When the 12 inch to the foot scale started using superheated steam, the books tell us that lubrication problems meant flat valves were unsatisfactory, and piston valves were needed. Why? I can see that it's easier to get a given port openeing with a piston valve than a flat valve in a given size, but I don't understand why lubrication makes the piston valave more desirable. Peter Chadwick Swindon ---------- Zarlink Semiconductor Limited is a Company registered in England and Wales under number 00705031 with its registered office at Cheney Manor, Swindon, Wilts SN2 2QW, England. This email is confidential and may contain information that is privileged and exempt from disclosure by law. If you have received it in error, please contact the sender immediately by return email and then delete it from your system; you should not copy it or disclose its contents to anyone. Emails are not secure and cannot be guaranteed to be error free as they can be intercepted, amended, lost or destroyed, or contain viruses. Anyone who communicates with us by email is taken to accept these risks. MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.