Russell in 5" g.

It can be done. Honest guv! has anyone else tried it? I'm about to, care of a slight scale up from 3.5. Anyone else interested? Beware, the frames work out at 50" long, boiler is easy, compared with a pacific, but expensive, and scaling it up means 8" dia, perhaps a trip into duplex steel... A new & interesting build.

Boiler is indeed 8 inches, give or take. A venture into castellated joints, and rollers. 4 mil thickness.

Are you seriously thinking of building an 8" dia duplex stainless steel boiler 4 mm thick with a castellated joint? I would not recommend this combination in any circumstances, and I would consult a boiler inspector before buying any materials. No qualified weld inspector will approve a castellated joint in any mild or stainless steel boiler. 4mm thick is very light for a stainless steel boiler design 8" dia unless WP is very low. In mild steel a 4 mm thick 8" dia boiler design would not likely be approved due to insufficient corrosion allowance over time, with the light stock.

Are you talking about the Welsh Highland Rwy 'Russell' narrow gauge locomotive?

Is this your first locomotive?

JD

I would query that for an 8" outside diameter x 4mm thick boiler the working pressure would have to be 'very low' A quick check, assuming a working pressure of 120psi, gives a hoop stress in the barrel of only 1,31 tons per square inch (hoop stress = pressure x inside diameter / twice thickness).

Even if a relatively low grade of stainless was to be used, say of an ultimate tensile of 28 tons per square inch, this would give a factor of safety of 21.4:1 based on ultimate.

There are other factors to consider though, such as the widespread opposition in the UK to stainless steel boilers whereas on the continent boilers tig welded from 316L stainless seem to be commonly accepted - even preferred.

Almost a week gone by and no word from OP.

I hope there wasn't a build attempt and a resulting mishap....

JD