1958 Merc Mark 55 lower unit removal

[JIM MCCARTHY]

I recently acquired a Merc Mark 55, probably a 1958 or thereabouts, and need to remove the lower unit. I have it unbolted and have the reverse lock disconnected, but the lower unit will not drop down more than about 1/2". From the diagrams I have found on the internet, it apprears the shift linkage is splined about midway and should separate on removal. I am loathe to take the "get a bigger hammer" approach and wonder what I am missing. Does anyone know what the problem might be?

 

[Chinewalker]

Re: 1958 Merc Mark 55 lower unit removal

There are only four bolts/studs holding it in place. One on each side fore, one under the AV plate above the prop, and an Allen bolt hidden up inside the exhaust outlet. If you get those four off and the unit doesn't drop, then something is stuck. That can only be the water tube (least likely), driveshaft (mostl likely) or shift rod (rarely). If a previous servicing didn't end with the driveshaft splines greased and a new o-ring installed, then the crank may have become one with the driveshaft. I've never had that problem with the shift rod as it is moved both directions in operation and that tends to keep it freed up. The water tube can become stuck in the grommet in the waterpump, but that isn't generally all that hard to undo.

Wooden wedges are sometimes handy for working things apart if you're certain all the fasteners are out...

 

[JIM MCCARTHY]

Re: 1958 Merc Mark 55 lower unit removal

Thanks for the help. I was able to get it loose without breaking anything. What prompted the removal is a shift problem that I suspect I am going to become intimately familiar with. The lower unit is locked in forward gear and rotating the shift rod will not bring it out of gear or into reverse. I'm guessing it's got to come apart and that the shift cam is going to be the problem. I would appreciate your thoughts.

 

[kawiski]

Re: 1958 Merc Mark 55 lower unit removal

I was able to get it loose without breaking anything.

So how did you do it?
I am having the same problem with my 2004 Mercury EFI 4-stroke 115HP O/B lower unit. I have the factory service manual and followed the exact direction, but the lower unit would not budge. I bought this boat new in 2004 and am trying to do my first water pump impeller change (over due!)... Any help?

 

[JIM MCCARTHY]

Re: 1958 Merc Mark 55 lower unit removal

I used the plastic shims you can get at Lowe's or Home Depot and placed them at the front and rear perpindicular to the lower unit housing between the exhaust housing and the lower unit housing. Two on each end. You place the thin ends facing each other so that when you drive them together it separates the housings evenly without twisting them one way or another. this assumes that your lower unit drops down enough to get the shims in. Piece of cake.

 

[coolguy147]

Re: 1958 Merc Mark 55 lower unit removal

make sure you have a good grip when it drops. when it drops it drops. becareful you dont strike the bottom cause i almost did