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  1. #1
    Senior Member 65ShelbyClone's Avatar
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    How To: flat lapping a cylinder head.

    I have been meaning to put together a quick tutorial on how to flat lap a cylinder head since the question does get asked on occasion. Tonight I finally did just that. Some of you may remember the dumpster jewel 3hp iron bore/ball bearing Briggs I found a while back. It's been sitting on the work bench partially disassembled, so I figured what better candidate. This procedure is general in that it applies to OHV heads as well. Without further ado...

    Materials:
    - Wet/dry sandpaper sheets, at least 400 grit.
    - Masking tape.
    - Layout fluid like Dykem if you have it or a permanent marker if you don't.
    - Soapy water. Light oil can be used too, but it's much messier, so I don't recommend it.
    - Gloves. Okay, and safety goggles.

    1.) You'll need a clean flat surface, the more precise the better. I have a real granite surface plate at my disposal that is flat to 0.00006". For the purposes of this tutorial, a piece of decent window glass will suffice. I have even resorted to a polished concrete floor for quick 'n' dirty work. Tape the corners of your sandpaper down like this:



    2.) Put a little soapy water on the sandpaper. This keeps the paper from loading up and makes it last a bit longer:


    3.) Make sure the head is completely clean with no gasket residue. You can put layout fluid/marker on the gasket surface now or wait until the next step. Put the head on the wet sandpaper and make a few swipes in a figure-8 motion, NOT a circle. Keep the pressure as even across the part as you can:


    4.) Have a quick look for high and low spots. Here you can see the high spots are mostly around the head bolt holes:


    Put the layout fluid/marker on there if you haven't yet:


    5.) Use some more elbow grease. Rotate the part occasionally as you lap it. Check progress as you go. Low spots will still have color like this:




    Keep lapping until you are comfortable that they are gone.

    6.) Finish up with a relatively fine paper. Like I said before, 400 is probably a good minimum grit. I started with 400 and went directly to 1000. This is why I recommend using gloves:


    7.) This is what I ended up with. Almost all of the hills and valleys are gone and the gasket has an RA more than fit for even a copper gasket:

  2. #2
    Senior Member Oldschool's Avatar
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    There is such thing as too fine a finnish.

    You need some roughness so the parts can geta good grip on the gaskets.

    I have never gone finer that 240.

    Then I lightly grease the parts with some copper antiseize.

    Funny thing about that stuff.....
    When it gets hot it carbonizes and bakes hard. Its not near as good as proper gasket sealer but it fills tiny voids and cooks there on an engine, In some cases it actualy becomes siezing compound if you use it on things like exhaust manifold nuts.

  3. #3
    An OMB original! Neck's Avatar
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    Nice Rock Dude!
    Its the members that make OMB, a great place!

  4. #4
    Senior Member mr.modified's Avatar
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    Easy to read, easy to follow...

  5. #5
    Senior Member 65ShelbyClone's Avatar
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    Quote Originally Posted by Oldschool View Post
    There is such thing as too fine a finnish.

    You need some roughness so the parts can geta good grip on the gaskets.

    I have never gone finer that 240.
    True, but that depends also on the type of gasket. Fiber gaskets may(or may not, I don't know) prefer a more "toothy" finish, but metal gaskets just about universally require a fine surface finish. The combustion pressures that 99.999% of minibike engines generate aren't high enough for it to matter IMO. I have typically found that metal-backed fiber gaskets leave bite marks in aluminum anyway. The gasket my example engine above had was just fiber...probably original asbestos from 1964.

    Quote Originally Posted by Neck View Post
    Nice Rock Dude!
    Thanks. I think so too.

    Quote Originally Posted by mr.modified View Post
    Easy to read, easy to follow...
    Mission accomplished then.

  6. #6
    An OMB original! Neck's Avatar
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    So.......We missed you at Funday!
    Its the members that make OMB, a great place!

  7. #7
    Senior Member 65ShelbyClone's Avatar
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    I didn't even know when it was. I always forget to see what's brewing in the Events forum. That food looks incredible...

  8. #8
    An OMB original! Neck's Avatar
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    You missed out, and we missed you!
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  9. #9
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    Only time I did this before, I used wd40 instead of soapy water. As you stated, was very messy. I will try soapy water next time. I have a clone head I plan to take .020 off in the near future. Only problem, is a lot of work.

  10. #10
    ogygopsis's Avatar
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    WOW! WOW! WOW! What a fantastic article ShelbyClone! Large clear pictures. Easy to understand instructions and lots of valuable information for anyone who works with small motors. Thanks for taking the time to prepare and post this How To. I could sit and read these kind of threads all day. More! More! Ogy

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