We have this lovely, but older 5" gauge rebuilt Scott going through the workshop this week for a full recommissioning. It's been sitting for a good many years, and there are several known issues, and almost certainly a few that will pop up as we progress.
The known issues were firstly, the safety valves. These were old and a little past their sell by date, plus the threaded bushes in the firebox were unacceptably stripped and worn. The cylinders (it's a 2 cylinder model), were each exhausting through individual and separate blast pipes, one directed up each chimney. so as it ran you'd get a chuff up the front chimney and then a chuff from the rear! Lastly, the fittings were likely to be made from brass, and would certainly be crystalised, and need attention if not renewal.
So first up were the safety valves. It was decided to carefully ream out the bushes a little first. There wasn't an enormous amount of metal left so care was taken not to open them out too far. John then bespoke made a new tap to do the job to fit the new size of the whole, (19/32 x 26tpi), and the bushes were tapped back to a satisfactory thread.
Next up are the cylinders. The intention is to sort out the rings and packing. The loco was blowing past fairly badly when test run, so we will probably re-machine the pistons to be fitted with specially made PTFE rings, as with the right tolerances we've found these to work really well. There's no risk of cracking as might be an issue with traditional cast iron, and the longevity of PTFE is excellent. The pictures showthe pistons as they were removed from the model, with lots of very mucky and degraded packing material, ad the re-machined pistons, orange o-rings in the ring slots, and the PTFE rings ready to go in.
Here's a little video clip to show it running on it's new rings in the workshop.
Next up, and slightly oddly, each cylinder currently exhausts independently up each chimney hole. So it alternately 'chuffs' up the front and back of the chimney as it runs. We are going to remove the exhaust pipes and blast pipes, and remake the whole setup, so that the cylinders both exhaust into a new central boss, which then splits the unified blast into the two blast pipes. It will sadly almost certainly have the effect of quietening the loco's bark down, softening the blast,.... but it will be right, so it has to be done!