On Saturday, July 10, 2010 at 1:38:27 AM UTC-4, James H. wrote:
On Fri, 18 Jun 2010 19:09:32 -0400, Jim Elbrecht wrote:

there have been enough folks down this path before to make
it profitable for someone to carry replacements.g

My Craftsman 917.388853 6.5 HP 21" push mower (Consumer Reports Best Buy in
2005) came with this "fancy" gas cap that supposedly "preserved the gas".

After a couple of seasons, the fancy gas-preserving cap couldn't preserve
itself, and it broke into pieces. I just put a gas-can cap on. It fit. But
the engine kept quitting after a while. So I loosened it up when I was
cutting the lawn.

I'm curious WHAT is the difference between a gas-can cap and a lawn mower
gasoline tank cap? (The threads are the same.)

Gas caps *should* provide a complete seal. Mowers, particularly the modern carb compliant ones, as well as other small equipment, have a cap that has a negative pressure valve in the top.

This means that as the pump draws fuel it creates a vacuum in the tank that opens that valve to let air in, but the valve closes so no gas fumes escape when there is no suction, when the engine isn't running - till it wears out, then you will see the rubber falling apart and smell gas fumes if it's left sitting in a non-ventilated area. In other cases it just gums up and plastic parts stick shut or open and it can be cleaned out, but usually there's some rubber that went bad unless your gas was terribly dirty.

You could certainly make a TINY hole in a gas can cap, plug that hole while it's not running and unplug the hole while it is, though you might still end up with a little gas splashing out when the tank is full.