Plumbing - How to calculate nipple length

I'm replacing the natural gas shutoff valve to my water heater with a 1/2" ball valve with a red handle. The existing valve is 2.0" long; the replacement one is 2-1/4", which means that one of the iron nipples fitting into it must be shortened 1/4". Problem is, I don't know if the existing ones are standard lengths or one is custom cut and threaded.
The visible length of the nipple going into the existing valve is 4-9/16". The visible length of the nipple between the output of the existing valve and the union just in front of the heater is 2.50". The actual lengths of these nipples are longer by the amounts that fit into the threads of the valve and the adjacent fittings. How much should I increase the length of a nipple over the distance between the end of a fitting and valve to account for the threads that go into the fittings?
Standard nipples sold in Home Depot and Lowe's vary in length by 1/2". That's why I think one must be custom cut and threaded.
One other concern is the lace of slack between the input gas supply and the heater. This suggests that those nipples must be exact lengths so when tightened there isn't a leak.
The final concern is using a brass valve with iron nipples. Is this okay with natural gas?
Here's a photo of the setup:
http://tinyurl.com/pyppcg
Thanks,
Ray
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Thanks, that's a very good photo. I don't really think the extra quarter inch is going to hurt you that much. The pipes by the water heater can be wrangled around, a little. The brass valve may also take threads in more deeply than the steel valve.
Brass valve on steel pipe is fine. Remember to use thread sealant like Rectorseal #5 non hardening on the male (external) threads only. It's not good to use teflon tape cause pieces may flake off and get into the gas valve.
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There's usually a bit of leeway in the pipes, so a small difference can be accommodated by just moving the pipe a bit. It won't bend, it'll just move out of the way.
If you need a specific length, the big box stores usually have a threading machine in the plumbing aisle and the employee will cut a nipple to length and thread it for you for a price.
R
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Why not simplify your installation by eliminating some of that hard line and avoid the "hard line to hard line" issues
Replace the valve, come out of it with one of the nipples in your current installation, follow it with a elbow and nipple down to a couple Tee's ... add a flare male & use a SS flex line into the water heater valve.
You probably have enough fittings in the current installation to do all the rework.....just need two male flare to pipe adapters and a SS flex line.
You'll be able to eliminate the union and not have worry about nipple lengths or thread make up.
cheers Bob
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Eliminating the union isn't a good idea. It's there for a purpose: to make replacing the appliance easier. In some jurisdictions, it's required by code.
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Flex has swivel ends, so the flex functions as a union. No problem. Sort of no problem, that is. Unless you're trying to meet code.
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On May 19, 4:14am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

reread my post
if the jursidiction allows flex lines (SS or enameled) my suggested changes will make replacing the w/h easier, safer & better.
The flex will serve the same function as the union and there wont be any new pipe joints to break or make.
Right now he's totally hard plumbed, not a great situation unless local code requires it. :(
If it was my installation, I re-plumb it, so both valves were independent and at the "end" of their respective pipe runs.
Do it right, do it once.
cheers Bob
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On May 19, 4:14am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Not in SoCal...flex lines are allowed for water heater, stove & furnace
btw the code is a minimum standard....
can anyone give a technical reason why a flex line would be ok for a stove or bbq but not for a water heater?
cheers Bob
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Flex line probably doesn't meet code for water heaters. Necessary for stoves, as they are pulled out for cleaning. WH just sits in one place. That said, when I moved into my place, my furnace was supplied with flex line. Worked fine for many years. I put in a 90+ efficiency furnace, and used black iron to pipe it.
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On May 19, 2:52am, snipped-for-privacy@invalid.com wrote:

Or in the worst case, if you need it to the exact length for this or any other application, HD will cut and thread their stock black pipe to any length.
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It turns out that Home Depot cannot thread nipples as short as the 4-1/2" I needed. The outside diameter isn't the problem; it's the length that must be inserted into the threader. (I don't know what that minimum length is.)
The new valve is actually 3/8" (not 1/4") longer than the old. So by playing with the nipples on both side of the valve, and not overtightening, I was able to get a good fits using standard nipples, which vary in 1/2" increments.
Thanks to all for your suggestions.
Ray
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We had complete confidence in you. You made us proud.
(Hope you sealed the threads with Rectorseal, or something similar.)
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they thread those 1.5 inch nipples where the threads on both ends nearly touch? I never knew that.
Of course the factory thread-cutters have internal jaws. I used to cut /thread pipe on an old Ridgid machine at a TruValu.
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Ray K wrote:

Uh, I think the only come in one size - but different colors. See here:
http://www.bodyperks.com/
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