I had problems with a set from Year-One last year on a car. I couldn't get them to seal to the H-pipe. After pulling things apart and looking for the problem I discovered that the repro mainfolds have a much more tapered looking flange. An original manifold has a more rounded edge. The design is set-up to be a ball and socket seal so you don't have to use a gasket. Problem is that the repro manifold edge isn't cast enough to look like the socket it needs to be. Sure GLAD I picked up a pair old original manifolds last week for only $400 so I don't have to deal with that problem again!