Well since you asked, the inlet and exhaust valves. Every 5000km or so I have to crawl under the vehicle, pop off the valve covers, and make sure the tappet clearance is between 0.15mm and 0.20mm. A model-year or two later they got "hydraulic lifters" which apparently don't need this. If the valves are too tight (less than 0.15mm), when they get hot they won't close fully which will make them even hotter because the hot compressed gas will leak past them and then they stretch and eventually break and when they fall into the cylinder it's bad news. Never had that happen thankfully. Rebuilt head busted a valve spring after a hundred km, that's what happens when you use Chinese parts (didn't even know at the time).
Glad I don't have to do that very often anymore, now that almost all of our travels are with the Tesla (well, not that we're travelling at the moment, can't go more than 5km from home).
Cheers.