mercruiser 3.0
 

Oil pressure sending unit was leaking a little. Previous owner had the adapter break so he put a brass nipple in the block and then a brass nut then sending unit. Not sure of the brass nut name but its like a butt connector in the plumbing world. Should I leave it alone and just reseal it or buy the correct adatper?

Also a block water plug was leaking a little so I tried to turn it to check if it was tight and it broke. Replace blue plastic plug or replace with a brass block plug?

if its not leaking, and its pipe thread, the coupler should be fine. The blue plastic plugs are known to break. Remove the whole fitting and heat it up with a torch to melt the plastic out of the fitting, you can chase it with a 7/16(IIRC) tap, install a new plastic plug and put back in the block. The used to just use a 1/4npt to 1/8npt reducer with a 1/8 npt plug. The sell some nice all brass drain plugs but the cost a little.

The plastic plug makes me worry. I dont want any leaks while out on the water. I will probably just put a plug in it. I can remove the plug to drain it in winter.

Oil sending unit with the nipple and nut just seems like Im adding another place for a leak.

they use those plastic plugs all over the engines, a fuel injected v8 with a bravo drive has at least 9 of them, teh most trouble I have with them is breaking off when removing them, They are o-ring sealed, so they rarely leak. You can replace it with a brass version
https://www.zoro.com/parker-brass-dr...2&gclsrc=3p.ds

Me personally I have no idea what your talking about but if I was you I would do exactly what spareparts says. The man is always on point. Listen to him and learn ,that's what I do.