[Road_Clam]

2019 EXC 300 TPI

I'm reading most are around 180-220 PSI, so I assume mine has had it.

If I'm going to do the top end. I will replace the head, is a high compression head a good idea? Or better stick with medium compression head?

So after reading your other "blink code" thread and this one my strong suggestion is to STOP any further replacing parts are your just running around in circles guessing and wasting money. To answer your question in this thread 90 psi is absolutely a blown top end. Most all the 300's are up around 175 psi with a healthy top end. You FIRST need to disassemble your top end for a close inspection. Also more facts and history on the bike, did you buy this bike new or used ? If used previous owner history ? Did the engine ever run well ?

 

[Road_Clam]

double post, ignore

 

[Road_Clam]

Its interesting that your piston has almost no carbon on the surface. This tells me the previous owner had recently replaced the piston. Do you have the oem service manual ? If not you need to get one. The s/m calls out the piston and cylinder working limits. I suggest you take your cylinder to a shop and have them precision measure the bore. Your best course of action should be to buy a new oem piston (Wossners are also very good quality) and send both the new piston and cylinder to be re-plated. Then you know for fact you have in spec piston to cylinder clearence.