1. To check the "key", you don't have to dismantle everything. No tools needed (if your phonic wheel has a triangular cut/notch).
https://www.fiatforum.com/threads/engine-timing-setup-1-4-8v-vvt.426347/post-4790015 Flywheel + phonic wheel.
To be 200% sure (double check/"sanity check"), you probe the pistons (via sparkplug holes), where they are (at TDC and TDC+90).
2. Valve "lash" Guide:
https://www.fiatforum.com/guides/valve-clearance-check-and-adjustment-1-4-8v.824/
Tolerance is 0,05 mm, but total (not plus/minus, it's too much). The point is not only to be in tolerance (and "call it a day"), but to set clearances equally.
3. Camshaft lobes lift same story. The problem is, when they wear unevenly. If there is a wear, but equal on all 8 lobes, it's a lesser evil.
About 0,1-0,2 mm missing (polished) makes a difference in engine performance (for example lack of power - hot engine).
Use dial indicator method (not caliper/micrometer).
4. Compression test is a good idea. For a starter (appetizer). In-cylinder pressure waveform would probably help to pinpoint the issue.
But this is still not popular (most mechanics don't use "scope"):
https://www.youtube.com/results?search_query=in+cylinder+pressure+waveform+bad+timing
5. If the head was damaged (valves), it's probably still an engine head problem (valves/seats/guides)...
By the way, old eLearn (CD version) clearly states, that timing belt in Grande 1,2 8V is done with timing tools (same as 1,4 8V + VVT), so "old-school TDC method" is not needed (I mean: using tools cannot cause such trouble). But you must use full method (not mix them: TDC + one jig from the timing tool set).
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