1969 Skylark 350 rear main oil leak replacement method suggestions???

Discussion in 'Tri Shield Performance' started by LLC, Jan 15, 2024.

  1. LLC

    LLC Well-Known Member

    350 rear main cap.jpg 350 rear main crank.jpg 350 rear main case.jpg I drove my 1969 Skylark for the first time since the day I bought it back in 1980. The last 2 years of my spare time has really paid off in finishing my lifelong ground up project. Unfortunately, her maiden voyage has shown the rear main seal oil to leak. After researching the repair process to correct this condition, the owners manual states that in order to change out the upper half of the faulty seal, that THE CRANK SHAFT HAS TO BE REMOVED.
    Does anybody have a cheat around this???
    I have been studying the rods and mains with the oil pan removed;
    1) Has anyone ever loosened all the cap bolts and let the rear of the crank hang on the front timing chain? I never have and before I do, I was wondering if there is a noticeable rear droop? Or maybe the crank doesn't hang or move at all? Will this process hurt the bearings or damage anything ?I am only looking to increase the existing rear gap at the top half of the crank seal groove, so a new rope seal can be threaded into this top groove. Right now there is no viable process if the groove is not made larger, which would happen if the crank could be slight dropped down, but only in the rear. Hopefully this slight crankshaft rear droop will not adversely affect the timing chain, or the adjustment of it once crank main caps are torqued back into place, after the new rope seal is replaced into the top groove??? Am I missing or overlooking something?
    Just spit balling here, but I would really prefer to not have to pull my engine and all the work that goes along with that (hood, headers, belts, radiator, .................)
    Since I have heard from many that this is a Buick engine design characteristic, there has got to be some ole time Buick Techs that have a way around this. Has anyone heard of fixing this with Fiberglass resin? If so, I am assuming that the rope seal is soaked in a resin mix first, ok, but the problem of a new uncompressed sized seal getting installed into a compressed semi circle groove still exists. Thus the desire to increase the diameter of this groove, by dropping the crank.
    Last edited: Jan 15, 2024
  2. Mart

    Mart Gold level member

    Are you sure leak is the rear main seal? Drop inspection plate, clean and verify.
  3. LLC

    LLC Well-Known Member

    Thanks Mart. Yeah, its the rear seal. High speed oil steaks confirm.
  4. 1973gs

    1973gs Well-Known Member

    Per the older GM service manuals, pack the upper rope seal and measure how much it was packed. Cut pieces off of the old lower seal and pack it into the upper seal leaving 1/16" protruding. Install new seal in lower cap. It's always worked for me. I did my Cutlass probably 15 years ago and it's still good. I've seen people try to replace the upper in the car using those tools to pull it in without success although I have seen it done successfully. If you want to use one of those tools, you'll need to have someone rotate the crank as it's being pulled in. I wouldn't even consider dropping the crank. Make sure that the PCV system is working. Make sure to lightly oil the lower seal. I use RTV on the cap side grooves and anaerobic sealer between the cap and block. Make sure that all surfaces that get sealer are cleaned of any oil.
  5. LLC

    LLC Well-Known Member


    Thanks for the method and recommendations. I had purchased 2 sets of each, both the rubber and rope types.

    - RUBBER looks pretty and probably is a better bet if you can REMOVE THE CRANK. If you can not remove the crank completely, you will NEVER get a rubber seal in place. Loosening the cap bolts and letting the crank hang a little or a lot, does not create a big enough gap to slide the new one in place. You see, the rubber seal is slightly taller and slightly harder than the term "rubber" would leave you to believe. I have no doubt that they are a great sealing material, once crushed properly to fit, but I will never find out. For until I remove my crank completely out, which I am not doing, you CAN NOT install them properly. For you see, the 2 side gaps, where the seal first goes into the case groove, and where it finally comes out of the groove on the other side of the crank, never get wide enough, from crank dropping, that will not damage the new seal. The 2 gaps created at the sides of the journal are too narrow, regardless of how far the crank is suspended. In order for the rubber seal to have a wide enough gap to install it without destroying it, the crank has to be almost removed. For even though the gap at the top of the crank is wider by how much you suspend it out of position, the gap at the two sides of the crank stays relatively the same distance from the crank, until almost removal (which is impossible, due to the timing chain cover and timing chain in the front. In the rear of the block, I have removed the transmission and ring gear for two purpose; 1) for crank dropping ability and 2) for access to slide the oil pan out rearward). The two side clearance distances don't really open up until the crank is severely out of position (Basically removed, which is impossible while the block maintains position, pinning the crank in place).
    - ROPE seal can be done whether just repacking existing rope seal as was done and excellently described prior by 1973gs, THANK YOU
    replacing the seal with a new seal.
    Drop the crank enough to increase the groove tunnel dimension around the blind or back side of the crank as much as a 1/4-3/8 inch at the rear journal. I used both of the old 2 seals sides of the existing seal for the top repacking. I used about 4 feet of 30 pound test with a #12 trout egg hook, knotted the hook in the middle and pulled both strands at once around the crank to help pull it through the narrow groove opening around to the other side. I embedded the hook through the first 1/2 inch of the old seal and tried to pull it around the crank. You need to use a very small eyeglass type screwdriver, the blunter, the better.

    THE TECHNIQUE: as you slightly maintain pull tension on the fishing line hook which is imbedded in the seal, you use the screwdriver to work the seal into the tight gap, pushing a little bit in at a time in a round and round fashion. If you maintain slight even pressure on the string line, the seal will appear to pull itself through, you are just basically just tucking it in a little at a time and helping to force it through the narrow side gap pinch point. Once the screwdriver is eventually worked around and around, while applying steady pressure on the string, the hook eventually appears on the other side, and the seal is in place. Just work the hook out and pack it using 1973gs directions.

    I repeat,

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