OK Fred, which tender air tank are we talking about?
The pre-1903 trainline air reservoir that sat on the rear of the tender frame like this:
Or the smaller tender air reservoir with triple valve that sits above it?
Another view c1898:
If you are modeling anytime after about 1903, you will need just the tender air reservoir, like this:
The big train line air tank has moved to the locomotive, atop the boiler, sometimes two on the 2-8-0s. Often a tool box was added to the rear tender deck, behind the tank, as on number 66 above.
If what you need is just the smaller tender air reservoir, Precision Scale make a correct part, PSC-7510, "Westinghouse air reservoir with high speed reducing valve, C&S, 10" x 24". While I like fabricating my own parts, I do well with things that are straight and square. Round things like air tanks give me problems. If I can find a commercial part, the time saved is worth the cost (to me).
Before you spend a lot of time detailing the Bachmann tender, see if Steve can print you a tender tank in O scale. The S scale test print that he sent me is gorgeous.
By the way, what time period are you interested in?
First decade, with straight stacks?
Second decade, with "Como" stacks?
Or 1918 to early 1920s with Ridgeway stacks?
Inquiring C&S minds want to know . . .
Jim Courtney
Poulsbo, WA