Rick,

Now that I had a chance to do some looking, to get back to your original questions:

"What tolerance should there be ideally between the ID of the bore and the OD of the piston?" - CN was calling for 1/16" diametral clearance between piston heads and bores. CN was also calling for repair when clearance between piston head and cylinder bore became 3/16" or more. For your purposes, this tolerance could probably be increased to 1/4" or a little more. PRR was calling for cylinders to be re-bored when variation in bore measurement reached .090" or more.

"What tolerance between the width of the rings and the width of the grooves and the space between the ends of the rings when installed?" - As new, .003" - .006" play between the ring and the groove, and 1/32" - 1/16" end gap on the rings is what I recall seeing as standard from many builders/railroads.

"Is building up worn pistons with bronze a recommended practice, as shown in this Baldwin spec?" - I would tend to shy away from this method of repair. I'm not saying this can't work, but our experience has been that it requires many "stars to align" for this repair to take and run with longevity - impeccable cleanliness of the parent material, and great skill on the part of the mechanic for the material to knit properly and without significant porosity. I recall we overhauled a couple of piston valves for a customer a few years ago that had their bull rings built up with bronze material many years ago. By the time the valves were removed from the engine and addressed on our shop floor, the bronze had broken away in many places, and the bronze that was still attached was only marginally so, with many areas beginning to lift and peel away from the OD of the bull rings. If piston/valve heads/bull rings are worn enough that it is time to repair to new tolerances, I would advocate for making new out of a solid piece of material.

Again, all this information was gathered at a glance, and there is likely more out there if you wanted a better average or "another opinion". Locomotive builder, AAR, and railroad standards will all often give you as-new tolerances, while railroad, and occasionally FRA/ICC, will often touch on worn out limits.

Hope this helps.

- Erich Armpriester
Strasburg Rail Road Mechanical Services

Niagara Piston Rings, when located in Lockport, NY a few years back, was spray welding cast iron pistons on internal combustion engines. They could bring them back to specs, and did excellent work. Not sure if the company is still offering this service?
Not sure if spray welding would be suitable in a steam situation, and not recommending it, but it might be something to research?

What would the material prep be for this vs using the bronze? Would the level of cleanliness required be similar? As for the process itself, what is the temperature required vs using the bronze?

I'm not personally knowledgeable, but a good demonstration of spray welding here (though a different part/device), in the series about rebuilding "a Stoker Engine for the famous Nashville, Chattanooga & St. Louis Railway No. 576": https://www.youtube.com/watch?v=5kcTeyw ... Fo&index=9

Scranton Yard,
The video posted by PMC is a pretty good presentation. They seemed to be going overboard on the heat, for cast, and one of the fellows did say they may have had it too hot in spots. That being said, I have never done the procedure and will bow to their experience.
For cast iron pistons, cleanliness would be key. Having poured babbitt in cast iron, it sometimes takes a lot of prep to "boil" the old grease and oil out of the grainier cast.
I have had pistons in the 7or 8-inch diameter range spray welded with cast material. They can be chucked in a lathe. A thin "skin" can be removed to get them to a better base material, then the spray welding procedure can be done on the lathe.
For required materials and heat in these cases, one should consult experts as the fellows in the video appear to have done.
In one case that I saw, in an internal combustion piston of about 7-inch diameter, the new material did not adhere at the narrowest area at the hole for the wrist pin. I am not sure what caused this?
With cast iron, heating the part is critical, then allowing it to cool slowly in a controlled environment is very, very critical.
In this area, it used to be difficult to get foundries to do "one-off" jobs. They were usually swamped with production work. I talked with a local founder recently. In the past, they were not interested in one-off or custom work. He said they would do it now. Foundries are probably happy to get the work. The same goes for brass and bronze foundries today. From that perspective, it might be better, as a long term repair, to have a new part cast, at least in the case of pistons.