Engine 302 Boss Block 2 Piece Rear Main Seal--sealing Problem

Discussion in 'Fox 5.0 Mustang Tech' started by brienjohnson200, Jun 16, 2013.


  1. brienjohnson200

    brienjohnson200 New Member

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    I have a early 70's style boss block, YSI Blower (700hp) in a Mustang which has a two piece rear main seal. I have replaced the seal twice and it leaked AGAIN!!!! I am using a Fel Pro BS 30136 Seal. The old seal was just basically weeping before, but I wanted to replace it at the same time as the clutch and now can't get it to seal at all. Does anyone have any ideas hot to get this stupid thing to seal up
    [​IMG] [​IMG] [​IMG]
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  2. Illuminator

    Illuminator Active Member

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    Ahh...I remember those, trying to remember how is not so easy...been over 20 yrs... I'll ask a few old friends and get back ..
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  3. Kumm3

    Kumm3 Founding Member

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    I had the same problem with my 70 Boss block. Never got it figured out though, switched to a Dart block instead.
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  4. Gearbanger 101

    Gearbanger 101 we love us some gays up here Super Mod

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    It's been years since I've seen one with a two piece main seal as well. lol. If I recall, I used to install them with one end of the seal sticking out slightly further than the other and gave the ends a little dab of RTV where they meet the edge of the block. After it starts to set, give the seal a liberal coating of oil. Make sure the small lip on the outer seal is facing the inside of the block as well. A lot of guys put them in backwards.

    Found this pic with a quick google search as a reference of how the seal needs to go in....

    [​IMG]
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  5. brienjohnson200

    brienjohnson200 New Member

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    I used gease instead of oil on the seal. I put a dab of rtv just where the 2 halfs of the seal come together. Should I also put rtv on the cap itself where it comes together with the block? Everything else was done right
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  6. mikestang63

    mikestang63 Mustang Master

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    Good Advice. That's what we used to do at the dealer back in the day. Stagger the ends about a 1/4 inch and put RTV on the ends. Make sure the channel is clean and the seal is greased up or you will wipe it out on startup.

    The blocks from the 6o's use to have a locating pin for the old rope type seals we used to install, you had that you remove that pin or you will tear up the new type seal, and put a dab of RTV in the hole for the pin.
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  7. Gearbanger 101

    Gearbanger 101 we love us some gays up here Super Mod

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    I would. RTV is pretty much good at any point where two irregular metal parts meet up. You don't need a lot, just a couple of dabs will do.
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  8. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    Correct if the pin is still there make sure you grind it down. I also always stagger at least 1/4" and I use "the right stuff" rtv on the ends. Make sure it doesn't rotate as you install the cap.

    I actually find those 2 piece seals tend to leak less. Make sure your not pressurizing the crank case and blowing the seal if the motor isn't vented enough.

    Sent from my XT907 using Tapatalk 2
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  9. brienjohnson200

    brienjohnson200 New Member

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    Motor shouldn't be pressurized. One beather in either valve cover and a vaccum pump.
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  10. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    How much vacuum are you pulling?

    With a vacuum pump installed you don't want to run breathers in the valve covers typically your run to a breather tank off the vacuum pump and then run a relief valve in the valve cover or down below at the vacuum pump depending on your plumbing

    Sent from my XT907 using Tapatalk 2
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