I also had this problem on a SixPack motor I built where the starter hit the ring gear and it turned out to be the flywheel made by Mcleoud . The ring gear was a bit closer to the starter and the gears would not mesh. I used a OEM flywheel and no more problem. My problem was compounded by a bit out of spec TIR. Check the TIR (runout)of you bell to see if it within spec .There is also a lil play in most starters where you can pull it back away from the flywheel just a tad when you tighten it to the bell.