My 17 RR came with a csp fuel cap , when I try and tighten it it’s like the threads are pulling through when you try and get it tight ,  I’ve had it fall off a couple of times in crashes now so need to sort it , will it be the tank or cap at fault 

Csp cap is ally , petrol tank is plastic, more likely the thread in the tank is stripped.

Just because it looks good does not make it right. I would try home made gasket to fix it or the original cap before buying a tank.

Sadly not got the original cap , I’m wondering if the previous owner had fitted the wrong cap , looking into it more the caps bottoming out in the tank and pushing the threads off