As far as what motor its for I was told:) originally it was for a J-series motor so I bought it sight unseen and when I received it I realized it probably wont work on my 521(J13). When I posted pics a while back everyone that saw it said it was for a L-series but I cant guarantee anything cus I've never owned an L-series motor...It looks like the shorter runners have bigger ports than the long runners,not sure why,maybe velocity? I do know this is how it was cast,it hasnt been ported at all. Hope that helps