mate i would check that the bendix spring on the end of the drive hasnt busted also check the drive moves freely backwards & forwards & isnt really loose on the drive screw thread check to see if the teeth era not to worn also check the ring gear teeth for excessive wear bendix drives had a habit of this problem of disengaging.

         Get hold of Auto Electric, think they're under links in this site. Get one of his gear drives. Problem is over. Also easier for old arms to hold in place while threading bolts in, weighs about a third of the originals.