With the surge of vendors wanting to make big bucks selling custom heads it is hard to know what you will get and what problems will occur.
In this case 2 C5 owners buy custom heads, CAM, headers, etc and once they have been installed we notice both engines suffer with high oil loss.
In the end the problem is found that bottom of heads milled ground off the ends of metal now exposing the threaded holes for the rocker bolts.
With that end now open the oil runs down the bolts, seeps through the threads and into the cylinders.
Here is what both C5 owners found with these TSP custom heads :-(
From underside :