Conference vicki::boats

    You are experiencing electrolysis if your problem is the same as I had
    on a 1978 Mercruiser I/O. I was losing a piston a year on a twin
    installation. What made them last much longer was to put a shaft zinc
    on the stainless steel shaft that came out of the piston body. There
    was enough of the shaft sticking out when in full down position to fit
    on a zinc. I put one zinc on each shaft, that is two per outdrive. Then
    leave the drive in the water so the zincs can work. In fact, now that I
    think of it, if you leave the drive tilted up, you are leaving your
    sacrificial zinc out of the water since it is the trim tab and it will
    be out of the water in the up position.

    Part of the electrolysis problem is caused by the rams in the
    trim cylinders not being (electrically) connected to anything.
    There is a service bulletin for older outdrives that involves
    fitting a stainless steel coil spring around the rubber
    bushing at the aft end of the trim cylinder.  This creates
    an electrical connection between the cylinders and the drive
    units.

    I can probably find the service bulletin if you need it...

    There should be zincs on the lower two transom mounting
    bolts.

    	- Lee

    The hydraulic lines to the pistons are stainless steel.  Shouldn't this
    provide grounding to the outdrive?  There are anodes (bolt cap type) on
    each side of the drive.  These anodes disintegrate in about two months. 
    I recall seeing the springs between the rubber bushings.  There a pain
    to install.  
    
    Are the newer pistons made of the same material?  Has anyone ever made
    the "conversion"?
    

    re .2

    <<There is a service bulletin for older outdrives that involves>>
    <<fitting a stainless steel coil spring around the rubber      >>
    <<bushing at the aft end of the trim cylinder.  	           >>

    In removing my outdrive lift cylinders in the past I saw this stainless
    spring and wondered what it was for....  I suspected grounding.  Now I
    know. 

    I also believe that there are sacrificial zincs over the heads of
    bottom bolts where the outdrive is bolted through the transom.  
    The person who had the boat before me had painted the outdrive, and the
    zincs!!  Probably don't work too good when painted.

    Matt
                          

Whats Corroding? Is it the piston shaft or the outer
casing? If it's the shaft, use the zinc clamp-ons like
you would see on a prop shaft on a conventional inboard.
If it's the casing, I suspect your problem is not
replacing the zinc bolt caps that are corroding away
every two months. (What about the trim tab?? It should
be zinc as well.)

Don

    The shafts are fine.  The casings are the problem.  Trim tab is fine,
    but its out of the water when outdrive is raised while sitting on
    mooring.  Should drive be left in the down position?  I've always left
    it up when the boat is not in use. 

    
    
    
       I've had the same problem for the past few years. Each year
    I would replace at least one powertrim piston. At the end of last
    season I again had one piston that leaked fluid and with the strict
    envrironmental rules, this is a real problem. This can be a finable 
    offense.
    
       When I checked the other piston jacked I found that it had a deep
    pit that would surely leak shortly after being exposed to salt water.
    I ended up replacing 2 pistons at a $100 a crack per rebuild piston. The
    owner at the marina where I bought the pistons suggested I install a
    Mercathode system. At $80 this will pay for itself in less than a year.
    The system is supposed to equalize the electrical charge surrounding
    the boat.
    
       Has anyone else installed one of these systems? What kind of results
    did you have?  Here's hoping it works.
    
    
    
    
                                               

Re. .6, as hinted at in other replies if the zincs are to do their work, they
must be in the water. The drive must be down! The trim tab should only be "fine"
on the day it is installed. It should start sacrificing itself as soon as it is
dunked in the salt water. If it is in good, fine conidition, it is not doing 
its job.

I knew I had a problem last year when my trim tab was ok, but my prop started to
show signs for electrolysis.

john

    I spoke to Ward Marine and discovered that there is no after market
    manufacturer of Mercruiser lift pistons!  Apparently Merc has it tied
    up with patents.  Anyone know where you can buy rebuilts?

    
    Joe:
     To prevent elrolytic corrosion on my OMC, there is a large zinc "egg
    crate" anode mounted on the hull and electrically connected to the 
    negative battery terminal. IT works extremely well. So well, that I
    have had to double them up as a single one dissolves by mid season
    THe electrolysis between your copper bottom paint and your aluminum in
    your outdrive are obvioudly causing the damage. 
    
     The mercathode system works, but it requires drilling two holes in
    your hull. THat meaens hauling the boat out, etc.
    
     I'd pick up a large zinc (lots of surface area), mount it near your
    outdrive (S.S. Wood screws) and run a S.S. Wire from the anode to the 
    Outdrive. Also make sure your cylinder housing and piston are
    electrically connected to your outdrive and engine.
    
     If we could still get the TBT based bottom paints, this problem would
    be minimized. Copper + Aluminum + Salt Water = Big Time Electrolysis
    
     Dick
    
    

    My merc manual has detailed instructions for rebuilding cylinders, says
    rebuilding kits are available for alot less money than new cylinders.
    The process aint all that complicated (what is, in a book?)
    
    Now, I have to admit that I havent yet had to do one, but from what I
    see I wouldnt hesitate to give it a try.

    Where do you get the kits?  Did you buy a service manual from Merc? 
    Are there any special tools involved?  Thanks for your help.