The 500 is recommended for the S50, beam engine and steam hammer.
The 501 is recommended for the 10V and 10H.
The 504 is recommended for the Double Ten and engines up to 1 x 1. Stuart claim it will steam the No.9 "for demonstration purposes" (and, we would add, providing the No.9 is decently built!).

These boilers tend not to give much trouble - we hydraulic test them to 90 psi for 20 minutes and have never had one fail. We have had a gauge glass fail on a 500 under steam, which was quite exciting.

The backhead fittings (water gauge top and bottom fittings, pressure gauge syphon, pump non-return valve) are all fitted with aluminium washers. These corrode badly and will usually break up if the fittings are removed. They are supplied in a range of thicknesses, enabling the fitting to be tightened down and still end up facing the right way. Many boilers come in with a whole range of hermetite, PTFE tape, fibre washers and worse grunged into the fitting threads!

The small water gauges are not fitted with a blowdown cock, which rather restricts their usefulness and means more frequent cleaning out with a piece of wire is required between steamings. The rodding plugs are fitted with aluminium washers and can get very tight - care is required when removing them (best done hot but NOT under pressure) otherwise they shear off.

The boiler pictured is a spirit-fired 501