A well-fitting, temporary crown that does not bother you and does not have noticeably excess cement stuck to the teeth is not considered a chatzitzah unless it is very temporary. Halachic opinions vary somewhat as to the length of time that is too temporary.

We follow the opinion that it would be considered a chatzitzah if it is to be replaced within a week after immersion. Therefore, if the crown cannot be replaced prior to immersion, make sure to delay its replacement for at least one week after mikveh.