Hi Jem, I think the first thing to do is decide what you intend to do with the engine. If you intend to keep it in a showcase then your choice of gauge is more or less determined by the space available for your showcase and the limitations of your equipment (including lifting equipment). If you intend to run the engine on a track then 5" is to be recommended as 31/2" is really too small to do serious work, more difficult to fire and not elligible for the GL5 ground level events. 71/4" is often too large for most peoples equipment and good tracks are few and far between. Pleased to hear this question being asked before committing to cutting metal - it never fails to amaze me how often people set off building a model without giving sufficient thought to issues such as this. Good luck, Barrie Purslow, WarringtonUK 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.