I'm replacing a cooktop and I'm wondering about materials for fittings past the cutoff valve.

Currently there's a 1" black pipe coming into the house under the cabinet, a brass shut-off valve, a brass nipple (1" MIP to flare), and a metal flex pipe going to the regulator on the cooktop.

The new cooktop doesn't come with a nipple for attaching the regulator to the cooktop (thanks, guys!), a 3/8" close nipple is specified, but they don't specify what material. The material in the cooktop appears to be yellow brass. The regulator is 1/2" FIP and is light, either galvanized or aluminum. So they specify to use a reducer.

No problem there, but I'm wondering if I should use brass fittings here or galvanized, or black pipe.

On the other side of the regulator I'll use stainless flare nipple to a new stainless flex tube, but I with needing to adapt a 3/8" flare connector to 1" valve. Again no problem, I'll remove the existing brass nipple with one that's appropriately sized, whether I need a reducing union or not, but again I'm wondering about materials here. Stick with brass?

Galvanized is not recommended and neither is copper. Copper because of corrosion and galvanized because the coating can come off and clog jets. Aluminum and black pipe are used all the time. The flex line approved for gas is OK too.

Plastic coated brass or stainless steel is OK.

Remember yellow Teflon tape for pipe threads or an appropriate pipe dope.

Thanks -- what threw me off a bit is the use of brass within the cooktop itself. For stuff like this I'm used to a laboratory setting where we've got pipe stock, and if we were out I'd just be ordering from Grainger or Murray. Doing it at home I tend to overthink.

When you say stainless flex tube are you thinking of CSST? Corrugated Stainless Steel Tubing (CSST) for Gas Distribution (http://www.toolbase.org/Technology-Inventory/Plumbing/corrugated-stainless-steel-tubing)
Most building codes as of 2006 no longer allow plain stainless corregated tubing, it must be coated.

I come from the same environment and you have the right to question. I guess, in part, looks are deceiving. Brass parts really aren't BRASS parts.

This hardware store seems to have it right. Find Gas Connector Fittings and other Gas Supply Lines at Aubuchon Hardware (http://plumbing.hardwarestore.com/52-296-gas-connector-fittings.aspx)

Construction apparently also plays a part. Seemed tubing is different than seamless.

Aluminum tubing is used for pilots, but they will use brass ferrules, but might use a different body.

We know that corrosion also is enhanced just by having dissimilar metals in contact.

It might be difficult finding these specialized parts at the local mom and pop hardware store. There is good reason to discourage messing with gas lines.

Some codes even discourage the use of flexible gas lines.