There are many fully threaded 3/4" reach plugs and they're usually found in aluminum heads. You don't want a threaded portion sticking out into the combustion chamber as the threads will fill up with hard carbon and you'll never get them out. A shop I worked in used to retrofit BMW R69S motorcycle heads with 3/4 reach plugs as the 3/8" reach plugs would strip the threads out of the head over time. This involved welding. The S models had higher compression.
