Can 1955 mark 25 lower drive unit fit 1956 mark 25

[dcantrell]

Hello I am working on a 1956 mercury mark 25 and what I can tell I believe it is a 20 hp motor. The lower drive unit needs replaced. I have a lead on a 1955 mark 25 lower drive unit which what I can tell that year the mark 25 was a 18 hp motor the drive shafts seem to be the same length on both 55 and 56. With all of this being said my question is are the 1955 and 1956 lower drive units interchangeable even though they were different hp any knowledge or advice would greatly apprieacated. One other question what fuel oil ratio should I be using in this 1956 mark 25 motor I have a 1997 mercury 15 hp that I run mercury oil at 50:1 ratio would the 50:1 ratio be safe to run in the 1956 mark 25 motor thanks for your time.

[dave-bernard]

the only difference that may stop you from installing it is the shift shaft seal bushing. But that can be moved from one lower to the other without a problem. other than that it will fit.

[dcantrell]

Thanks for the reply.

[20mercman]

If you wish to install a 1955 lower unit on a 56 and later tower, you will have to either change the shift shaft bushing/holder, (it threads into the lower unit and keeps the shift shaft in place and seals the shaft) The diameter of the bushing is larger in 56 and later. It will also have smaller hole in the housing for the rear bolt as they switched from a 5/16" stud to a 3/8" stud. You will either need a very thin walled socket or need to enlarge the relief area around the hole where the stud goes through to get a socket on and of course drill out the hole in the lower unit to fit through a 3/8" stud.

Steve

[1946zephyr]

I would run 24:1 on that.

Would the innards work from one, to the other? That may be a good time to re-seal the lower and check all the gears out, anyway. Plus you won’t have to do any mods to the lower unit housing. You do have lower the lower to yours, correct?

[dcantrell]

Yes I do have lower unit from my motor I have overlooked the parts and most of them are in good shape. I thank those of you that replied to my question I will be getting 1955 lower unit and transfer parts or modify them to fit. When I get motor back running I will run 24:1 thanks again to those that responded comments and answers have been very helpful.

[jw-in-dixie]

Why 24:1? Isn’t that a 50:1 motor? Just curious. Thanks.

[20mercman]

As for oil in the Mk-25’s, These are all full roller bearing motors, and Mercury years ago stated that they can be run on 50:1 ratio with no harm. I use 48:1 usually, easier math, but the idea is the same. There is no major difference in the internals of this motor than the 1981 Mercury 20hp that ended the line for this fine family of motors. Too rich of a mixture can cause loading up at idle and poor idle quality. Please, not trying to open a can of worms on an oil debate. I use 24:1 on after a rebuild and break-in, but after that, they don’t need the rich mixtures.

Steve

[1946zephyr]

Well, larger OMC’s are fully jeweled like the Mercs, so why would they recemend 24:1 for those but yet the Mercs can run 50:1?

[dave-bernard]

as I remember during the change over period some omc’s were "full jeweled" except for the wrist pins. I may be wrong I know more bout Merc’s than OMC’s

[Author]

Posts