mounts.
5 - bring the flanges together until the prop flange just contacts the transmission flange. At that point measure the gap using the feeler gauge all around the flange. at 12, 3, 6 and 9 o'clock (ensure that the flanges are clean!)
6 - adjust the mounts to bring those measurements very close, within a few thousandanths. again you are using the bottom nut on the motor mounts.
7 - return to #4. Adjustments in #6 it may have altered the centerline alignment.

so you cycle through 4-5-6 until you are satisfied.

8 - tighten down the top nuts on the motor mounts to lock in your adjustments.
9 - return to #4, 5 and 6 one last time to verify that you did not disturb alignments when you tightened up the mounts.

One caveat, #6 is MUCH easier said than done. It is quite tricky, tedious, and can be time consuming. Dont rush it. Have a beer ready for when you are done. you will need it, and you will have earned it!

oh and for the record, the boat should be in the water when you do this.

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post #3 of 6 Old 04-11-2013
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Re: 2GM20F Shaft Alignment...

I may be faced with this in the future. I thought if one wanted to check the alignment all
one would have to do is unbolt the prop shaft flange from the transmission flange, slid the prop back a tad and insure the distance between the two flanges is the same all way around and remains the same while either the prop or the transmission is rotated.
I understand messing with the engine mounts if the two are misaligned.

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post #4 of 6 Old 04-11-2013
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Re: 2GM20F Shaft Alignment...

One of the hardest technical problems to solve on a boat in my experience. You should get the "runout" (wobble) of the flange to as near zero as possible as measured using a dial indicator and also get the flange faces as near parallel as possible using feeler gauges.

The shaft should be firmly fixed and centered in the cutlass bearing and the shaft packing should have the shaft centered within the packing gland but the shaft log to packing gland connection is a hard but flexible hose and as a result the shaft will "sag" when the flange is loosened. Managing this "sag" to make sure readings on the indicator and feeler gauges are both acceptable and repeatable has driven me up the wall a few times.

I still don't feel comfortable that my alignments are technical skill rather than eyeball luck. I haven't ruined any cutlass bearings yet (knock wood) and don't seem to have unreasonable vibration while running under power. But...

If anyone has any hints on how to do this better I sure would like to hear them.
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post #5 of 6 Old 04-11-2013
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Re: 2GM20F Shaft Alignment...

If the rear motor mounts are on the engine rather than the gear,(as aft as possible) it's possible to find it tricky; front then rear then front etc. success is when the axis of gear lines up with shaft. the fine tune is when the friction on feeler gauge is same all around the flange. Not having the engine weight even on all four mounts will cause rocking and mount failure. If the coupling is not correct on the shaft (like lath fitted) the rest is moot.
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post #6 of 6 Old 04-11-2013
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Re: 2GM20F Shaft Alignment...

Nice description on doing the alignment.

Anther possible source of vibration could be your engine mounts. I believe I have a decent alignment but for he last year or so I get a lot of vibration when I get my 2GMF above 2500 RPM. My boat is an 84 and the motor mounts are original and I believe they are wore out. I have four new ones and my engine is currently sitting in the cabin on a wooden base ready to swap out the mounts. I really think and hope this will cure the vibration.

I priced Yanmar replacements but ended up buying third party D.F. mounts that I think will work fine and which cost $400 less for the set than the Yanmars. Will know in about a month when I get launched.
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