Replacing sillcock

How would I replace this frost-free sillcock with a new one? Should it be soldered?

The pipe on the left of this pic is the sillcock.

http://a.imageshack.us/img838/499/sillcock.jpg

It depends on whether your new sill cock has a pipe thread on the end or is sweat.

If it's sweat, I would cut the pipe to the right of the subject matter in that photo and solder a 1/2 inch ball valve in between the supply pipe and the sill cock. The ball valve would allow you to shut the water off to the sill cock to replace the washer in it (or whatever other servicing needs to be done to the sillcock).

Don't believe anyone who tells you to take a ball valve apart before soldering it into place... you're way more likely to have a leak in the ball valve by taking it apart prior to soldering it than heating it up to solder it in place. As long as the ball valve is OPEN when you solder it, you should have no problems. (Personally, I like to remove the handle from ball valves before soldering them just to prevent my burning the plastic on the handle with the torch.

The trick is to make sure the ball valve is open when you solder it, and ensure that the air inside the piping has a path by which it can expand other than through the joint you're trying to solder. So, in your case both the ball valve and the sillcock should be wide open when you're soldering. Also, don't close either the ball valve or the sillcock until they've cooled down to near ambient temperatures.

No, you don't have to solder to install a new sillcock, but that's the most reliable way of making a plumbing connection that I know of.

call a plumber.

I would recommend a "shark bite" fitting here but, it appears your going from 3/4" to 1/2". I don't know for sure but, you could see if shark bites come in the size adapter variety.

Use a backup wrench on that female adapter and unscrew the hosebibb from it.
Then thread in a new one with the same length.
Like I said use a back up wrench because if you allow that compression joint on the other end of the female adapter to turn it will leak.

I found a new sillcock by the same brand (woodford) and the threads looked the same. The problem is the new one is 3/4" shorter than the old one. I thought there might be enough play for it to work but there isn't and it just won't reach.

I guess I will have to try soldering.

Alleluia!

The sooner newbies learn to solder, the fewer compression fittings there are in this world, and that's a blessing for all concerned.

Solder a ball valve upstream of the new sillcock so that you can shut off water to the sillcock to do any repairs to it without shutting off the water to your whole house.

Before buying the ball valve, check that it has a packing nut. That's because lots of ball valves are now being made without packing nuts. It seems that manufacturers are believing their own advertising and are operating under the misguided belief that their valves are made so well that they will never leak from the stem. I have more faith in a packing nut than what I'm told in the advertising.
The packing nut is what this diagram calls a "Thrust Washer & Nut":



You can see a packing nut on this ball valve right under the handle. The packing nut on every ball valve you're likely to see will be right under the handle like you see here:



If you don't see the flats of a hex shaped packing nut, then the valve doesn't have a packing nut, and I wouldn't buy it.

Locate the ball valve a few feet or so upstream of the sillcock so that when you next need to replace that sillcock you'll have enough empty pipe between it and the ball valve that you won't have any problem getting up to soldering temperatures.