Ian,

The grate support ring can only be fitted from underneath with the boiler suspended from a crane, so when it's in, it stays in. The ring is quite simply that - a ring which rests on four brackets rivetted to the foundation ring, and which in turn supports the cast grate segments.

The grate is usually fitted through the firehole, but if the boiler is suspended it *can* be fitted from underneath. The grate consists of cast iron segments, four shaped a bit like a capital "D" which interlock with the curved edge against the firebox inner wrapper, leaving a square aperture in the centre into which a number of straight segments are fitted. The segments weigh a fair amount (the larger "D"-shaped ones perhaps a hundredweight, and the straight ones perhaps 3/4cwt. Manoeuvring them through the firehole and into place is not easy, especially since the firehole is only just large enough to admit your head and shoulders!

As far as cleaning the grate is concerned, clinker and large crud has to come out through the firehole with a shovel, whilst ash and small debris can be raked out of the ashpan through the dampler opening at the rear of the superstructure.

I don't think any breakdown cranes (certainly not British ones) ever had a tank directly under the boiler, since this would cause all sorts of problems. Many of the early ones, like GWR 2, have a cast steel slab on which the boiler sits, which forms part of the tailweight. When boilers got bigger, the later cranes (such as the 1939/40 45-tonners like S1560, whcih is the source of my "new" boiler) had a steel plate an inch or two thick in order to keep the overall height within gauge. The ashpan opening is always at the back of the superstructure. GWR 2 can accomodate the larger boiler (just) since it was built to a much larger loading gauge.