Mamod steam loco - boiler

I decided to start the restoration/rebuild with the boiler, for no better reason that the replacement 32mm gauge wheelset that I want is out of stock. 

The paint was rather tatty;

So, out with the stripper, this stuff is apparently a Nitromors equivalent;

I resprayed it with VHT, Very High Temperature, black;

Then I made a big mistake. The instructions on the can tell you to bake the painted part at 200 degrees C to harden the paint, so into the oven it went. The problem is that the boiler is soft soldered, and soft solder melts between 180 and 190 degrees which I didn't know at the time. So, when I took it out of the oven I could see a couple of beads of solder around the dome insert;

All of the joints still seemed to be solid, so the next step (after taking advice on a Mamod Facebook group) was to pressure test the boiler. For that I needed an adapter which screws into a safety valve insert to connect the boiler to an air line. I also needed a way of blanking off the steam pipe under the dome. I made a blanking plug on the lathe;

This sits on the top of the steam pipe with an O ring underneath;

A spring sits on top and is compressed by the dome cover, making the seal;

I also fitted a new sight glass and O ring, and sealed the other hole with a 1/4" BSF bolt and fibre washer;

I then filled a bucket with water, attached the boiler to my track (bike) pump and placed the boiler in the water;

I then used to pump to pressurise the boiler to 40psi. This showed up a few leaks around the washers, but a turn of the spanner fixed those. There were no other leaks.

I then filled the boiler with water and again pressurised it to 40psi;

Again, there were no leaks and no deformation. So, I think I got away with my initial mistake, thankfully!

I tried it at 40psi for two reasons. Boilers should I believe be tested at 1.5 x working pressure. So if I choose to use a high pressure safety valve, which is set to 20-25psi, then I'd need to test at 37psi. The second reason is that I don't know how accurate the pump gauge is, so I decided to play safe and go slightly higher than I needed to. One thing that should be noted is that Mamods are essentially toys, and as such operate at quite low pressures. If this boiler was from anything else then I would visit my local model engineering club and get it professionally tested as I am not an expert by any means. Also, this blog is about how I work, and is not intended as a guide or a how-to.

Anyway, I've made a basic mistake in that I should have checked soft soldering temperatures before baking the paint. I've also used VHT paint, which is I'm told unnecessary as enamel or brake caliper paint will apparently do just as well. But, I've also learnt from it, and if I hadn't made the mistake I wouldn't have pressure tested the boiler. I'm confident that it will hold steam safely, so now I can think about the next step.

Paul.