Shimano #10KC8 or TGT1018 Worm Shaft (Worm Gear).  Check the schematics for your reel for the correct part number.  The worm gear is one of the most often parts to “wear’ out on level wind reels.  If your line guide is not traveling from side to side smoothly or stops at some point, the problem is most likely the worm gear.  Don’t have schematics to check?  Just see www.reelschematics.com!  If you need more help, just send us an email and we will get right back to you!