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What accuracy do you need to turn a flywheel?

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  • What accuracy do you need to turn a flywheel?

    I need to get the flywheel skimmed on the MG Midget 1275 motor I'm using in my Morris Minor project car. Just how little runout would I need to be able to dial in in order to turn the clutch bearing surface myself?
    If I mount it on a faceplate which I've actually never used on my Sheldon, should I plan on skiming the faceplate first in order to get the required accuracy?
    How about brake rotors, how feasible are they to do in a home shop?

  • #2
    I think its customary to always skim the face plate to true it up, then put some hash marks on it to make sure it mounts back onto the spindle in the same place next time.
    I dont know about the fly wheel.


    • #3
      Flywheels are not machined flat, but rather have 1/2* angle (IIRC)ground into them with the high spot in the center. This is to promote gentle engagement of the clutch, since the contact starts at the center and moves out as the pressure plate clamps down.


      • #4
        I would get it done on a grinder made to do flywheels.


        • #5
          I concur with the gentleman who recommends taking the flywheel to an automotive machine shop to have it done and done right the first time. One aspect of flywheel turning is that hard spots show up when cutting with a tool bit and the tool bounces over the hard spots, leaving bumps in the surface, the grinding wheel does not leave any bumps.


          • #6
            I've cut dozens of flywheels and at one time or another read all the specs. All thats needed is a smooth skin cut off the face to remove all wear glazing and heat checks. The surface has to be parallel and concentric to the crankshaft mating features.

            I've never cut a crown in the face because the spider on which the clutch friction material mounts has some built-in spring.

            Usually I chuck a spud duplicating the featues of the flywheel of the end crankshaft in the 4 jaw and bolt the flywheel to it.


            • #7
              I put a new clutch in my Land Rover about 20 years ago. It gets a lot of use when snowplowing, foward and reverse about 200 times to plow my driveway. It still works fine. I broke the glaze on the flywheel at the time with an orbital sander and some 80 grit emery paper.
              Free software for calculating bolt circles and similar: Click Here


              • #8
                Like Evan says if it doesn't have grooves gouged in it just break the glaze.

                We have a small rotary blanchard grinder at work that we do them on,the first thing I do is lay the flywheel face down on the magnet and dust off the crankshaft mounting face first,this way if the flywheel is warpped by making sure the shaft mount surface is parallel to the face,this saves lots of time grinding,then its just a matter of flipping it over and grinding the face.I use a 36-50 grit aox wheel,no clogs and the finish needs to be a little course anyway.
                Oh forgot on the rotors,the local automotive parts houses do them for $7.00 each here,Really not worth the effort.

                [This message has been edited by wierdscience (edited 02-07-2004).]
                I just need one more tool,just one!


                • #9
                  The auto house in my neck of the woods does them on a small Blanchard Grinder with the 1/2 degree slight tip on it as mentioned.

                  $7.00 is a good price for grinding a flywheel, and the work is to spec.

                  CCBW, MAH


                  • #10
                    I have done lots of fly wheels from tractors, cars, and semi trucks. logging truck drivers can do the best job of roasting the hell out of them.

                    I just get them running flat (they get warped from heat so if you get it within .002-.005 I call it good)set the cross feed about .010-.012 and take .010-.020 a pass ,turn them slow, it's realy a gravy job, you can sit back and have a coffee.

                    I like doing flywheels, I would never farm them out, besides some shops cant handle the big monsters I have cut.

                    most cost $30-$40 , one I did cost around $150 but it had 32 bolts broke off in it that needed taken out.

                    [This message has been edited by tattoomike68 (edited 10-31-2004).]


                    • #11
                      take it to an automotive machine shop where they have a flywheel grinder(the correct tool for the job) and save yourself the trouble. It'll take them about 10 minutes tops. There aint' no crown in a flywheel either, at least not in any of 'em I've seen in 25 yrs of making my living a mechanic before I got smart and got out of it. Also, heat checks in a flywheel don't mean a thing. If they do, plan on replacing it, cause most of the time, by the time you cut deep enough to remove them, all that's left of your flywheel is a big pile of chips on the floor. A lathe is the WRONG tool in this application.