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Thread: cylinder boring

  1. #1
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    Default cylinder boring

    Want to put new pistons in my panhead. Made a ring fixture to mount in a 4 jaw chuck. OD and ID run true, has 4 holes in the face to mount the cylinder. Trued the ID in the lathe and made the hole a slip-fit for the sleeve end for the jug. Faced the fixture so the cylinder base would run true to the bore. I mounted the first cylinder and indicated near the top of the bore, and it runs out .008". Ran the indicator down the bore and it's good within .002" in 3 or 4 spots. The bore is stock (3.4385"). Is this something anyone has seen? I thought the bore would run truer than that provided the face of the fixture was true to the axis of the lathe.

  2. #2
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    Default

    I made a similar fixture that bolts to the top of the cylinder. I find that the base flange is seldom flat. After boring, I true the flange.

  3. #3
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    Default

    Ideally, Folks,...

    Your fixture should stress the cylinder as if it were installed upon the cases with heads.

    HDs distort from fastener stress at their long spigots, more and more dramatically as the bore increases.

    By the time an HD cylinder is .060" over stock, stress plates for the final hone-fitting are more than prudent.

    ....Cotten
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    Last edited by T. Cotten; 09-03-2019 at 11:32 AM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  4. #4
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    Default

    OK, thanks for the tips. Just a couple questions. First, for Cotten, what's that flange piece with the screws in it for? And for Larry, when you true up the flange, do you fixture it in the lathe by bolting through the head bolt holes to the surface on the jug that mates with the head, or do you make some kind of expansion arbor to fit the fresh bore on the cylinders?

  5. #5
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    Default

    That's lagged to the bench, Omar!

    The "torque plates" have notches for the bolts in the fixture, so the assembly can be held firmly for a convenient and accurate torque.

    The idea is that the cylinder should go back to 'round' when installed in the motor, at the same torque spec.

    Once again, although always beneficial, it is at larger over-bores that this technique becomes critical.
    Horror stories of fragged cylinders or severe overheating could have been avoided.

    ....Cotten
    PS: Carbs distort too.
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    Last edited by T. Cotten; 09-03-2019 at 11:28 AM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  6. #6
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    Default

    Bolts to head and can act as a torque plate.

  7. #7
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    Default

    Ever wonder why, Folks,...

    Superior brand pistons for OHVs were offered up to a hundred over?

    My favorite was high compression 74" pistons in 61"s.

    When you approach a 'square motor', the spigots distort so much you can't get the piston in without re-applying the plate.
    Then you remove the plate and install the cylinder with piston Indian-style.
    Torque the base, and it releases perfectly.

    ....Cotten
    PS: Larry!
    Torque-plating the head deck certainly helps with ring seal and power, but its the squirrely spigots that need 'torqued' most.
    And you're quite right, any bend on a flange contributes.

    Metal moves.
    Last edited by T. Cotten; 09-03-2019 at 02:03 PM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  8. #8
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    Default

    OK....Thanks again

  9. #9
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    Default

    For what it's worth, I made a torque plate for my 1928 Ariel (81.8mmx95.0mm BxS) and measured the distortion after I had bored the cylinder and removed the torque plate (held by 4 bolts). The cylinder was oval by 0.0011" at 1/4" below the top flange, 0.0002" at 1/2" and round to better than 0.0001" at 1". That is, had I not used the torque plate it would have been oval by this amount, compromising ring sealing in the region where compression pressures are highest. I used the same plate when I honed the cylinder to final size.

  10. #10
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    Stress-plates were the standard of the automotive industry in the mid-Eighties, Folks!

    To give the MOCO a little credit, the fellows who did the school gave me the sixty-over spec.

    But an independent observer who sleeved some Chief cylinders for me detected spring-action, even when returned to stock bore.

    Sleeves are a spring within a spring.

    ....Cotten
    PS: I have been compelled to assemble some 1" Linkert throttlediscs while torqued.
    Last edited by T. Cotten; 09-03-2019 at 03:14 PM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

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