Paul you're right ! There was a decal (or rubber stamp) on the right valve cover between # 2 and # 4 sparkplugs. I took a pic of it when i dismantled a '57 Imperial which was wrecked since 1967 so all parts of this car were original. But the decal was in bad condition and i couldn't read the text which began with "Use only .." i believe. There was also a drawing of the end of the sparkplug which shows the lenght of the threads. You can see the pic at http://www.ifrance.com/c-i-f/spk.htm -- Philippe COURANT (Pau, France) Imperial 57 Crown convertible Buick 58 Roadmaster sedan - American Car Club de France (ACCF) : http://www.accf.com - Chrysler Imperial France : http://www.ifrance.com/c-i-f - Cadillac " Standard of Excellence " : http://www.ifrance.com/accf-cad - SportsCars : http://www.ifrance.com/accf-sprtcar