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Rookie steam modelling - valves behind Prairie cab steps - what are they?

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A slightly red-faced enquiry... I'm new to steam loco modelling after a good few years of chopping RTR 4mm diesels*, hence I have some questions (and a request for pics) to help me out with my latest project.


I'm modifying a Hornbix 61xx into a (hopefully) passable representation of a 5101, including adding the lamp-iron recess in the bunker rear wall below the coal fender. Aside from the usual smokebox door handle, handrails and lamp irons, I'm looking to model some of the "kettle workings" on this one, and I've obtained a few photos including two of my chosen loco - 4129 in late-crest lined green.


Sooo... what are the valves behind each cab footstep please? I have a vague idea that one is the live steam injector and the other is a water injector, but I'm guessing - I have to admit that I'm not familiar with the intricacies of steam loco construction and operation; I just like them rather a lot. I'd rather like to model those valves and their associated pipework, in something like a representative fashion.


Further questions, if I may:


(1) Are these parts available in 4mm, and if so, made by whom? and

(2) Does anyone have any detail photos of each side's valves and pipework please? The photos I've found to date are atmospheric but of their time, hence picking-out the finer details isn't easy.


As always, the benefit of others' knowledge and experience will be much appreciated! Over to you, folks :-)


*I'm still modelling diesels too, but an increasing proportion of them seem to have hydraulic transmission!

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Both are injectors, one is live steam only and the other is the exhaust steam injector which also has a live steam facility. They both inject cold water from the tanks into the boiler via the clack valves. The clack valve is a one way valve to stop the steam escaping, on GW most engines have top feed and the clacks are mounted either side of the safety valve.


The injectors operate by turning on the water supply and then turning on the steam until the injector picks up. IE the water stops coming out of the pipe behind the step, the injectors are heard to sing whilst working. When the boiler is full the steam is shut off and the water allowed to run for a bit to cool them. Injectors will not normally work if the water is to hot.


There have been some embarrassing incedents over the years, where helpful tanker drivers have supplied hot water for locos on mainline steam runs.





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These two might help, but I think the first is a (small prairie) preserved adhoc:


Your best bet for good pics would be to commission something from 6106 from this lot:
I don't think anything is available in 4mm except as part of a Finney kit. (Or SEF?)
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Hi All,


The injectors on a prairie are both live steam. On the large prairie types there is an 8mm and a 10mm. I can't remember which way round they go but externally it won't really matter for modelling purposes. An exhaust injector is quite a big thing and wouldn't fit in the space provided on the prairies due to the balance pipe and wheels getting in the way...


The way I did mine is to chop about the Gibson pannier injectors and if you look at my Little Didcot thread (link below) you will see my complete project to produce No. 4144.


I hope this helps!


All the best,



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Crikey, I appreciate this community - overnight results are so rare these days... Thanks to all who've posted! Yes, it all helps :-)


Killybegs, those pics are perfect - thank you. I hope you don't mind, but I've copied them for my own use, so I can enlarge them a little to admire while the soldering iron burns away yet more parts of my fingertips.


Also, thanks Castle for the tip regarding the Gibson pannier tank injectors - sounds like a plan, I'll follow that up.


Right, back to 4129 then; should be done by 2016 at my current rate of progress...

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