mine is 500 upright , i streched my arm in and screwed the inner part through the hole , then got a ajustable on the outer nut tightend it up so the inner part pulled right up to the inside wall .
then i loosend the outside nut off and put a little sealant round the edge of the hole and , pushed the washer on, then the nut and tightend it all up .
the hole i drilled was tight so i had to sand round the edge to make it slightly bigger.
then i put the washer on the immersion and screwed it in tightened it up and thats it.
DO NOT OVER TIGHTEN THE IMMERSION OR THE FLANGE.
i used ( NAIL & SEAL) sealant its the best , boat builders use it to put inlet pipes for engines in the hulls.