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  1. #1
    CB750 Enthusiast John Luke's Avatar
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    1981 CB750C base gasket leak after rebuild.

    If I remove the cylinder heads to replace the gasket will it decrease the compression when I put it back together? My thinking is the rings are seated and if I remove the head it will mess up the seal. Are there any ways I can fix the leak without removing the head? Iíve heard of people using sealants on the seam to sop the leak.


    Thanks?


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  2. #2
    CB750 Guru amc49's Avatar
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    ???????

    On 2/1/20 you posted this.....

    'I have totally rebuilt my 1981 CB750C.'

    You were talking about the engine when you posted it.

    Removing head does nothing to rings at all unless the work is done by a butcher.

    'I’ve heard of people using sealants on the seam to sop the leak.'

    And if you went further you found that those all failed to work. That is an absolutely incompetent idea, the head AND cylinder MUST come off to do that again, assuming you are not confusing the base gasket with the head gasket, and the way you are wording your post we can't be sure there.

    Of course, yours and do as you will there.

  3. #3
    CB750 Enthusiast John Luke's Avatar
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    Yes Iím talking about the base gasket.


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  4. #4
    CB750 Guru amc49's Avatar
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    Not trying to be mean there but the idea simply will not work. Once you have an area soaked with oil no gasket sealer on earth will stop it leaking.

  5. #5
    CB750 Enthusiast John Luke's Avatar
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    Lol u fine. My main question is if I remove the head and the cylinders will it cause the rings to not seat properly due to removing the pistons?
    Because at some point Iíll need to replace the gasket. Iíll probably use a copper spray to help seal the new gasket.


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  6. #6
    CB750 Guru amc49's Avatar
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    You change the gasket with pistons still in place.

    ANYTIME you remove the cylinder block with the intentions of keeping the same rings in place to use over you are asking for it. I've done it but you are on pins and needles the whole time as any little scratches you put on rings tend to create issues. You might put masking tape around the rings so that in cleaning gasket off you don't scratch rings from constantly rubbing them against studs. Do NOT remove any studs either, typically they break off or pull threads coming out and then you are in even more trouble.

    You stuff clean towels down in under each piston and slowly chisel away at the gasket until done, it can be maddening. I use a razor blade a lot.

    It CAN be done but not for the fainthearted.

  7. #7
    CB750 Enthusiast John Luke's Avatar
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    Iím going to attempt to fix it. I canít even think about selling a bike thatís leaking oil.
    Is there a possibility that I over torqued the head? I torqued it to spec then did a 10+ mile ride to break it in. Let it cool overnight then retorqued it. Like maybe when I re-torqued it could I have crushed the gasket? In some places the base gasket almost looks slightly buckled. Iím going to buy a new base gasket and head gasket Iím gonna use copper spray. Click image for larger version. 

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  8. #8
    CB750 Guru amc49's Avatar
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    Use gasket dry. The copper coat lets the gasket slip under load and it moves as it squishes around. You need new o-rings at the bottom of the studs too, there is an oil leak issue at the back of the cylinder and why those rings are there. Later ones use no rings but silicone ONLY closely around the cylinder studs, too much and again the gasket slips.

    The way you retorqued did nothing unless the tool used is defective.

  9. #9
    CB750 Guru amc49's Avatar
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    http://www.cb750c.com/publicdocs/CB7...udbulletin.pdf

    Applies to all early DOHC, the early engines were the worst ones.

  10. #10
    CB750 Enthusiast John Luke's Avatar
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    I used new oil seals.


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  11. #11
    CB750 Guru amc49's Avatar
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    That means nothing if it leaks of course. FYI, new they get no gasket sealer on the gasket, dry only. One of the last engines I took apart had no seals there at all, they simply put a small circle of silicone right there around the bottom of studs. No leaks.

    Up to you to analyze why yours leaked.

    At a second look it almost looks as if the gasket is not tight there like the cylinder not all the way down 100%. If you got something holding it up from going down all the way then of course it won't seal.

  12. #12
    Moderator dirtdigger's Avatar
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    I agree that it looks like the cylinder is not actually seated all the way down. Either a small bit of gasket left over or dirty alignment dowels.

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