A crack at the "Y" junction has a rather odd origin. Heat cycling of the manifold causes it to change shape slightly over time. The outer arms eventually get closer together, so the two end manifold bolt holes will not line up properly. You may find it difficult to remove or reinstall the manifold over the two end studs. This misfit will cause stress that can lead to cracking of the manifold at the "Y" junction. If you find this shrinkage, use a rat tail file to elongate the end holes in the manifold to achieve a free sliding fit on the end studs for installation. And if you have to do it again five years later, so be it. I believe later production manifolds have an addition thin web cast between the arms to inhibit such cracking. I do not know if this reduces the shrinkage problem, but it should reduce the tendency to break.

Photo above, being replaced in December 2012 with a used manifold with weld repaired bottom flange. Not the first time replacing this part, but it was the first time I took pictures for record. Photo below replacing the welded one 27 months later when the rear branch broke again.

This long term creep in shape of the manifold will also lead to misalignment of the exhaust ports between head and manifold. If this a concern, you can use the manifold gasket as a template to scribe around the ports in head and manifold, then grind away excess metal to match the ports for best exhaust flow. Photo at right from March 15, 2015.

Photos below from April 14, 2016 and April 30, 2016. I finally got tired of replacing used exhaust manifolds that break in this manner, bit the bullet and bought a new one from Moss Motors. Notice that the new casting does not have a reinforcing web between the arms, so it is cast like the earlier parts. They missed a good opportunity to do it right with the new casting.


The new replacement part boken in March 2019.


The second new part broken again in July 2021. This is irritating that they seem to break just a few months after the two year warranty expires. New parts are not cheap.