One of our guru members at the club had a TT 4 stroke do similar. He had crashed it previously but it passed a thorough examination and clean before being fitted to a new plane. A couple of flights later, while running the engine on the ground it blew apart with a similar centreline crack as you show. Luckily, no injuries to anyone or the plane. All we could put it down to was a hidden crack, caused by the previous crash.