For fresh water use, am I stupid to use the less corrosion resistant 304 stainless steel for making some cooling water plumbing fittings? I can't get 316 locally.

Use the 304. For what you are using it for it is more than adequate.

You using the fittings on your boat or for home use?

304 will do just fine in fresh water. You may see some tiny specks of surface rust appear, but it won't hurt anything.
The major difference between 304 and 316 is the chrome content. More chrome, the more corrosion resistant it is.

I used to build custom hospital/laboratory equipment. It was all constructed of 304/308 stainless. We had some units come back for overhaul after 10+ years in the field. All they had was minor specks of surface rust. They were originally polished when the left the company. Once repaired we would repolish them and you couldn't tell the difference.

Fittings are for the boat - predominately fresh water. Might take one trip per year down to the coast.

304 will work fine for fittings. If you have access to a company that does electropolishing I would recommend having that done as the process will improve the corrosion resistance.

304 will be fine. I would reccomend getting it passivated after final machining.

304:
C 0.08 max, Mn 2 max, Si 1 max, P 0.045 max, S 0.030 max, Cr 18-20, Ni 8-12.
Corrosion Resistance: Excellent atmosphere resistance; slightly better than type 302.

316:
C 0.08 max, Mn 2 max, Si 1 max, P 0.045 max, S 0.030 max, Cr 16-18, Ni 10-14, Mo 2-3.
Corrosion Resistance: Best of any standard stainless steel. Greater resistance than typ 304 to reduce acids, sea water and other corrosive media causing pitting type corrosion.


Reference: Materials Engineering, Material Selector.

example out of McMaster Carr
1" x 6' lg
303 70.58
316 82.52

mail order an use good stuff wwwmcmaster.com

Nothing like McMaster Carr!

For material selection go check this out. It's a very usefull site.

http://www.matweb.com/index.asp?ckck=1

I'm looking for 1 1/4" tubing. I'm going to make a fitting that goes inline just upstream of my seawater pump with a 1/2" angled inlet, pointing toward the pump allowing me to inject water from a garden hose to run my boat on the trailer. The inlet piping is a cluge now with all sorts of elbows, adapters, a three way valve, and enough short length of hose and clamps to stock a harware store.

I've not found any other clean way to run my boat on the hose with the TRS and transom water pickups.

If it works well mocked up I'll mount a couple of hose adapters to make it a permanent arrangement.