Lower unit

[oxmax360]

Hi all I am about to break down an remove my lower unit, I was out and was wot and then rpm shot up and then it was dead on the water, was hoping I spun the prop, but as my luck goes lower unit gears, any how my question is, is there a trick to removing it with out pulling that water tube out by accident, I have heard that happing and you have to remove the power head 125 force is the motor I am working on, thanks

 

[Frank Acampora]

Re: Lower unit

No tricks. If the water tube is gunna come out, its gunna come out. Nothing you can do to prevent it short of drilling a big hole through the side of the midleg and holding it.

It is more common for the plastic adapter to stick to the tube and come out of the water pump, if that's any consolation. Grease it and the tube liberally before attaching the lower unit again to prevent it from happening in the future, and remove the lower unit regularly as preventive maintenance.

 

[oxmax360]

Re: Lower unit

Thanks Frank, as much as I hated to hear that, waiting on my lower unit will update when I am done,UPDATE Got it off no problem, now when the new one comes in hope I dont have any trouble putting it back together, any tips on puting it back on?

 

[oxmax360]

Re: Lower unit

Update got the lower unit on today tested it on the hose as far as shift in forward and shifted in reverse so far so good, I am however a little concerned about the fact that the lower unit came off an 85 hp and I mated it to my 125, I am almost positive it is the same maybe a small difference, any one else know if there is a difference let me know thanks.

 

[Frank Acampora]

Re: Lower unit



White is single exhaust, black is dual exhaust. Either will fit on a pre-Mercury engine.

If it is not a Mercury lower unit, Any differences do not matter. Later units had a bigger water pump which is always a plus, but the earlier water pumps were adequate to do the job. Bolt patterns, prop shaft, exhaust snout are all the same. As a matter of course, you should have replaced the water pump impeller while the lower was off the engine, that way you will have no worries

The only difference you may find is that the 85 MAY have a 2 to 1 ratio. If your 125 lower is pre 1990 or 89, it will have a 1.76 to 1 ratio. If it is post 1990 or dual exhaust, it will have around 1.85 to 1.

IF the 85 is 2 to 1, you may need to step up in prop pitch to keep engine RPM in the operating range. The positive is that you may also see an increase in top speed.

As far as mounting the lower, it is heavy so an extra set of hands will help but is not absolutely necessary. You will need to "Feel around" for the crankshaft and you may need to turn the propshaft (in gear) to align the splines.

 

[oxmax360]

Re: Lower unit

I did put a new water pump and impeller while it was off, when you said step up in prop what size do you think I may need Frank, thanks your wisdom is awesome.

 

[Frank Acampora]

Re: Lower unit



What you need is dependant upon what RPM is now, what prop you are using now, and how the hull performs with the engine/ prop combination. Very difficult to correctly answer without numbers.

I can tell you that when I swap a 2 to 1 lower unit in place of my 1.76 to 1 ratio, depending upon boat size, I go from a 17 to a 19 or a 19 to a 21. My goal is to keep engine speed at or above 5000, but except in the case of my go-fast, less than 5500.



All these boats have a 2 to 1 ratio lower unit:

The glastron with either a 90 or a 105 (really 90) turns a 19 pitch prop in the vicinity of 5000 and tops out at 45 with either engine.

The cuddy with a 140 runs about 4500-5000 with a 19 and when I take a load, I drop back to 17 pitch. It tops out at 35. Currenty I have a 150 waiting to be mounted on the transom because I think the 140 does not have sufficient torque for this boat. I plan to start with a 19 pitch on the 150 and go from there.

The g-fast uses a 21 pitch prop with either a 90 or a 125. With a racing cleaver, the 90 turned 5500 and 55 MPH the 125 turned 5900 and 60 MPH with the same prop.

 

[oxmax360]

Re: Lower unit

Thanks for your info frank, I will start with a 19 and see how it does, will update when I am done.

 

[oxmax360]

Re: Lower unit

Update: 17 pitch prop ran the best, got 5500 rpms and 35 to 36 mph on gps, new lower unit works great.