Here's how I did the seal on my 350F. Other petrol tank cap seals may be similar, I don't know. Both my 350F tanks have the same cap so I think all 350F's at least are the same.
First, Open the tank lid. If you look closely you will see four triangle shaped pieces hidden up under the cap (circled in the pic). These are the spring tabs which hold the cap parts together.
Next, locate a weapon like this one. You will need a sturdy little pic to dig out the four spring tabs.
Now, get diggin'
If you get two of the four tabs out that are next to each other than the part should come out.
...And this is what the part looks like out
Now you can go ahead and install the new seal trouble free in the comfort of you recliner. Remember, when you take the old seal off and install the new one that
all the "disks" go inside the new seal....you'll see what I mean when you get it off.
This is also a good opportunity to clear out the vent hole.
INSTALLATION:
All you do is press the part back in the same way it came out. I put mine back in, realigned with the old marks on the lid to make sure it went back in the same way it came out.
Matt.