The BB on my Flying Scot kept on cross threading. In the end I put the non drive side in first which helped the cartridge to line up and allow the cup to screw in correctly .
I hope that your seat post is less trouble than mine . I tried all sorts of solutions, caustic soda ,rust removing liquid and electrolysis. I eventually cut the top off and put hacksaw cuts down the length of the post . It was hard work trying to get right to the end of the post and cut right through the inner wall of the frame . The post eventually moved inside the frame so I had to drill and fit self tapping screws so that I could pull it out . The post once out had a nasty area of corrosion on it .