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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.

  13. #13
    CB750 Enthusiast John Luke's Avatar
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    Thatís potentially one of the reasons I had a real hard time removing the head due to the dowel pins being rusty after I got it off and tried to remove the dowels I probably and most definitely bent them a bit probably causing it to not seat properly. Well time to buy new gaskets and new dowel pins. Ugggggg


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  14. #14
    CB750 Guru amc49's Avatar
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    Please think and post more clearly. Removing the head has nothing to do with the dowel pins we are talking about. You are talking about ones in the top of cylincer but the problem area we are now talking about is on the BOTTOM of the cylinder.

    Aren't you the poster who posted a while back about sitting the head on top of cylinder with no gasket to see daylight in between the two? I am concerned about somebody having forced the cylinder loose from lower case with enough force to bend the cylinder block. If somebody never pulled the last bolt holding cylinder to case then a screwdriver prying up on block that will not come loose can do extreme damage that does not show up until you look close. Like knocking the cylinder block out of dead flat.

    Any time you damage dowels you either fix the dowel to again be fine or it is damaged enough to need replacement.

    See if like a .002" feeler gauge can be slipped in anywhere that gasket looks loose, if so it all comes back apart.

  15. #15
    CB750 Enthusiast John Luke's Avatar
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    So, you are correct. But I did remove all the nuts and bolts before getting the cylinders off. The locating pins on the cylinder were so oxidized that it literally was frozen together. Eventually I got to the point where I could use liquid wrench hoping it would free it and it did. But those locating pits were nasty. Then I did what I now know was a stupid idea. I used pliers to rent he locating pins for the cylinder. That scraped them up.

    I apologize for not being clear. Thank you so much for all the help. Literally couldnít do it without you.


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  16. #16
    CB750 Guru amc49's Avatar
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    You can take a file to the pins if the OD is not too scarred up, I have a hundred times. They need to be round and slip into both the opposing holes. Pins have to remove if say you are surfacing a part, it's a risk you have to deal with. If you think they are a bit too long take a bit of length off using say bench grinder. All they have to do is go into both holes to keep part much closer to center, they do not have to be the exact length.

    If you pried the cylinder block up anywhere that gouges some aluminum then it MUST be dressed back down to dead even flat or slightly lower to let the block come down all the way flat against the lower case.

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