Oil leak - trans bell house drain hole

Discussion in 'Troubleshooting & Diagnosis' started by Chuck71RS, Sep 21, 2017.

  1. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

    Nov 24, 2012
    Houston, Texas
    Original block still has an oil leak at the rear (bottom drain hole, trans bell house cover).

    Leak 1: shop built long block used 4-piece type oil pan gasket. Replacement oil pan in 1986. Leaked not long after first start

    Leak 2: Installed Fel-Pro 1-piece and new rear seal (don't know if front seal is thick or thin). Tightened with inch-lbs torque wrench. Dry for a while then started leaking

    Leak 3: Dropped pan, checked, new rear seal, reinstall. (Removed rear seal had a small slice but could have happened during removal). Leak began shortly after starting.

    Can not see or feel any oil on the pan rails forward of the rear.
    No oil coming off the oil filter area.
    No oil from the valve cover
    No oil from oil pressure sending unit

    Should the front seal be thick or thin? What is the measurement of the needed thickness?
    What can cause the above problem?
  2. cadillac_al

    cadillac_al Veteran Member

    Apr 22, 2015
    It sounds like the front seal is fine and the rear seal is leaking out the inspection cover hole. I have a leaker too that I put together a few years ago. I figured I screwed up the rear seal. I didn't offset the parting line of the seal. I don't remember my reasoning now but I will be offsetting them all from now on. I think I got by in the past by not offsetting them so I figured it wasn't necessary. I hate making rookie mistakes late in life.

    You should inspect the crankshaft seal area next time you change the seal. I think GM went with the thicker front seal around '75.
  3. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Nov 4, 2004
    Ringgold, GA
    Have you used the same oil pan? The stock originals are pretty good but can get warped over time. I'm not sure about the gasket thickness, you can call Fel Pro, they should know or maybe someone with experience will chime in. Pictures in another post show you have an automatic transmission? Drop the dust cover from bottom of the transmission housing and you should be able to see the rear of the crankshaft. My guess is that you'll see a small oil leak there, at the rear main seal. They're a real bear to get sealed properly. My analogy of a sbc rear main seal is to that of a radial aircraft engine; if it's not leaking oil, it's out of oil. Good luck with this. :)
  4. budro6968

    budro6968 Veteran Member

    Apr 2, 2016
    Jax Florida
    When you had the pan off did you peen the bolt holes? I make it a point to place the pan rail over a 2X6 and take a ballpeen Hammer round side down on each hole and tap with another hammer to dimple the holes out a little. I like to use a little rubber contact cement to stick the gasket to the pan or block which ever way you like to do and silicone the corners. I guy I knew raced a lot so he would coat the gaskets with a thin coat of silicone and let it set up completely first before assembly. Then rub a little oil on the silicone and put it together. He rarely had a leak. It made the removal of the pan and rocker covers easy and could reuse again and again. Don't use a torque wrench to tighten. Use common sense and don't over tighten. I see so many that over tighten and crack the gasket.
  5. COPO

    COPO Veteran Member Lifetime Gold Member

    Sep 15, 1999
    Ontario, Canada
    If you have the thin 1 piece then there's no way the thick would fit. I have the GM factory 4 piece with no leaks. Something is wrong if it leaks.

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