bolts are sized by diameter of thread not head ,it will be 8mm bolt with 10mm flange head ,if part of thread is sticking proud you can weld a washer to it then a nut to the washer ,the heat also helps free it , failing that you drill out and use an easy out /stud extractor drilling centrally is the key, if you have to drill all the stud out you can then drill oversize and helicoil it back to size