Where to start looking ...
Usually, such a thing is caused by excessive pressure built-up in the crankcase. This is caused by combustion gases somehow entering the crankcase. And this can happen via two ways: past the piston and past valve stem seals.
Just assuming your piston is OK, did you install genuine Yamaha valve stem seals or the sh*tty aftermarket stuff? Are they still correctly located?
Also, rethreading the lower bolt hole of the oil filter cover is tricky: the oil to the crankshaft passes along the bolt. If you thread in the wde paryt of the hole and screw a bolt in there the passage is wholly or partly blocked.....
So, this is also a thing to make sure.
And, last but should be first: is your oil level not too high. Checking is close to sorcery and therefore liable to go wrong with overfilling as result. (Run engine under load for at least 15 mins, let idle for 1 minute, switch of and immediately check level, this is the only way to do it correctly).
Auke
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