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Steve Sr.
Steve Sr., 20 yrs -Mechanic/Manager thru 2003
Category: Boat
Satisfied Customers: 79
Experience:  20 total yrs in service outboards and sterndrives - certs in Chrysler outboards only -
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I just replaced the impeller and water pump on a 7.5 Mercury

Customer Question

I just replaced the impeller and water pump on a 7.5 Mercury outboard motor (1975).
I am having a hard time putting the lower unit back up into the motor tail housing.
I have tried to move the gear shift handle forward/neutral/reverse.....but I can't seem to push the lower unit back up in place.....any advice?
Thanks, Customer
Submitted: 8 years ago.
Category: Boat
Expert:  Steve Sr. replied 8 years ago.
Hello ,

Those puppies can be a little tricky.

What you have to do is, take a trial stab, forget about the water tube, and shift shaft(sort of)

all we are going to do this time, its practice to get a feel for the Crankshaft by way of the Driveshaft, and what its going to feel like when it slips into place.

FIRST, get you a flash light, and look up inside the motor leg and get a good look at how everything is laid out and what / how its might feel when your drive shaft is up against it, which way it can slide / move etc, also make sure no old seal, grommet, etc is blocking the drive shaft from where it is going...

Next, slowly stab her up in there, and keep move the drive shaft up until it starts touching and you can't move it any further up into the motor, now just light pressure, sort of feel / lightly drag the drive shaft around up there and try to feel when it starts to hit the splines of the crank.

You should have already removed the spark plugs, have the engine cowling off, and be able to use one hand to turn the engine in a clockwise rotation every time you think the drive shaft is close or aligned.

Once it is in the splines you will know by turning the flywheel and it will try to turn the drive shaft when its lined up and slipping in.

Ok, that was the easy part. This next time we are going to do the same thing, while making sure that our water tube and shaft shaft is aligned.

Make sure that you apply just a little oil or lightly grease the water tube grommet so it will slip in easily.

Make sure that both the engine, and the lower unit are shifter into forward gear.

Note:(may not apply to this 7.5) Most of mercs clutches are beveled in a way as to, if you turn the prop while in fwd gear, and it turns, with out locking, you will roll the shift shaft out of forward gear, and into neutral, and if you turn the prop and it does lock, your turning the drive shaft backwards.

You don't want to do that, you could flip one of the impeller blades over backwards, and if you do, it won't pump properly, an din the smaller engines, even more so.

So the idea is to place lower unit into fwd gear, turn prop once or so to see what I am talking about.

What your going to do is apply a slight pressure on the prop in backwards rotation while you start to stab the drive shaft, don't actually turn the prop, just hold slight pressure against it.

Now keeping the lower unit at the proper angle, start to feel for crankshaft again, keeping lower unit in best angle as possible as to where it will be once bolted up.

Keep that light pressure on the prop /propshaft / while feeling around with driveshaft.

Once you think that you may have her lined up, turn lose of the prop, and use that hand to turn the flywheel (CLOCKWISE), else you know what will happen.

In the event you still are not lined up, place your hand back on the prop with light pressure backward(COUNTER CLOCKWISE), and start feeling around again, until you get the driveshaft started into the crankshaft.

Hopefully, your a patient man :) Now, that you have the driveshaft started /aligned into the crank. We want to make sure that our water tube is aligned, if not, we may need to let drive shaft slip down enough to re-align the water tube. This will depend on the length and curve of your water tube, it actually should be aligned before the driveshaft slips in, but with age and service they can get bent, squished, pushed, pulled, etc.. so look at ti closely when you first slide the lower unit up in the motor leg.

Also you will want to have a bolt close by so once everything is aligned, that it will be in reach.

Now, we have driveshaft, water tube both aligned, hopefully, your engine is equipped with a plastic reverse lock cam that fits up onto the shaft shaft and will pretty well keep it in alignment all the while your doing the driveshaft stab.

Now as you slip the lower unit up into the leg further, you may need to wiggle the shift shaft just a tad to make sure that the splines align as well.

Hopefully, I haven't left anything out, and your fixing to screw a bolt in-through the lower unit housing and up into the motor leg , securing the unit in place. Go ahead and get a couple of bolts started.

Next go ahead and grab your flash light, inspect everything the best you can to see that water tube is lined up.

Next , try to shift the engine, BEFORE you tighten everything up. When shifting, turn the engine's flywheel in a clockwise rotation.

If water tube is aligned, and the engine now shifts, proceed to replace all bolts and secure them.

Next, give her a tank run to make sure your pumping water, and then Off to the lake you go!

Hope this has helped.
Steve Sr.
Steve Sr. and other Boat Specialists are ready to help you
Customer: replied 8 years ago.
Hey Steve,
Thanks a mil for all your guidance. Actually yesterday after many hours of frustration, I managed to bring the unit up and the shaft in except that when I gave it a try, the shifting was all off i.e. shifting forward was neutral, neutral was forward etc....
Also just a dribble of water came out through the water outlet but then nothing. I think I have to take it out again, look at the impeller again and realign the shift rod as you indicate above. Do you have any advice as to how the impeller blades should look like once in its place? blades going clockwise or counter-clockwise? etc...
Thanks again,Customer
Expert:  Steve Sr. replied 8 years ago.
Good dealCustomer

Sure, the correct way to insure the impeller blades are facing the right direction would be to lift the impeller housing of of your impeller and take a wee bit of oil or petroleum jelly lightly coat the impeller housing inside.

you should already have impeller in place and laying on baseplate.(make sure key is in key way on shaft and impeller)

Slide housing down drives haft, and let it rest on top of the impeller blades.

Next, grab up a red rag,(shop rag,any rag) and wrap it around the top of the drives haft, the rag is so that you can have a good grip.

Now turn the drives haft clockwise and at the same time push down on the impeller housing, forcing the housing down over the blades, the blades will bend and slip up into the housing.

This will face the blades in the correct direction, and is by far the easiest way to install impellers.

Reinstall fasteners and tighten.

Thats about all there is to that part. Hope that helps.

PS: yea, never turn that drives haft backwards after you install the impeller, the impeller blades will flip backwards if you do, and for whatever reason, they sometimes will stay that way, forever, laugh, go figure...then she won't pump :)

Steve Sr.

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