Mammoth Uncensored Word List:
19 words ending with th, 12 letters long

blabbermouth centillionth commonwealth decilliardth hairsbreadth handsbreadth nonilliardth octilliardth overstrength septillionth seventyfifth sextillionth sixtyseventh snaggleteeth snaggletooth tonsillolith trochelminth twentyfourth weathercloth