My £0.02

Assuming the number plate is not painted, just use silver solder. Then guaranteed secure at all smoke box temps. I once saw a comsol soldered chimney band ping off due to melting. Engine was working pretty hard though….. I have also riveted this kind of stuff in place with fine 3/64″ brass rivets then carefully countersunk and dressed off a the front. Then paint it and nothing shows.

As Paravoz  use rivets fit and forget.

My fowler ones are screwed on with brass roundheads as per original. I spent some time bending the photo etch plates to follow the curve of the door so the screws are not under any real load. The full size were cast with a suitable compound curve to the back.

Dave it is not the blast of steam from the blast nozzle that heats the Smokebox (not firebox that is the other end of the engine) as that is aimed straight up the chimney but the flue gasses coming out the boiler tubes though there is usually a baffle plate inside the door to help keep the heat away.

Loaded slate trains on Ffestiniog rly were helped by gravity, the locos hauled empties back. I reckon they work harder now as they have to pull heavy passenger trains uphill.

Steam in the smoke box will be at relatively low temperature as it has expanded in the cylinders. Even before expansion it isn’t hot enough to get a smoke box visibly red hot.

On 17 February 2025 at 15:01 duncan webster 1 Said:

Loaded slate trains on Ffestiniog rly were helped by gravity, the locos hauled empties back. I reckon they work harder now as they have to pull heavy passenger trains uphill.

True!!!

Steam in the smoke box will be at relatively low temperature as it has expanded in the cylinders. Even before expansion it isn’t hot enough to get a smoke box visibly red hot.

It’s exhaust gas from the tubes that’s the source of heat, not steam.  Locomotive tube boilers are very inefficient in that most of the heat generated by the fire escapes up the chimney.   The boiler, tubes and firebox are all kept at boiling water temperature, so no danger of them reaching red-heat unless the water leaks!   Not so the gases coming out of the tubes; they are at much higher temperature, and, particularly when forced, contain fragments of burning coal that blast into the smokebox.  The amount of heat and temperature increases when exhaust steam is used to force the draught, burning coal in the firebox faster & hotter by pulling more air through the grate.

I’m sure Brian is right that a loose smoke-box cover greatly worsens the problem in full size by providing the air needed for unburned coal to burn completely inside the smoke-box.  That really would get things hot!

None of my engineering books gives the exhaust temperature at tube output end for reciprocating steam locos.   A Marine engineering book quotes 400-500°F at battleship funnel-tops, bad for lookouts on a nearby mast!  Doesn’t say what the gas temperature is as it leaves the boilers a 100′ below, I guess double or more.

That said, I suggest red-hot smoke-boxes are much more likely in hard-driven full-size locos than in models. Be surprised if Comsol could get too hot because small boilers lose heat faster than big ones, and there’s not much space or time for coal to burn inside model smoke-boxes, plus, as Brian says, models are unlikely to let air in due to poor fitting doors.

Does anyone know how hot model smoke-boxes get?  Could be easily be measured.  Hard driving a heavily loaded model loco up an incline whilst pointing an IR thermometer at the smoke-box would soon reveal if Comsol is in danger of softening.   Or, in the absence of temperature facts, note that screwing and riveting entirely avoids the risk of a Comsol failure.

Dave

Loco boilers, both model and full size, tend to have efficiency around 70% unless they are overloaded. Jim Ewins did lots of tests on boilers. The results need to be taken with care as its difficult to measure gas temperature close to a source of radiant heat. You need a suction pyrometer, which is a bit of overkill

There is also the results of tests carried out at a South African University in ME somewhere.