B20 sleeves arent like other Bs, they are siamesed cast iron. Other Bs are separate with siamesed cast aluminum. The cast iron makes the B20 cylinders brittle and prone to cracking esp. under boost, so you have to be good with your tuning.
Also check out this pic of a cutaway of a recent skunk2 IM. Apparently they changed thier supplier in China and theyve been using epoxy to cover up casting blemishes.