I have a 2015 Triumph Street Triple R with 4500km. A few day ago I observed an oil leak just near the left exhaust gallery. I have wipe it but appears every time. It is for sure oil. I read a ton of topics regarding Triumph's oil leaks, but i am still confused with this one. I cannot figure where it came from. It is only the spot seen in the picture.
Any ideas, please?

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The trail of oil looks like it is coming from above, probably the valve cover gasket. They are a bit tricky to reinstall after a valve clearance check. Can leak oil if it gets pinched or folded.

This was my first opinion, the valve cover gasket, but, after carefully checking, I saw no oil trail from it. And the bike has only 4500km; no valve clearance done yet.

When was the last time the valve cover gasket was disturbed in relation to when the oil leak first appeared? I have the same bike and recently I had the valve clearance checked, and no leaks (so far). The work was done at a dealership.
I did recently do a valve job a 1050 triple engine and you are right the gasket can be tricky, the service manual also called for a few locations where silicone sealant was required. That maybe where the leak is coming from.

There was no work involving the head gasket. The bike was for the first time put in circulation in the spring of 2016. I bought it last year when it had almost 4000 km; I think it is already leaking from some time, because I first noticed when oil came further down on the exhaust gallery.

If you want to fix it then you're only options is to bring it into a dealer, or pick up a service manual and start wrenching. You could contact Triumph and get their thoughts on it, but they may basically tell you to take to an authorised dealer.
I would suspect it it is the head gasket, but I don't know for sure.

I mentioned on the other forum that this looks like it’s coming from the spark plug well drain hole. If so that would mean the inner valve cover gasket the part that seals around the spark plug wells (holes) is leaking.

I took some time to ride it more ... checked again: oil on the same spot; like coming from nowhere. If it came from around the spark plug, shouldn't I see a trail of oil from above (from the yellow sign to the magenta)?
Unfortunately, I don't have a Triumph dealer in my area; the closest one is 600km away :-( The oil change, I did it myself and reset the service interval with TuneEcu. So this would be my first Triumph issue ...

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If you look into the spark plug hole then yes you should see oil if it’s coming from there. However there is a drain hole that comes from around the spark plug and goes through the head and comes out in the area of your red mark on each cylinder.

Today i took the bike to a mechanic i trust and he replaced the gasket. Unfortunately after he turned the engine on and left it run for a while we noticed oil was gathering in the same spot. He said it might be a small crack in the body of the cylinder head (fabrication defect? he presumed), but when i asked if the spark plug hole is in that area, well... we couldn't find it.Can you, please, attach a picture with the spark plug well hole? He showed me the same holes on a Kawasaki ZX10 that had this oil leak problem, but we could not find them on my 2015 STR.