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  1. #1
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    Default Oil leak at prop shaft on 77 Merc 7.5 HP OB

    I have a 1977 (according to a SN chart i found on the web), Mercury 7.5 hp outboard.

    I had noticed that there was a bit of oil evident on the lower unit. I was going to change out the oil at any rate, and drained it,, that's when I knew I had trouble, the oil is clean, but there was not much of it.

    I removed the prop, and it appears that the prop shaft seal is leaking, as there was oil collected in the cavity that is behind the prop. And yes it is 90 weight oil, the motor has not been run in over 4 weeks. The shaft turns smoothly, and I do not feel any play in the bearings. I would like to keep it that way.

    I do not have a manual yet,, but I am wondering how much work is involved. I ordered a manual yesterday, but if anyone has some insight to this process I could sure use it.

  2. #2
    Fleet Admiral Texasmark's Avatar
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    I'd wait on the manual. I had a 7.5 of that year but never had to do anything to it.

    Usually the rear carrier assembly, which contains the seal can be pulled and not disturb anything else. Then it is a matter of driving out the seal much the same way as you would replace the outer race on a trailer's wheel bearings. The engine would be placed on the floor, or workbench, horizontally with the propshaft sticking straight up. That would keep all the parts in place that you don't need to, nor want to disturb.

    I have, in the past been able to pick the seal out of an engine and replace it without disassembling anything. It takes a little time to do it but it can be done. The secret is to break the metallic vertical part of the seal "support" and once that is done, a sharp pointed object can be carefully inserted between the housing and what's left of the seal's support which is now flat (and against the housing) and will collapse as you apply pressure. Then it's just a matter of prying it out.

    The new seal just taps in from the outside insuring that you get it installed in the correct direction....remember how the old one looked (was installed). I'd cover the propshaft splines with paper as you slide the new seal over it to keep the splines from cutting it. A little silicon spray (auto store) on everything should help.

    If you didn't damage the housing when removing the old seal then you can just wipe out the housing and tap the new one in place. If you did, then I'd put a light coating of a waterproof/oilproof cement (Permatex #2) around the outer diameter of the seal prior to tapping it into place.

    When you fill after the work, it would be best to use new gaskets on the screws and only use the correct gasket from your Merc dealer. For the larger engines they are light blue in color. Don't know about yours.

    Mark

  3. #3
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    I am pretty anal about doing things right the first time, so I will be waiting on the manual, but it does not sound like to much of a process for me to handle.

    Thanks for the speedy reply. Great information, Hopefully one day i can help someone else on the forum.

  4. #4
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    You'll either need the special tool for unscrewing the threaded gearcase cover, or very very carefully drill the cover in a few places then break it out. If you destructively remove it, take care not to damage the bearing carrier. If this motor is a "Salty Dog" it's very likely the gearcase cover isn't gonna unscrew anyway, it'll be all bound up.

    Then, with a slide hammer attached to the end of the propshaft, pull the entire proshaft/reverse gear/bearing carrier assy out of the lower unit. If it's reuctant to come out, apply a bit of heat to the outside of the gearcase and this'll expand enough to git 'er out.

    The propshaft will just slide out the back of the bearing carrier assy, leaving reverse gear/bearing assy pressed into the bearing carrier. You should be able to tap the seal out from the back side without removing reverse gear/bearing. If you can't, gently heat the O.D. of the bearing carrier and reverse gear/bearing should fall right out.

    Use care when handling the alum bearing carrier as it's quite easy to break or damage the part that the gearcase cover screws down against.

    Drive out the old seal and drive/press in the new one. Take care not to damage the propshaft roller bearing while you're driving out the old seal. A screwdriver, bent to a slight angle at the very end, makes a cheap, effective seal-driving tool. You may be able to pry it out of the front but be real careful as the bearing carrier might not be able to take the prying without breakage.

    Clean the inner I.D. of the gearcase including the threads for the gearcase cover; clean the O.D. of the bearing carrier, install a new O-ring on the bearing carrier.

    Note if the gearcase internal threads are gunked-up with salt or other hard deposits, you'll have to scrub them out with a metal-bristled 'toothbrush' and/or clean out of the threads with a mechanic's scribe/pick. Otherwise the gearcase cover won't properly thread into the gearcase. Before you reassemble it's a good idea to check and make sure the cover threads down properly. This will ensure the gearcase cover develops proper torque to the bearing carrier instead of binding on the gearcase.

    Spread a thin coating of Permatex #3 Aviation-type gasket dressing on the threads of the bearing carrier and also the internal threads in the gearcase. Carefully tap the whole mess back into place.

    Note before you reinstall be sure that the shift cam plunger hasn't fallen out of the end of the prop shaft. If it's in there loose or fell out & laying in the gearcase, install (with the pointy end out) with a dab of thick grease to hold it in place.

    Spread some Permatex #3 on the threads of the gearcase cover and screw back into place. Note if you don't use the special tool you'll have to tighten with a pin punch or equivalent and this is nowhere near as tight as it should be (60 ft-lb). Although it might work for ya, be sure to re-check after 1st test run.

    Here's a link to the special tool, Mercury P/N 91-99222T:

    http://store.eastcoastmarineservice..../91-99222t.htm

    Here's a diagram which should help:

    http://www.mercruiserparts.com/Show_...USING+ASSEMBLY

    HTH..............ed

  5. #5

    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    I had the bottom end off for an impeller change on my '76.The prop had come off easy enough.I then soaked penatrating fluid on the threads for a couple of days.I made a tool and impacted it off. Warning, the thread is LH.The bearing was a Torrington B-98.It's as easy to change as the seal.

    You should be able make a tool out of 2" tubing,if you don't have a lathe and welder.As you need an OD of 2". Sadly, pipe sizes don't work out.

  6. #6
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    OK thanks for the hint, the tool retails for $57 and change just now, I will talk to the local welding/machine shop and see what they can come up with design and price wise. They enjoy a challenge, and I spend lots with them..

  7. #7
    Fleet Admiral Texasmark's Avatar
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    Quote Originally Posted by Whitefox View Post
    OK thanks for the hint, the tool retails for $57 and change just now, I will talk to the local welding/machine shop and see what they can come up with design and price wise. They enjoy a challenge, and I spend lots with them..
    Over the years I have had a lot of challenges associated with antique farm equipment. I have a machine shop down the road that likes something different too. They enjoy seeing me come through the door, do excellent work, and charge very little for the service they perform for me.

    Mark

  8. #8
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    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    I guess I have not yet given them enough projects,, they still charging me :o

  9. #9

    Default Re: Oil leak at prop shaft on 77 Merc 7.5 HP OB

    Quote Originally Posted by emckelvy View Post
    You'll either need the special tool for unscrewing the threaded gearcase cover, or very very carefully drill the cover in a few places then break it out. If you destructively remove it, take care not to damage the bearing carrier. If this motor is a "Salty Dog" it's very likely the gearcase cover isn't gonna unscrew anyway, it'll be all bound up.

    Then, with a slide hammer attached to the end of the propshaft, pull the entire proshaft/reverse gear/bearing carrier assy out of the lower unit. If it's reuctant to come out, apply a bit of heat to the outside of the gearcase and this'll expand enough to git 'er out.

    The propshaft will just slide out the back of the bearing carrier assy, leaving reverse gear/bearing assy pressed into the bearing carrier. You should be able to tap the seal out from the back side without removing reverse gear/bearing. If you can't, gently heat the O.D. of the bearing carrier and reverse gear/bearing should fall right out.

    Use care when handling the alum bearing carrier as it's quite easy to break or damage the part that the gearcase cover screws down against.

    Drive out the old seal and drive/press in the new one. Take care not to damage the propshaft roller bearing while you're driving out the old seal. A screwdriver, bent to a slight angle at the very end, makes a cheap, effective seal-driving tool. You may be able to pry it out of the front but be real careful as the bearing carrier might not be able to take the prying without breakage.

    Clean the inner I.D. of the gearcase including the threads for the gearcase cover; clean the O.D. of the bearing carrier, install a new O-ring on the bearing carrier.

    Note if the gearcase internal threads are gunked-up with salt or other hard deposits, you'll have to scrub them out with a metal-bristled 'toothbrush' and/or clean out of the threads with a mechanic's scribe/pick. Otherwise the gearcase cover won't properly thread into the gearcase. Before you reassemble it's a good idea to check and make sure the cover threads down properly. This will ensure the gearcase cover develops proper torque to the bearing carrier instead of binding on the gearcase.

    Spread a thin coating of Permatex #3 Aviation-type gasket dressing on the threads of the bearing carrier and also the internal threads in the gearcase. Carefully tap the whole mess back into place.

    Note before you reinstall be sure that the shift cam plunger hasn't fallen out of the end of the prop shaft. If it's in there loose or fell out & laying in the gearcase, install (with the pointy end out) with a dab of thick grease to hold it in place.

    Spread some Permatex #3 on the threads of the gearcase cover and screw back into place. Note if you don't use the special tool you'll have to tighten with a pin punch or equivalent and this is nowhere near as tight as it should be (60 ft-lb). Although it might work for ya, be sure to re-check after 1st test run.

    Here's a link to the special tool, Mercury P/N 91-99222T:

    http://store.eastcoastmarineservice..../91-99222t.htm

    Here's a diagram which should help:

    http://www.mercruiserparts.com/Show_...USING+ASSEMBLY

    HTH..............ed
    I have the same motor it sounds nlike. the seriel number is 3280414 and I cant find anywhere on the net to tell me what year it is. also have the same leaking and I know it is coming out of the shaft but there is an aluminum part in front of the seal that wont move. Any guidence would be helpful.

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