piston rings
The manifold is not a long process but there are the right placement of washers and fasteners to learn and checks of gaps, and the best procedures. Once you learn it, you can R&R the manifolds in <1 hour (assuming they are not warped and you have your gaskets, sealants, extra studs just in case, etc.).
BTW, any oil coming past bad oil rings can oil-up the compression rings and make the compressions read pretty good. Make sure you do it after it has set at least overnight and not been started at all, to let as much oil as possible drain off the rings.