I'd always thought the reason for thicker plates was the unsupported areas
you tend to get with the back, throat & smokebox tubeplates. A backplate can
have big (ish) areas where there are no stays, the throatplate needs to be
thick if using the Curly method of a butted join (I use a full flanged
throatplate and think I'd be ok with 16swg but still use 13!). I'm sure what
I've built in this sort of model size (Tich, Invicta & Scotsman) all use
16swg for all inner firebox plates? Not got a model or their drawings to
hand though! For my tubes, I use 10mm microbore household plumbers pipe, not
sure of its swg? A 10m roll is very cheap (for copper!).

----- Original Message -----
From: "John Baguley" <baggo@...>
To: <halfinchlivesteamforum@...>
Sent: Monday, May 12, 2008 8:54 AM
Subject: [Half inch live steam forum] Re: question on boiler material


> Hi Charles,
>
> I am not sure if there is actually a spec laid down over here for
> metal thickness versus boiler size. However, the 'standard' seems to be :
>
> barrel, wrappers, and crown stays - 16swg (1.6mm)
> Smokebox tubeplate, throat plate, backhead, firebox plates - 13swg (2.5mm)
> Tubes - 18swg or 20swg (0.9/1.2mm)
>
> For a small boiler you can get away with 16swg for the throat plate,
> firebox tubeplate and firebox door plate. Makes flanging the plates
> much easier as well. I've read that the only reason for using 13swg
> for the backhead was that LBSC used to thread his fittings straight
> into the copper without using bushes and the thicker metal gives more
> thread. I would not recommend that practice nowadays though!
>
> John