Broken Drive Shaft?!?!

[TMS1155]

Have my merc alignment tool since 1976,Dont know how many hundreds of boats Ive used it on but being parallel to the bottom has nothing to do with alignment. Think about all the differant transom angles there are, Gimbal housing is mounted to transom. I think if there was something wrong with the way this tool was made it would have been redesigned by now.

[Tinkerer]

The shaft still needs to be paralell to the bottom of the boat.
How do you know that the rear mounts havn't sagged?

[Tinkerer]

If the rear mounts sag you can still get the engine to align properly by lowering the front mount.
The problem is that as you lower the front mount to get the MERC alignment tool to show you that the engine is aligned ( the tool will slide right in smoothly ) You are changing the angle of the output shaft and making the U joints flex. This side loading of the U joints Will blow the rubber coupler. I had a TRAINED MERC. tech. tell me that the engine was perfect. ( the new coupler lasted 30 minutes.) I lost three couplers in two weeks. I then figured out what was wrong and fixed it. I now pin my rear mounts so they can't sag.

[Tinkerer]

Isn't there a different transom assembly for each different angle transom?

[blackhawk]

What Tinkerer is saying makes sense. The alignment tool would go in smooth but at a slight downward angle and you probably wouldn't realize it. Another reason to run offshore mounts.

What causes the rear mounts to "sag"?

Freeryd in attempt to keep your thread from totally being hijacked I would completely drain the oil while the motor is out and add a oil drain kit that replaces your drain plug. Check everything over and look for the obvious. Compression and leak down is a good idea but you can get screwy readings on a cold motor. If it still runs strong don't worry about it, drop it back in and run it. Inspect the bellows and gimbal bearing while the drive is off and definitely check the alignment before installing the drive . The coupler price of $300 sounds good especially if it's the longer coupler.

[blackhawk]

Isn't there a different transom assembly for each different angle transom?

No I believe they are all the same. I would assume the bearing would swivel to compensate for the slightly different transom angles. Shouldn't matter since it is constantly swiveling for trim angle and steering.

[Tinkerer]

The thing is with trim and steering is the shafts are always in the same plane ( the ends of the shafts line up at the same height and angle when the drive is trimmed neutral)
When the rear mounts sag the front of the engine is lowered to get the alignment proper per the tool.
Now the output shaft from the engine is tipped down and the U joint end is higher than the input shaft of the drive. This works the U joint causing side loading of the gimbal bearing and coupler.

[MOBILEMERCMAN]

Steve, the more you talk the more I question your experience. When the rear mounts "sagged" you should have replaced them. 3 couplers shame on you. Quite an expensive learning curve. Was this your boat or a customers?

I hope you got it worked out now.

[Tinkerer]

So you don't agree with what I write?
What I said is true. You just must not have run into it yet.
The typical boat I work on is NOT stock. They usually have at least half again more HP than stock.


That was 20 years ago - and you didn't read what I wrote.
I said that a MERC tech. said everything was perfect. This was a high HP app.
The reason for the coupler going bad was not an alignment problem according to the MERC guy.
The reason was the rear mounts were flexing ( sagged ) this caused the coupler to go.
Replacing the mount would not have solved the problem. I learned how to set the proper rear engine height and then locked the rear of the engine in place so that IT CAN'T move.
I havn't lost a coupler since.

[Tinkerer]

I made an attachment for my alignment tool to check the rear mounts for sag.
I bet you don't have one. It wasn't hard to figure out. If the alignment tool is not at the right angle in relation to the drive it shows up instantly.

[Tinkerer]

I bet that you think that if the alignment tool slides right in smooth everything is OK.

NOT

[Tinkerer]

I can lower the front of an engine two inches and still get the alignment tool to show that everthing is perfect according to your MERC tech.

[MOBILEMERCMAN]

I find at times people create there own problems.

I am glad you figured it out.

[Ratickle]

I made an attachment for my alignment tool to check the rear mounts for sag.
I bet you don't have one. It wasn't hard to figure out. If the alignment tool is not at the right angle in relation to the drive it shows up instantly.

Can you take a pic and post it? I cannot for the life of me figure out what engine height would have to do with alignment the Merc tool would not pick up.....

The engines have trim and tilt and never run at exactly zero, in my experience, so maybe a sketch?

The OMC's used to tilt the entire engine, but had the big boot.

My TRS, and the 3A boat, is entirely different to align than the Bravo boat.

Anyway, I'm not picturing it even looking at my manuals.

[blackhawk]

The thing is with trim and steering is the shafts are always in the same plane ( the ends of the shafts line up at the same height and angle when the drive is trimmed neutral)
When the rear mounts sag the front of the engine is lowered to get the alignment proper per the tool.
Now the output shaft from the engine is tipped down and the U joint end is higher than the input shaft of the drive. This works the U joint causing side loading of the gimbal bearing and coupler.

Not sure if that was a reply to my post. I totally agree with you I was commenting about the gimbal assy and different transom angles.

[blackhawk]

Can you take a pic and post it? I cannot for the life of me figure out what engine height would have to do with alignment the Merc tool would not pick up.....

If your rear mounts sag your alignment will be off as the center of the coupler will be lower than the center of the gimbal bearing. However, if you lower the front of the motor enough the alignment bar will slide in easily since the gimbal bearing swivels. If you didn't know your rear mounts had sagged you would think it is aligned because the tool slid in easily. However, the tool went in at a slight downward angle. This angle is so small that you wouldn't notice it. At least that is how I'm interpreting it. Makes sense to me.

[TMS1155]

If your rear mounts are bad or not installed properly, including spring washers then you cant align engine properly no matter how you redesign or add pieces to the alignment tool. The mounts have to be in correctly to insure proper height of rear of engine. Then and only then can engine be aligned correctly. If you are pinning or staking rear mounts for HP setup they still have to be the same height as original mounts or it will not line up right. The alignment only aligns the coupler with the gimbal bearing on an alpha or bravo drive. The angle of the bottom, the transom or the bell housing has nothing to do with it. From the gimbal bearing to the drive the alignment changes any time you trim or turn the drive and has nothing to do with spinning the hub or splines on the coupler. When the drive goes on the U-joint shaft goes right up against the gimbal bearing and never moves between first U joint and coupler unless eng mounts are bad or loose. Hope this explains it a little better.
On the TRS drive the alignment tool with the disc that fits the bell housing is really only used to center the tail shaft of the trans to the gimbal housing while aligning eng. There is no rubber hub coupler on them and the gimbal bearing is pressed into the tailstock and shouldnt move.
No, they do not make different transom housings for different angles, at least not stock.
Ever see a drive saft on a car that was perfectly straight? U-joints are made to work at different angles.

[MOBILEMERCMAN]

Thanks Tom.

The mounts have recently been s/s from Merc. If you replace them now they are different and no longer use the spring washer. They come only in pairs and need to be changed in pairs.

[TMS1155]

The new style mounts are only out in the last few years. I was talking about the problem He was having 20 years ago. If there were no springs or bad mounts You couldnt get alignment right, Didnt matter if you had 120 hp or 800 hp on the alpha drive.

[Tinkerer]

DING DING DING - Blackhawk has it figured out. He IS correct.

TMS - You are wrong. the gimbal bearing swivels so even if the rear mounts drop 1/2 in inch you can still get the engine to align with the Merc tool by lowering the front of the motor. If you don't believe me try it for yourself.

Show me a car that uses a double U joint like a boat does...
The double U joint will allow sideways loading - a single U joint will NOT.
Sideways loading will flex the shaft in the coupler and blow the coupler.
The adapter I made is very similar to the TRS plate that goes onto the alignment tool.
It checks the alignment of the engine, gimbal bearing and the outdrive hat.
If the rear mounts are sagged the tool will not align with the hat.