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Maths help needed! marking crank pulley degrees.

Discussion in 'The Hokey Ass Message Board' started by lowsquire, Jan 3, 2010.

  1. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,564

    from Austin, TX

    Ive put a TDC mark on my 99A Flathead and made a pointer, but I would like to mark ten, twenty and thirty degrees so i can get an idea of timing at idle and total advance, As the engine is in the car and only has an inch in front of the pulley, I think the best way to determine the marks without a degree wheel on the front of the pulley (Becuase I cant get in there to see it) is Via good old mathematics..

    So with a crank pulley Diameter of 5.175 inches (131.4 mm) how would i work out how far around the pulley ten degrees is?

    I know it will have Pi in the equation, but its been twenty years since i FAILED maths at high school..

    Im sure someone out there can do this in two seconds...
  2. Andy
    Joined: Nov 17, 2002
    Posts: 4,765


    5.175 x 3.1416 = 16.2577 for 360 degrees. Divide by 36 for 10 degrees = .4516" for 10 degrees
  3. 5.175" X 3.1416 = 16.258" circumference. 10 degrees = 1/36 X 16.258 or .452".
  4. 131.4 millimeters x pi = 412.805275 millimeters
    Divide this by 36 to give 10 degrees.
    Answer is 11.466813 millimeters on the curve.

  5. thunderbirdesq
    Joined: Feb 15, 2006
    Posts: 6,984


    If you don't feel like doing math, get a little artist's protractor and hold it up to the pulley. Mark 10*... Done!
  6. duste01
    Joined: Nov 5, 2006
    Posts: 1,213


    three way tie!
  7. flamed34
    Joined: Dec 30, 2009
    Posts: 733


    Mark out the .4516" on a piece of paper or tape (it would be easiest to dial this in on a digital caliper and mark to the jaw opening)...pull it around from your 0 and mark it!
  8. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,564

    from Austin, TX

    Damn you guys are Quick!!

    Thankyou very much. who needs online calculators when you gots da hamb!!

  9. very good points
  10. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,564

    from Austin, TX

    yep the positive stop method was used to set up the pointer and TDC ,before i put the heads on.
    yeh I understand the measureing around a curve bit, will do it later today and take some shots to help others out hopefully.
  11. Divide pulley into quarters . They are 90 degrees. Divide them (the 1/4s)in half. That is 45 degrees. Divide them in half . That is 22. 1/2 degrees . Divide that in half and you get 11 1/4 degrees. The side of that line closest to the tdc mark is 10 degrees or so darn close it wont matter. All you need is to cut out a circle of thin paper the same diameter as your pulley. Then fold it in half then half that to a 1/4. Fold that again to 1/8 (45 degrees ) fold that again for 1/16 (22 1/2 degrees) amd once more gives you the 11 1/4 degrees. No math no measuring just simple paper work. I use this when finding hole spacing on round objects like spacing the screw holes on my wooden steering wheel. I just fold till i get the spacing I want. When i unfold there are all the spots evenly spaced and easily seen.
    I made this one for a different reason hence the diamond shaped holes and I started with a square but it only takes a minute to do. Made the second one fom scrap paper today. Takes less than a minute/

    Attached Files:

    Last edited: Jan 4, 2010
  12. super plus
    Joined: Dec 14, 2006
    Posts: 566

    super plus

    Borrow or buy a degree wheel
  13. Wow.

    I think measuring 11.5mm from TDC is the easiest way for sure... :D

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