Stainless to low carbon, 309 or fab the other part from stainless?

One of the pieces I have lined up is a cooling system part for a tractor (the Peterbilt) $450+ for a new one. Got a piece of SS exhaust (right diameter, little heavier gauge) bent to shape. I need to put two different sized bungs on it. The joint design I can handle, it's the materials I'm wondering about.

The old one was steel to steel and looks like a lawn sprinkler (unreal how many holes there were once the paint was knocked off). The bungs are solid, two different sizes, use NPT for threads. Well, I don't have NPT taps and I don't have a tool for my lathe to make new ones (no cobalt blanks long enough and no insert bars small enough).

The tractor uses anti-corrosive coolant, how much of an issue would it be to run the carbon steel bungs?

Also, would 309 be the best choice?

I'm going to cut the bungs off, wire wheel, chuck in the lathe to clean off the old welds, was thinking about cooking them for a bit with a torch or toaster to make sure all the oil/coolant is burned out.

OR

Anyone have a source for SS bungs premade that I could just order in for less money than two NPT taps would cost?

Lots of (commercial) plumbing supply places carry stainless pipe fittings. Get a coupler, cut it in two, and you have two stainless NPT bungs. They're not even much money. In my part of the world it's Crane who has the most stainless fittings in stock.

Even if I had the taps, I would not want to tap NPT threads over 3/8" in stainless. And I work with it all day long. And neither would I want to mess up my nice SS exhaust all messed up with some carbon crap bungs.

PS - what size pipe threads?

In all honesty, I don't know the size, I'm sure I could make them both the same and just step one down to the smaller hose. Remember, it's coolant, not exhaust, so the heat factor will be lower.

I'd cut them if I had the tooling on hand, though I despise turning inside threads on SS, and love turning NPT even less (what a bitch to set up)

I do as little plumbing as possible, wasn't aware SS fittings would work... they'd likely be a different grade, would 309 work, or should I go 308?

I believe they're around 3/4 and just over 1/2"

If you have a lathe, just get a nice stainless pipe coupler and cut it in half. I do that all the time for stainless pipe fittings. Way cheaper than making them from bar, and much faster. You can turn the OD to fit whatever hole you can bore in your tubing. Or you can fishmouth them if that would work better. Thanks to China there are now real cheap stainless pipe fittings, that you can make into all kinds of things. Here is a stainless manifold I knocked together from a bunch of pipe nipples for a fraction of what a pre-made unit would cost.

Attachment 7408Attachment 7410Attachment 7409

Took too long to post. :D You can get fittings in 304 or 316, at most pipe supply places.

The ones on there are female... though after reading this, I'm wondering to myself; 'self, these are bungs to which fittings screw into, to which hoses clamp to. Why not just weld in tube the right diameter and put the hose directly on that?'

Any good online sources for these you'd recommend? Pretty slim pickins around here for off the shelf anything without driving half hour-45 minutes

Shipping would probably cost more than the part. Sure if it's just hose fittings, use some pipe or tube. Pipe would let you turn some barbs for better retention.

http://www.amazon.com/dp/B004XEKNBA

Like Rambozo said, 304 and 316 are available. Locally they're mostly 316 for me. Either 316L or 308 filler would be fine. Whatever you have, assuming the exhaust was made of 304.

If you're welding steel to stainless, then 309 is technically correct. It has extra chrome & nickle to make up for the metallurgical soup you make joining mild steel to stainless (turning your soup into "not quite stainless but better than stainless dissolved in mild steel"). But in real life, 308 will work if you have some already; your soup will just not be quite a stainless.

Bungs down to 1/2" are common in the plumbing supply places, which is the only reason I asked. Smaller might be hard to find. Remember pipe sizes are nowhere near what you actually measure. (1/8" pipe threads are about 3/8" OD for example).

The shop that made the on-the-road replacement without the fittings ran a bead around each end instead of rolling it. I was going to do the same on the larger pipe, the smaller... was thinking I could build up and turn down (harder to get a smooth bead around that smaller diameter). Guess it will depend on what I can find locally.

Seems like a lot of work compared to turning down a bit of pipe. Plus turning barbs is kind of fun.
Alternatively back to the fittings, you can get a barb-to-whatever and take off the whatever and weld it on. Same plumbing supply place. (Or some gasket / steam places, and especially oilfield places come to think of it.)

The barb is just a retaining mechanism for the hose. It does nothing for sealing. One should be plenty if you put your hose clamp ahead of it.

Main concern with evenness of the barb is stress on the hose. It does present little in the way of sealing, but if it protrudes or has pockets, etc. it could contribute to a later failure. Good point on using thicker and turning it down, I'll have to look into that option.