It depends on when they were made. The models made since February are actually only capable of about 5/8", before February they came with the strongest magnet we offer for these pumps and 3/4" was the limit. Keep in mind that both magnets are waterproof, if you have a thinner walled overflow, the simple solution is to mount them on the overflow. The 6025.501 extension magnet will boost the hold of units made since February to 3/4". The newer magnet holders that are weaker have a larger diameter outside magnet holder where the old one for 3/4" is inside and outside the same diameter.