BTW - when I initially couldn't figure out WHY I was blowing couplings I took the boat to the MERC. expert. He said everything was perfect. I blew a coupling that weekend. I had had enough and tore it apart to figure out what was wrong.

If the tool is not close to parallel to the bottom what happens is the output shaft at the front of the U joint will not be in line ( on the same plane ) with the input shaft of the drive. A single U joint cant get out of plane but a dual U joint can. If a dual U joint gets out of plane then it causes side loading of the U joint and that causes side loading of the output shaft inside the engine coupling and IF you have enough HP like I usually do it overheats the coupling and it melts the rubber. A U joint is in plane when it is straight and both input and output shaft are inline, one end is not higher than the other. IF the engine output shaft is higher at the U-joint due to sagged rear engine mounts it causes the U-joint to not only flex because of trimming or steering but it also has to flex to correct the difference between the height of the two shafts.

You guys need to look at a picture of the engine, gimbal, drive alignment before You comment about this. That or go back to school.

IF you don't believe this possible just try it for yourself. Remove 1/4 inch of the shim on the rear mount and then lower the front mount about an inch and You should be able to get the alignment tool to go in with some tweaking. IF the tool goes in then it must be all OK right.

NOT

What shim on the rear mount? There is a fiber washer and a double wound lockwasher. That is ALL! You want to adjust the rear, you gotta fill the transom and cut a new hole. If there are no shoulder bolts for the coupler, and there usually aren't, it is difficult to get the coupler on true. I sit down with a dial indicator and check the run-out on the coupler. You can also use a 4x4 piece of lumber and a come-along to raise-lower the front of engines without dragging it to a hoist or forklift.

Here's the Mercruiser manual boat builders use for pretty much all the rigging installing motors and drives: A lot of cool stuff in it.
http://www.boatfix.com/merc/Install/gas/86017211.pdf

Here's one for the 8.1L if anyone needs it: https://www.scribd.com/doc/9560841/M...allation-guide

To the OP, I have some oe shoulder bolts for coupler mount if you dont have them

Thank you. I actually just picked some up this morning.

Thanks everyone for all the help. I think I will get this figured out!

DEREK - The whole point is that over time the rear mounts CAN sag. ( that is the 1/4 inch - I am not saying they sag this much - more like an 1/8 inch.) 1/8th inch at the rear mount will cause you to lower the front mount about 3/4 inch and make the output shaft angle up badly.

the rear mount can NOT sag. unless the entire transom assy is sagging. the inner and outer are rigid and non-adjustable in relation to each other. i have never seen or heard of the mount surfaces on the inner transom assy bending downward or wearing any measurable depth where the spring washers sit. the same with the mount face on the flywheel cover.

something to check... Make sure you have the correct alignment tool. There are two. The long sleeved coupler takes a different tool than the coupler without the sleeve.

On a second note. Whenever I have re-installed a motor after the stringers have been replaced and it has solid mounts/ aluminum angles on the stringers I have always bought new angles and drilled new holes to get correct alignment.