I can't comment on the specific engines mentioned. I am building two 4" scale SCC Burrells using the LSM drawings and castings from John Rex in Pontefract. The drawings are poor, many items and dimensions not detailed and many errors ranging from the minor to show stoppers. The castings from John Rex are very good, they need a fair amount of fettling and are undersize in places, but machine beautifully with no hard spots. Rather than write impenetable paragraphs I'll make a few notes.
Due to the drawing errors, and a desire to change the design, I am modelling the whole engine in 3D CAD. I'm idle and only expect to make parts once, and correclly. I'd rather spend time getting the design right than waste time and material in the workshop. I am building in a rather haphazard order, but I expect parts made years apart to fit together, so proper engineering drawings are a given. Another reason for modelling the design is material availability. While much imperial material is still available an exception is sheet and plate, it's only metric. I had to do some careful redesign when changing the hornplates from 1/4" to 6mm sheet.
I bought all the castings in several tranches over a period of two years. That means I have the castings available as they're often not in stock at the suppliers. On the downside I now have some castings I'm never going to use, and I've binned a fair few castings after machining as not up to standard and/or not prototypical. Of course what is prototypical is a moot point. The fullsize engines were hand built and are all different if only in detail.
I would think it is rare to find a copper boiler in 4" scale on the grounds of cost, strength and working pressure. My steel boilers will run at 170psi, the same as the fullsize engine.
Nobody has mentioned machinery. To build a large engine you need commensurate machines; you're not going to build a 4" engine on a mini lathe! It's not just the weight of the engine and parts; I'm beginning to find it a struggle moving rotary tables, dividing heads and lathe accessories.
One thing you need to decide is what you're going to make and what will be bought. Will you buy ready rolled and welded rims, laser cut spokes and what about the gears? Do you have facilities for machining large spur gears, bevel gears and worms and worm wheels? If these items need to be bought look carefully to see if the supplier lists them or do they only supply castings?
Being a fully paid up member of the awkward squad I want to make everything myself; to do that you need more than the average machinery set and a lot of time. The only thing I haven't made are the steel boilers and a few, mostly hidden, fasteners. I am making all of the external BSF bolts, studs and nuts so I can get the correct pre-war proportions.
I would also commend TractionTalk. There are some excellent build diaries and you get a lot of help from fellow builders and fullsize owners.
Given you're in Norfolk I would recommend the Model Engineers day at the Forncett Steam Museum, a few miles south of Norwich, on Sunday the 6th of October. There should be a number of model engines on display and in steam. There will also be work in progress on display as well. I'll even promise not to bore the pants off you!
A 4" scale engine is big:
Andrew
Edited By Andrew Johnston on 05/09/2019 11:53:55