Max

Hi there all I’ve had a Derbi senda 50 for a couple months now and recently bored it out to 70cc I have changed the carb out and was running fine no issues what’s so ever, I soon then put a new exhaust on from technigas and I moved a single coolant pipe out the way and bike was having cooling issues. I bleeded the cooling system and it was still over heating now I’m not sure what really was the cause of it but I think it may be lack of oil causing lack of lubricant but after 2 days of the fitment of the exhaust the top end blew up. As in the bottom compression ring on the piston is stuck into the bottom of its casing this was caused by a heat seizure. I need to rebuild it now with another 70cc bore kit any advice of how to avoid this I’m pretty certain it’s not enough oil to the engine as my cooling system is fine, if it is oil is it worth running pre mix instead of injection in the future or can I run injection and pre mix slightly ?