I spent 10 weeks apprenticing for a marine mechanic, during which we did several alignments. His spec was 4/1000 (4 mils) or better. Otherwise it was as arf145 has described. I believe we would work as follows:
- before starting, all other shaft issues should have been checked and dealt with: cutless bearing is good, no alignment problems with strut, shaft isn't bent, coupling is solidly mounted to prop shaft and bolts secured with wire or loctite.
- with the bolts securing the mounts to the boat rails slightly loose, we shift the engine side-to-side to roughly line up horizontally
- we quickly adjust the mount lower bolts up/down to roughly set the vertical alignment
- at this point we can now pull on the shaft to mate the couplers. Usually the misalignment gap at this point is visible and we repeat the horizontal then vertical adjustments to improve that.
- now comes the 4 mil feeler gauge. We mate the coupler then feel around between the faces. If the gauge can be inserted freely at any point, the engine needs to be shifted slightly. Again we try for horizontal alignment first (worst gap is at top or bottom) then we can snug up the mount bolts to secure the mounts to the rails
- finally we tweak the lower bolts on the mounts to shift the engine up or down
- when the coupling faces are mated and we cannot freely insert the 4 mil feeler at any point around the coupling... the alignment is good. At this point we tighten the mounting bolts fully and spin down the upper bolts, tighten, and check the alignment one more time. If that's good, then we can bolt the coupling halves together. Done.
I am surprised that we don't have a higher-tech solution to alignment issues than this 19th century process, but it is what it is...
I haven't been impressed with the quality of the usual engine-mounts, and how they fail in 3 to 5 seasons, or tear after a rope fouls the prop. If anyone knows of a longer-life aftermarket mount, I'd love to hear about it.
