Join Date: Apr 2011
Location: Oriental NC
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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.
1977 Bristol 29.9