Hi Ian,
I confess that I hadn't thought of turning between centres, something to consider if I need to make any bigger rings in the future. I am quite lucky in that many of the locos that I intend to make have the same boiler or smaller so my mandrel will work for all of them. The smaller ones will just need the ring tightening which I can do by hand. The N10 and no doubt most of the other tank engines only have half a boiler at the cab end so turning between centres isn't and option.
You must be much better or patient with a file than I am, my use of a mandrel was to get away from having to file (not very accurately in my case) the remaining quarter of the ring. But as ever when we start to discuss things I have just remembered that in some of my more vintage versions of kits that now reside with Gladiator there are some solid brass plain and machined tubes. While I may struggle to use these on the loco when I get to building them I realise that they would be ideal for use as mandrels
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Not very clear in the photos being all brass the two that are turned actually have very nicely turned transition ring which would alleviate the need for DIY assuming of course that you can manage to attach all the boiler fittings to the heavy heat sink of a tube without melting them.
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For anyone who is remotely interested, they are from the following 'kits' (left to right):
George Norton J21
07 Models N8/9 - 07 was the forerunner of some of the George Norton Range, so real ancient history.
London Road Models G5
