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Thread: Rear. Main. Seal.

  1. #1
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    Rear. Main. Seal.

    1988 stock 5.0L
    Always dripped down the back, from the bottom of the bellhousing. Replaced the rear main a few years ago, started leaking right after putting everything back together. ZR and Zippy replaced the intake gaskets last year, also got a new PCV valve, screen and grommet. Valve covers are dry too. I've double checked the oil pan gasket and it seems dry also.

    Talked about this in a thread last year and loved RedSN's comment about how even Ford considers a rear main leak as 'normal'
    http://www.torontomustangclub.ca/for...-Intake-Gasket

    I'm ready to do it again and want all the insurance I can get to eliminate this leak. From my research this is what I've come up with:

    - fel-pro crank repair sleeve 16250 (didn't know about these last time)
    - green loctite between this and the crank
    - fel-pro seal 40620 (or similar: Ford Racing M-6701-B302)
    - fel-pro installation tool 16300
    - grease (dialectric?) on inside of seal spring/groove
    - RTV (of some kind) on outside of seal

    Anything you think I could be missing?
    Last edited by m_owing; 09-29-2021 at 09:23 AM.

  2. #2
    Club Supporter hammerhead's Avatar
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    maybe oil pan seal...?
    1979 Pace Car 302 4spd
    1981 Cobra t-top option - power to be determined, in the works

  3. #3
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    please document the job and update later with result. I have a fairly fresh motor on a stand waiting for install and this prompted me to consider doing the seal before dropping it in the hole.

    I know you've done the PCV, but years ago I was certain I had a real main leak and it was the PCV not properly seated.

  4. #4
    Admin ZR's Avatar
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    If leak is between crank and seal itself, yes opt for the Fel Pro repair sleeve making sure to use lock tite.
    If leak is between block and seal, replace seal only. They make a standard and premium seals, neither have proven 100% reliable. When doing clutches, if seal is dry we've opted to leave as is and not replace seal.
    Back side of the seal, I use white lithium grease to fill the cavity full to prevent spring from flinging out. Oddly enough just had a pick up in for axle seals, previously installer did not use grease, spring had popped out and did damage to the bearing / axle face when it became lodged in outter edge of the bearing race.
    Not sure why Ford did not make the rear main seal more like an axle seal with a steel lip so it's for sure in perfectly straight.
    When removing old seal, be x x x tra careful not to damage / mark the crank itself, very common to see when we get a persistent leaker in someone else has had their hands on.

  5. #5
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    Quote Originally Posted by ChickenLips View Post
    please document the job and update later with result. I have a fairly fresh motor on a stand waiting for install and this prompted me to consider doing the seal before dropping it in the hole.
    I know you've done the PCV, but years ago I was certain I had a real main leak and it was the PCV not properly seated.
    Will update. I'll also check the PCV again

    Quote Originally Posted by hammerhead View Post
    maybe oil pan seal...?
    Updated my first post. I've checked the oil pan many times and it seems clean and dry.


    Quote Originally Posted by ZR View Post
    If leak is between crank and seal itself, yes opt for the Fel Pro repair sleeve making sure to use lock tite.
    If leak is between block and seal, replace seal only. They make a standard and premium seals, neither have proven 100% reliable. When doing clutches, if seal is dry we've opted to leave as is and not replace seal.
    Back side of the seal, I use white lithium grease to fill the cavity full to prevent spring from flinging out. Oddly enough just had a pick up in for axle seals, previously installer did not use grease, spring had popped out and did damage to the bearing / axle face when it became lodged in outter edge of the bearing race.
    Not sure why Ford did not make the rear main seal more like an axle seal with a steel lip so it's for sure in perfectly straight.
    When removing old seal, be x x x tra careful not to damage / mark the crank itself, very common to see when we get a persistent leaker in someone else has had their hands on.
    When everything is apart I'll look to see where the leak is coming from. I'll buy everything and decide when it's time. Do you put any RTV or silicone anywhere?
    How does grease prevent the spring from moving?

  6. #6
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    My rear main likes to leak in colder weather.
    Warmer weather usually nothing.
    Same seal since 92 when Ford rebuilt the engine under warranty.

    I keep the PCV valve and screen clean.
    Not going to touch it as it's probably a expensive job.
    Might even leak more if some mechanic doesn't do the job right,lol.

  7. #7
    Admin ZR's Avatar
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    Quote Originally Posted by m_owing View Post
    Will update. I'll also check the PCV again



    Updated my first post. I've checked the oil pan many times and it seems clean and dry.




    When everything is apart I'll look to see where the leak is coming from. I'll buy everything and decide when it's time. Do you put any RTV or silicone anywhere?
    How does grease prevent the spring from moving?
    Back of the seal packed with grease prevents the spring from coming dislodged. Same technique with axle seals etc.
    I install as designed, dry n clean on outter edge.

  8. #8
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    Quote Originally Posted by LX89 View Post
    Not going to touch it as it's probably a expensive job.
    Might even leak more if some mechanic doesn't do the job right,lol.
    When driving the oil either drips on the exhaust or the exhaust is close enough that it burns it up and stinks. That and I'm growing weary of always having to make sure there's cardboard wherever I park. By the end of the season I usually have to add almost a quart.

    Quote Originally Posted by ZR View Post
    Back of the seal packed with grease prevents the spring from coming dislodged. Same technique with axle seals etc.
    I install as designed, dry n clean on outter edge.
    I get that's what it does, I'm curious how it physically does it. Does the grease fill in the space so the spring can't move, or create some kind of hydraulic pressure?

    Thanks again everyone.

  9. #9
    Club Supporter hammerhead's Avatar
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    spring will pop if the seal isn't installed or pressed in evenly thru the install - too much twist can cause the spring to pop also...
    1979 Pace Car 302 4spd
    1981 Cobra t-top option - power to be determined, in the works

  10. #10
    Club Supporter hammerhead's Avatar
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    one other thing to consider as well is crank case pressure can push oil out of weak areas as all mentioned above
    1979 Pace Car 302 4spd
    1981 Cobra t-top option - power to be determined, in the works

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