Replacing lower unit drive shaft Alpha gen 1

[500dollar744ti]

So I had a driveshaft strip out on my lower unit. How do I go about replacing the driveshaft with one from a parts lower unit?

 

[500dollar744ti]

Nevermind, I found some instructions in the manual via the top of this sub-forum, however this looks like it will be a pain in the butt!

 

[Rick Stephens]

It is painstaking work. If you don't get it right you will wear it out in no time.

 

[Bondo]

Ayuh,...... It's an involved process, 'n requires a few specialized tools,....

 

[500dollar744ti]

Here is my stipped driveshaft.


I pulled the driveshaft out of this busted lower unit only to find that it had good splines but the thrust surface had been worn down by the roller bearings rendering it useless. I also messed up the bearing carrier, locking ring and a few other things getting it out.



I'm doing this without the special tools mind you, so you can imagine the pain of disassembly. After I found the driveshaft in the other busted lower unit was useless, I only had one more busted lower unit to take apart hoping to salvage a good driveshaft. Since both of the ones I pulled the shafts out of had busted cases, I figured I would get my sawzall out and cut the case apart so I can get an in-tact locking ring, bearing carrier and other misc items.



I was able to extract a good driveshaft from the third busted lower unit and press the good bearings onto everything. Now I have a functioning lower unit put together from other broken units. I was able to take three bad ones and make one good one. It seems to work properly and doesn't make any abnormal noise on the boat.


And finally, I am left with some scrap metal.

 

[achris]

Did you check and adjust the pinion gear height? Did you check and adjust the gear backlashes? Did you use new seals throughout? Have you pressure and vac tested it?

And you need to remove that 'o' ring off the top of the drive-shaft.

Chris......

 

[500dollar744ti]

I did vacuum and pressure test it. I admittedly did not adjust the backlash and pinion height. I measured those before and installed everything using the same measurements. I was banking on the fact that it had been set up correctly in the first place.

 

[achris]

I did vacuum and pressure test it. I admittedly did not adjust the backlash and pinion height. I measured those before and installed everything using the same measurements. I was banking on the fact that it had been set up correctly in the first place.

You moved a drive shaft from one housing to another... Needs to be set up all over again. The heights and backlashes are set to the particular housing and set of gears.. Each housing is slightly different, hence the set up procedure. (gear and bearing manufacturing tolerances are much tighter and usually, but not always, don't need the shims changed) ....

Chris.......

 

[JustJason]

Agree with Chris, any time a shaft gear or bearing is changed out the backlash needs to be checked and the sterndrive needs to be re-shimmed. If it is neglected.... You might get a day, you might get a year, but it is only a matter of time before the gears go out and you are back to square one.

 

[Bondo]

You might get a day, you might get a year, but it is only a matter of time before the gears go out and you are back to square one.

Ayuh,.... As much site mechanicin' I've done, I gotta agree,....

Ya might get one more lap out of it(My job), but it needs to be rebuilt Right to last,...

 

[500dollar744ti]

Well I'll run it and see how it does. I was cleaning house and originally planning to scrap all three broken lower units. I purchased a replacement but thought if I could possibly make a good one out of the three bad ones, I might get some time before I have to install the replacement.

One of my issues I'm having is that lower units are failing on a regular basis. I have everything I need to fix a problem on the water but the lower unit breaking is the one thing that leaves me stranded. I have the BoatUS unlimited towing and that has been worth its weight in gold but it's even annoying waiting for them to come haul me to shore. Since I've had this experience, I will not be shocked if this one also blows up in short order.

I'm setting up a 15hp kicker motor so I have some other option to move the boat when the lower unit fails. Then I keep a spare lower unit at home.

How about the SEI lower units? They are guaranteed for 3 years but if one fails, does the replacement get a 3 year guarantee from the time of installation or only from the time when the first one was purchased.

For example, let's say I purchase the SEI lower unit in 2015, it is guaranteed for 3 years. In 2017, the lower unit fails and I make a warranty claim to replace it. Does the replacement have 3 more years of warranty coverage until 2020 or is it only guaranteed until 2018?

 

[Bondo]

Does the replacement have 3 more years of warranty coverage

Nope,.... From the date of the original sale,.....

 

[achris]

...One of my issues I'm having is that lower units are failing on a regular basis...

What engine you have in front of your drive? High horsepower/torque engines tend to push Alphas to the edge. If you don't have a big engine, are you doing 'failure analysis' on the dead lowers? What is the cause? Are you using Mercs' High Performance Gear Lube? If not, what are you using? When you rebuild, are you following ALL the setup procedures, and have the correct FACTORY tools to do the job properly? If you say NO to any of these questions, that could easily explain your going though drives.... I've rebuilt MANY drives, very very few have any problems, but I have a few thousand dollars worth of special tools with which to do the work....

Chris.....

 

[500dollar744ti]

It is a 350 Magnum rated at 270hp, it runs really well and the boat goes 51mph at 4400rpm, I think it may be a lot of torque on the drive and a heavy boat causing the issue.

Yes, I use the $15/quart Merc high performance gear lube. I pressure and vacuum test every time I have the drive apart. I have never rebuilt one of these drives. This is my first time taking one apart and it was only to replace the driveshaft.

All the other broken lower units have been bought as "rebuilt" units, or recently, I just buy a used unit, preferably from a 4-cyl.

One of the drives that broke was an SEI complete unit that I purchased second-hand. The lower unit broke and the drive was 1 year out of the warranty period. The upper unit survived and I am still using that as of today.

I find that if I buy a "fully rebuilt" lower unit for a lot of money, it seems to last just as long a corroded, junky looking lower unit with half a skeg missing and 2000 hours on it that cost me next to nothing.

 

[achris]

If you have a 'heavy hand' with that engine, consider going to a Bravo set up....

Is 4400 max revs at WOT? If so, consider going to the next prop down (2" LESS pitch)... That will make your engine run a bit freer and reduce the torque loading on the drive...

Chris....

 

[500dollar744ti]

I do have a 'heavy hand' but sometimes when fully loaded for a long trip, I really need to get on the throttle to plane out. I added bennett trim tabs which helped tremendously but still need to full throttle when pulling up skiers with a boat full of people.

I've been keeping my eye out for a good deal on a complete Bravo 1 setup but have not come across anything I can afford yet.

4400rpm is max revs fully loaded, it may creep to 4500rpm with just me on the boat. My engine cover shows the correct WOT revs to be 4200-4600, I'm running a Solas stainless steel 19p propeller. I think a 17p would over-rev.

 

[achris]

... I think a 17p would over-rev.

Only if you allow it to. Just start pulling back a little when you hit 4800.... And if you're pulling skiers, the book (written by Merc) recommends you prop down anyway....

Chris......

 

[500dollar744ti]

The conclusion is that it didn't last but a few hours, pretty much what you guys predicted. I'm going to put my replacement lower unit on. Just thought I'd let you know the outcome.

Also, what is the reason for not using the o-ring on the lower unit driveshaft?

 

[achris]

.....Also, what is the reason for not using the o-ring on the lower unit driveshaft?

It has a tendency to hold the lower seal open (top housing) and allow water into the splines in the bottom of the upper drive shaft. Seen it plenty of times. Rusted out splines = new upper drive shaft + the full setup of the top housing = expensive....

Chris.....
Note: Later model lower drive shafts and replacement drive shafts don't even have the 'O' ring groove. (Merc service manual #6, section 3, part B, page 50)

 

[500dollar744ti]

Ok Chris I have a brainbuster for you, if you'll give me your opinion...

I took my lower unit off today, the one that I swapped out the driveshaft on, and found out that it is still functional, no broken parts I mean. I pulled the upper unit and took the top cover off to find the gears completely stripped and part of the shaft broken.

The lower unit driveshaft also looks like it was on the verge of stripping out again. Here's the pics.



Do you think this was caused by a botched attempt to fix my lower unit with another driveshaft or could there have been a problem in the upper that caused the first driveshaft to strip, then almost strip the second driveshaft?

i checked engine alignment to see if there could possibly be some issue there and it's perfect, I can slip the bar in with one finger.

Thoughts anyone?