Re: 1978 Mercury 20hp lower unit
First I would get Mercury service manual and/or Seloc manual off ebay. The Mercury service manual you can get from Mercury.
I think the biggest item is getting the cover nut off (the big threaded ring that encircles the prop shaft around the inside of the lower unit housing). I made a tool to engage the cover nut slots out of pipe or you can buy one. Cover nuts can also be stuck pretty good, so lots of PB blaster can be useful. If you are making a tool, having a new replacement cover nut as reference is useful to get the fitup correct.
Otherwise no special tools needed. If you are mechanically inclined, it is a reasonably straight forward job if you follow a manual.
Good luck and great motors!
