I have a 1975 Honda CB360 - which had a petcock leak.
I ordered a replacement from Common Motors Collective - and went to replace it - to find almost no resources on doing so. It's shockingly simple, which probably explains the lack of resources, and doesn't require many tools or a whole lot of expertise - just patience.
You will need:
Gas can and aluminum funnel
Glass jar (empty) - I had an old pickle jar it worked well
Wrench - 19mm for this petcock (yes metric)
(If you want to replace your fuel lines - as you will remove them to do this - then you'll need the new fuel line and scissors. I also recommend - since you're pulling the lines anyway - to add in-line fuel filters as they're inexpensive and can be replaced very quickly when needed. Plus they save the carburetors)
Step 1: Drain the Gas Tank
After making sure the petcock is set to closed - so gas isn't running into them - after noting which line connects to which carburetor (there are 2 on my bike) pull the fuel lines from the carburetor(s) and drain into the glass jar or gas can. Then feed the fuel line into the gas container and open the petcock. Wait while the tank drains through the lines into the container. When the tank is empty, close the petcock, and drain whatever is sitting in the curve of the fuel line.
Pull the lines from the petcock and store them in your glass jar.
(if replacing your fuel lines, cut the new lines to length to match the lines you pulled off, including the fuel filter if you are adding one)
DO NOT DRAIN GAS INTO A NON-GAS CAN CONTAINER
If, like in the photo above - your petcock is leaking too much to drain through the lines, use the glass jar to fill then dump into the gas container. Gasoline will eat through many plastics.
Step 2: Remove the Old Petcock
Using the wrench - loosen the petcock until you can pull it straight down off the tank. If you missed any gas you'll likely know it- keep your rags handy. I prefer to use the rag to hold the petcock to limit how much gas gets on my hands. The petcock has a tube and filter that go up into the tank - so pull it off straight down.
Step 3: Attach New Petcock
When replacing with the new petcock- be careful with the pipe and filter. They should slide up into the tank before you begin to tighten the nut. Make sure the nut is fully tightened to the tank stem and the petcock or you will get a weep - and it will get messy.
Step 4: Reattach Fuel Lines
In my case – it was attach new fuel lines with an added fuel filter on each line.
I found it was easier to attach to the 2 carburetors first, then to the petcock. Use the same paths as the old lines - noting which carburetor connects to which part of the petcock from before.
Step 5: Refill the Tank
Using the gas you drained - or new gas if the gas you drained seems funky or dirty - refill the gas tank as you would any time, and check the petcock for leaks. I recommend filling in parts - in case you find a weep or leak in your new part or lines. After the tank is refilled - and no leaks are spotted - you can clean up your mess and start the bike.
Step 6: Ride!
(Pretty self explanatory)