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Chain drops off outwards between plastic guard and last cog on crankset. This effect i mostly managed to remove by moving front derailleurs inside (not happened since). But looks like the core issue is not solved. Sometimes under applied torque chain getting of the cog and just slips few teeth with cracking noise. It's not happening on the stand. I drove not more than few hundreds kilometers.

I cannot move front derailleur even more, as frame will start rubbing the chain.

Drivetrain: Shimano Claris 3x8. Shimano chain CN-HG71 7/8 with KMC masterlink. Chain is new and waxed, front and rear derailleurs are new and installed by me. Wiring is also new.

Crankset is a bit older than other components but does not look obviously bent. However I do not know how crakset behaves when I'm applying the torque.

How I can diagnose core issue ?

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The front derailleur needs to be correctly adjusted for height, rotation and then for the H limit screw adjustment/cable tension

There is a good guide to the procedures here

https://www.parktool.com/blog/repair-help/front-derailleur-adjustment

I would also strongly recommend that you review the Shimano documentation. https://si.shimano.com/#/en/DM/GN0001

The front derailleur helps keep the chain on the largest ring, so if it's not properly adjusted it's not unusual to fall off.

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"How can I diagnose the issue?"

I would suggest bypass the front derailleur for a little bit. I don't think you should be certain it is the derailleur. I beleive you said the problem happens on the largest cog? Take the chain off, and move the FD to the smallest cog. Then go around the FD completely and put the chain on the largest cog. Ride around for a bit. Do you notice any slipping? Does the chain come off? If so, your problem is not the deralleur.

If you ride for a while and everything feels good, the problem is the derailleur.

It is possible that you won't have the clearance to do this test, and that in its lowest position you cant get the chain on the highest cog. If so, then you have a harder choice to make, because taking the derailleur off completely is a bigger task. It will give you some good information though.

Are the teeth of the problem cog sharp at all? If so, it may be time to replace that chainring. Also, check the bottom bracket. If there is a small bit of play, this could be why you are dropping the chain only under load. It could be lots of things, but bypassing the derailleur will help you figure out where to look.

Good luck.

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