12mm spanner westy. Bill had to modify the flange on mine to get it to fit onto the head, and two nuts were very difficult to tighten up, and required a cut down open ended spanner with the head ground down thinner. Mine also needed a dent in runner #1 but that's a known issue with the V1 manifolds. If I do have issues with the v1 I'll just end up buying a new V2 manifold. my only concern is how long it lasts when given constant abuse, but only time will tell on that one!