Okay, here is what happened, and you can go ahead and tell me I am an idiot, that much is not news to me.
I am working on my girlfriend's 2003 1.8t passat which blew a head gasket after she ignored a leaky radiator hose. But this isn't about her, it wouldn't do any good to tell her why the car broke down, she claims that anything under the hood is my responsibility, so we have to go with that.
So I replaced the head gasket and installed new timing belt, hydraulic tensioner, and idler pulley. I'm not exactly sure what went wrong, but let's summarize by saying that the timing belt slipped and the pistons not only came into contact with the valves, they broke one of them off and there are now some gouges on the top of the piston in cylinder number four. I examined them pretty closely and the worst one is perhaps two millimeters deep. There is no cylinder wall damage at all, the only injury is to the top of the piston. I got a decent cylinder head from a junkyard passat and the machine shop has inspected it and pronounced it good. My plan now is to install it as is, with a new idler and hydraulic tensioner, hoping that this time the idler pulley won't loosen up like it did last time. Best I can tell is I failed to torque it properly.
Here is my question. Can I get away with just replacing the cylinder head and head gasket, and ignore the gouges? Or should I expect that I need to pull the block and do something with that gouged piston?
Any advice is most welcome.
My gut reaction says yes, you CAN do that. Keep in mind though, the pistons hit the valves with a lot of force. Inspect them closely. Make sure you don't see any cracks in the piston surface. Also if the gouge happened to raise any material above the piston surface, you'll want to smooth that out.