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Thread: Rear Main Seal - Help - Viton vs Fluoroelastomer - Sealant or Dry

  1. #1
    Senior Member CDXXVII's Avatar
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    Rear Main Seal - Help - Viton vs Fluoroelastomer - Sealant or Dry

    "Here is sort of short the story." I have been chasing a leak. Curious that I have found three for certain and have yet to find the forth.

    Valve Cover gaskets are now new and dry. They were replaced with the blue silicone gaskets and have sealed great.

    Oil Pan Replaced by Ford Racing. Found that one of the welds in the sump was compromised and leaking. Oil Pan is now new and dry.

    Transmission upgraded by Liberty's Gears and the leak was address with their proprietary threaded lower plug. Transmission is now dry.

    The leak appears to be coming from the Bellhousing between it and the "Flex Plate" (pretty sure that is not the correct term.)

    I am not happy with my McLeod Twin Disc Clutch. I have learned to drive it "properly" and do not have much of a problem with chatter at all when driving. Backing up is a complete other story. There is no good way to feather the clutch and backing in is no fun. Given that I am out and about most weekends and always back in to parking spots the chatter has become very annoying.

    I am planning to change out the clutch in a few months and while I'm at it I am going to go ahead and replace the rear main seal. The engine is a Ford Racing Z427 with only 6000 miles. "351 Windsor Block"

    I am looking at a couple of Rear Main Seals and trying to be certain to pick the correct one.


    Ford Performance M-6701-B351 ( Viton ) Direct replacement ( Failed )

    Fel-Pro 2921 ( Fluoroelastomer )

    I can not find good info. on install. I was thinking of using a sealant between the seal and the block and dry between the seal and the crank.


    Any help and guidance appreciated.
    F5R #7841: Anniversary Edition MK4, Ford Racing 427, Edelbrock EFI, Gas-N Pipes, Stainless Headers, TKO600, 3.31 Moser 3-Link, 17" Halibrands

  2. #2
    i.e.427's Avatar
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    It may not be the rear main seal. Most 302 crankshafts have though holes for the flywheel bolts. Confirm that threadlocker was used on the flywheel bolts as thread sealant. You may be getting the oil on the flywheel and may have already contaminated the clutch disc. Maybe that is where the chatter is coming from?

    Frank

    Sorry guys you are correct. Brain thinking 302, hands typing 351.

    Corrected.
    Last edited by i.e.427; 07-10-2019 at 08:49 PM. Reason: brain fart
    i.e.427 is located in the Southern California Inland Empire. We help Factory Five customers and builders realize their dreams by providing assembly, fabrication and maintenance for their Factory Five projects.

  3. #3
    Senior Member CraigS's Avatar
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    I think 302s have holes all the way through but 351s do not. When you installed the rear main cap did you put a light smear of sealant on the cap to block mating surface?
    Edit to add. How long have the VCs been dry? Possibly you're are seeing residual oil from when they leaked? Have you checked the intake manifold to block wall across the back of the engine valley?
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

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    Senior Member CDXXVII's Avatar
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    The engine is a Ford Racing crate engine. I did not install the original rear main so I do not know how it was sealed.
    The valve covers have been dry for six months now along with the intake manifold along the rear.
    Everything is really clean and dry. Same with the oil pan.

    Quote Originally Posted by CraigS View Post
    I think 302s have holes all the way through but 351s do not. When you installed the rear main cap did you put a light smear of sealant on the cap to block mating surface?
    Edit to add. How long have the VCs been dry? Possibly you're are seeing residual oil from when they leaked? Have you checked the intake manifold to block wall across the back of the engine valley?
    F5R #7841: Anniversary Edition MK4, Ford Racing 427, Edelbrock EFI, Gas-N Pipes, Stainless Headers, TKO600, 3.31 Moser 3-Link, 17" Halibrands

  5. #5
    Not a waxer Jeff Kleiner's Avatar
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    Craig is correct; 302/5.0s go through the flange and to oil and OEM 351s are blind however being a stroker crank either scenario is possible. Frank is spot on RE: the possibility of oil contamination on the clutch being the root cause of chatter.

    Good luck!

    Jeff

  6. #6
    Senior Member rich grsc's Avatar
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    Not sure I've ever seen a Ford rear main leak in a new build. Now 75-100,000 miles, possible.

  7. #7
    Senior Member CDXXVII's Avatar
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    My clutch is a McLeod RST Twin Disc. The chatter was much worse when it was brand new. I spoke to McLeod and they said this was not surprising on such a light car. They also suggested getting my billet steel flywheel resurfaced with a more aggressive grind. Seemed an odd response to me. I did use blue loc tight on the flywheel bolts but I can not verify if the go all the way through. When I last had the transmission out the clutch seemed dry but the bellhousing had quite a bit of oil or trans. Fluid coating the inside surface. There was clearly something leaking into the bellhousing. I assumed it was from the transmission.

    Quote Originally Posted by i.e.427 View Post
    It may not be the rear main seal. Most 351 crankshafts have though holes for the flywheel bolts. Confirm that threadlocker was used on the flywheel bolts as thread sealant. You may be getting the oil on the flywheel and may have already contaminated the clutch disc. Maybe that is where the chatter is coming from?

    Frank
    F5R #7841: Anniversary Edition MK4, Ford Racing 427, Edelbrock EFI, Gas-N Pipes, Stainless Headers, TKO600, 3.31 Moser 3-Link, 17" Halibrands

  8. #8
    Senior Member CDXXVII's Avatar
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    On a possibly related note. Something new that I have noticed is that I am getting a bit of oil spitting out from around my pcv valve grommet. I did not notice this early on. It kind of started happening around the same time as this particular leak. Could I have high case pressure? Should I try removing the pcv and just installing a breather?
    F5R #7841: Anniversary Edition MK4, Ford Racing 427, Edelbrock EFI, Gas-N Pipes, Stainless Headers, TKO600, 3.31 Moser 3-Link, 17" Halibrands

  9. #9
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    Viton is a trade name (DuPont I think) for fluoro-elastomer rubber compound. Both of the parts you mentioned are probably the same material. Most often are brown color. Viton is good for up to around 400F and very petroleum friendly/resistant. Good stuff.

  10. #10
    Senior Member seagull81's Avatar
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    Change your PCV valve to be sure you aren't pressurizing your engine and causing a leak.
    Steve
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  11. #11
    Senior Member Avalanche325's Avatar
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    Old school rubber seals - lubricate or they will leak
    New style Teflon seal - install DRY or they will leak.

    THIN layer of sealant on the outer surface.
    Thin layer of sealant on the rear bearing cap surface.
    351 stock crank has blind holes. Maybe / maybe not on an aftermarket crank.

    Crankcase pressure is usually worse when the engine is brand new and then settles down a little. If it is really bad, a common thing is that it pushes the dipstick out at high RPM. You can put an open breather on but you will always have oily valve covers. Don't be surprised if you can see some significant mist. Remember that when the engine is warming up you will be vaporizing any moisture in the oil.

    I ended up putting a Lokar locking dipstick in. Also a Moroso separator on the drivers side. I still have a PCV valve on the passenger side.

  12. #12

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    Every oil leak on SBF always ends u[ on the front of the block plate, and inside the bellhousing. Those leaks can be very difficult to find. Try and find a shop in town that has a smoke machine. They can inject a petroleum based smoke into the crank case. Where the smoke leaks out is where the oil is leaking out. As said above, a rear main seal leak is rare. Not impossible, and not unheard of. But rare.
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