how do i get these horrible semi-circular things out?

got 2 left, both in middle pipes and cant get them to shift, tried wd 40, realease spray, crow bar, but no joy.
skin is now wearing very thin on my knuckles, lol
just need to get these off, then i can remove my engine for stripdown.
any advice is appreciated

mark

Soz mate no idea, they've always just feel out for me.

sorted now, dropped whole engine and exhaust, got at them that way. hopefully easier to fit than remove.....

i found a little superglue helps alot when refitting, otherwise mega pain in the arse as they kept falling out!