167. Mortgage by estoppel.

A grantor cannot dispute the validity or effect of his own grant, and accordingly a mortgagor who has no legal estate at all1 is estopped at common law from denying that he had a legal title2. The estoppel binds the mortgagor and all persons claiming under him whether for value or not, other than a bona fide purchaser for value from the mortgagor without notice of the earlier transaction3. The title of the mortgagee does not prevail over the title of the true owner and the persons claiming under him; if, however, the mortgagor subsequently acquires the legal title