The 38s have a one-piece cast engine bed or frame which is as near to indestructible as makes no difference. The parts that wear, like the valve chest and cylinder liners, the pedestal liners, wedges, compo beam bushes and firebox slides can all be replaced, and have been, regularly. The claim that we only did "patch jobs" is not true. When I was at Eveleigh '01 was stopped every Christmas for its boiler inspection and heavy overhaul. Amongst other things we put in a new front end, new tubes and superheater elements, new bullrings, new pony truck wheel sets, new coupled wheel tyres, new grates, carriers and rockers, new tender wheelsets, new tender slope sheet doubler plate, and so on. Every other major component was removed and overhauled. Hardly a "patch job".
And in between doing all that we managed to rebuild 3830, look after 3112, and do as best we could with Bob - SMR18.
There's an interesting story behind the replacement of the tender wheels, but wary of starting a flame war I'll leave that alone.
Cheers,
Mark.