Rebonjour Squid.. only have vague memories of ty yams it was a while ago ;..does it have a gasket (copper) if so you can anneal it by heating it on a gas ring and then quenching it in cold water make sure you support it and dont melt it. Otherwise it's (from memory ) 4 studs...I have a memory of m8 threads with 13 AF nuts & washers...studs pass through the barrel and clamp the whole lot down,,,if this is the case and you havn't disturbed the base gasket why not do it by feel .....even pressure working diagonally .lubricated threads work better than dry (up to 60% more efficient).. Provided you don't overtighten it the worse thing to happen would be an air leak in the primary compression or at the head gasket...so be sure you have kill button handy...cant remember if it was these bikes but I can remember "lapping" jointing face of heads with valve grinding past and thick pane of glass..mention this in case head is "warped"....hope this encourages you also hope to see you at Vielles Tiges I think its 4/5 Sept Claude & charles Coutard run this excellent trial at Aurel ..not too far for you they prefer older bikes but also accept old riders like me!. Cordialment Chewy (or Lucky Luke en France)