Rotor Housing Culprit?
 

I'm in the process of my first engine rebuild, on an 87 13B Turbo. Both rotor housings have grooves by the exhaust port, with the one on the front rotor housing (pictured) being the worst. From Bruce T.'s video, he mentions that grooves less than 3/16 of an inch wide and 9 thousandths deep are usually usable, although not up to mazda specs. This one appears deeper (still waiting on my dial indicator to arrive in the mail).

I'm just wondering what causes these in the first place... all apex seals are intact, and they are quite a distance from the edge of the housing. If these are indeed too deep to reuse, has anyone got an S4 front rotor housing in better shape laying around for sale?

http://www.casstel.net/~sriley/rotor.jpg

I'm assuming this is from a blown combustion O-ring, but could it also be from the turbo? http://www.casstel.net/~sriley/uhoh.jpg

That looks like something snuck into the intake (rock, small hunk of metal, etc) because of the damage on the plate.

lol, I am in the middle of rebuilding my engine and my exhaust manifold looked just like yours. Mine was a blown o-ring and cracked center housing.

wow, looks like youve got some pretty bad chatter marks on your housings