Barrie I'm sure conventional wisdom says leave well alone. The depth is pretty minimal but the potential damage pretty catastrophic if you get it wrong. If you look in the tubes on any operational loco you will see very similar. This is certainly true at our club. Treat it like paint - it protects the surface of the flues when cleaning conventionally. If it is carbonm and it probably is, then heat transfer will not be retarded very much as carbon is an excellent conductor of heat. Not as good as copper, but still pretty good. Jesse in Tennessee 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.