Hi, Max Hemmens Caton engine threads, are just slightly smaller that to days std of 1/4 by 40 tpi, don't try and force a todays std union nut, or you will srip the threads,!!!
however I do have a caton steam valve with one union, that is available , if you want it, ( no charge) for a member , Roy: roy.burton1@ntlworld.com.