For Zenith chassis with a dial belt the tuning shaft has to be removed or at least slide back far enough to slip the belt on the shaft from the inside of the chassis. A "C" ring on the shaft must come off to slide the idler assembly as well as loosening the flywheel. I have found that the belts seem to be too short despite being the correct part number. I suspect that N.O.S. belts may have shrunk with time. I have also had good results from throughly cleaning old lubricant from the entire tuning mechanism, polishing the shafts and sleeves with crocus cloth and using a grease for office machines. Further, configuring the idler to bear down on the belt from the outside increases the friction on the tuning shaft pulley and stops all the slipping. You may have to re-orient the idler spring to accomplish this. Be sure that the pulleys are all inline so the belt is not offset. Do not allow lubricant to get on the belt. You may have to replace the chassis grommets that support the tuning condenser for the belt to at proper length.
Regards,
Charles