Should I replace these cassette and chainring? I saw my bicycle chain very dirty so after many months since last time I cleaned it and I checked the general state of the drivetrain. I totally forgot that a couple of years ago the chain was already somewhat stretched.

I found what looks like a very worn out chainring, a very stretched chain and a cassette in a mixed state.

Chain is 1.8% elongated.

Chainring:



Cassette: it looks like the 2nd sprocket is the most worn out, the 1st, 3rd and 4th not so much. It is a Maillard Helicomatic so I cannot easily find new sprockets, and probably not new in any case. I may need to replace the whole wheel. I wonder when this bicycle has been manufactured...



Which parts is good idea to replace now, given the actual status?
That chain should have been changed ages ago - at or before 0.75% ideally.  At 1% you will need a new cassette.  To get to 1.8% is astounding, and means that the cassette and chain have worn into each other,

The chainrings look OK, because there's a lot more teeth engaged compared to the cassette, however if the chain slips at the front after replacing chain and cassette, then the chainring needs doing too.

You might also want to examine your two jockey wheels and consider replacing them.  The main difference of that is improved shifting and little to do with skipping, but given the rest is that worn you may as well change all the wear items.

Keep your chain cleaner too - that's a contributing factor on transmission wear.



If I were you, I'd give the bike a proper refresh.


New brake and shift cables, inners and outers
New brake pads front and rear
New bartape


Depending on wear you might even consider new tyres and saddle, but they're more expensive so not a priority item.

Give your bike a proper clean and degrease (that means using an old toothbrush.   While doing so, examine the frame for cracks and any damage that was hiding under the dirt and oil.