The /Ɵ/ sound is spelled with <th>. The /ʧ/ sound is spelled with <ch> or <tch>.
This is a contrast between a fricative and an affricate, both voiceless and in the same region of the mouth. It is surprising that it is not more of a problem, but this may be because /Ɵ/ is an uncommon sound in English and does not occur in many other languages.
The mean density value is 2.2%. The list makes 33 semantic distinctions, a loading of 72%.
Arthur archer Beith beach Beith beech berth birch berthed birched berthing birching birth birch Cath catch doth Dutch filth filch hath hatch kith kitsch lath larch month munch north nautch path parch Perth perch pith pitch teeth teach tenth tench thane chain thanes chains thaw chaw thawed chawed thawing chawing thaws chaws theory cheery thick chick thief chief thin chin thins chins think chink thinking chinking thinks chinks Thor chaw thug chug thugs chugs thumb chum thumbed chummed thumbing chumming thumbs chums thump chump thumps chumps withe which withe witch wroth roach
John Higgins, Shaftesbury, November 2010.