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Thread: Neoprene Rear Main Oil Seal

  1. #1
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    Neoprene Rear Main Oil Seal

    The engine overhaul gasket set I got comes with a Neoprene Rear Main Oil Seal instead of the original rope seal. After I installed it the Crankshaft it is very tight and very hard to turn by hand because the seal is very tight. Is this normal for the seal? I've have rebuilt V-8's with the same type of seal and they turn over much easier. Thanks

  2. #2
    Super Moderator bmorgil's Avatar
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    Definitely something wrong. This is from a prior post I wrote. They were having similar trouble with the rope seal.

    "With a rope seal the crankshaft (no rods or pistons hooked up) should turn freely with about 10 to 15 ft lbs of rotating torque once it breaks loose and starts moving. With a rubber seal it should spin fairly free, you should be able to spin it by hand. As you add pistons and rings, each assembly should add about 5 ft lbs of rotating torque. So you could end up with around 35 to 40 ft. lbs. of rotating torque on an assembled lower end with no spark plugs installed. If it goes much over that you should take a look. A bearing may be in wrong or there may be interference somewhere."

    That said, it sounds like you may have an oversize rear main seal or, the crankshaft surface is oversize. An oversize crank surface is quite possible. The seal surface needs to be 2.310" or so for the rubber seal to work. I have read there is a lot of variation. The rope seal did not require a precise dimension. The crankshafts varied quite a bit in the early days. It is important to check the main seal dimension on the crank. If the crank measures 2.310", I would send the seal back and try another brand and/or source. I have read there are two different sizes running around out there. I haven't seen them. I use the Fel-Pro gaskets and the KW seal. Fel-Pro only provides a rope seal to avoid any problems.
    Last edited by bmorgil; 09-22-2022 at 08:56 AM.

  3. #3
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    Thanks for your response and you're right, it is the wrong size. I called K/W this morning and told them what my problem was and they assured me the problem wasn't with the seal, they have had no complaints about it. So then I did a little more research and that's not quite true. Check out this link. It's exactly the same problem I'm having. https://www.thecj2apage.com/forums/r...opic48307.html

  4. #4
    Super Moderator bmorgil's Avatar
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    Be sure you measure the crankshaft surface. If the crankshaft diameter at the seal is not correct, no rubber seal will work. I did read a while ago about "cheap" rear main seals having a wide lip and the "good ones" having a much thinner lip. It is important you push the seal into the block and cap all the way down.
    Last edited by bmorgil; 09-21-2022 at 06:10 PM.

  5. #5
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    If I am reading the info in this link correctly, there were 2 different seals used during the production of these engines.
    https://www.rfjp.com/p-1903-cranksha...37-felpro.aspx
    Jeff
    '51 CJ3A
    '47 CJ2A

  6. #6
    Super Moderator bmorgil's Avatar
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    Yes the original military engines used the rope seal for sometime. The service manual only refers to the change as "on early L134 engines a wick type seal was used", "on later L134 engines a steel backed lip seal was used". The manual indicates that the new style seal can be used in place of the old style. The problem seems to be the loss of the dimensions in the aftermarket. It is important to measure the crankshaft. The early military L134 crankshafts will only work with a rope seal.

    If you are using a rubber seal the crankshaft needs to be 2.320" - 2.310" and the block needs to be the newer civilian style. If you are using a military block and/or crankshaft, you need to use the rope seal. My machinist finds most L134 cranks out there are 2.310". If you have the right block and crank, the KW seal worked for me.

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