Re: Gravity or siphon gas line
No argument here.
I've always mounted the engine to the seat tube first and went from there. Whatever it takes to mount the front is what you have to do. I prefer fabricated and welded to any type of 'clamp' that relys on a clamping force to secure it to the bike frame.
Some frames, like the Nirve Switchbalde for example, or other stretched designs require a custom front mount because the down tube is so far forward of the engine. The 'Blade' also requires an engine off-set to the left which dictates a new rear mount but even so, that's where I start.
A few posts back aluminum frames were mentioned. I personally have no experience motorizing an aluminum bike but I fully agree with the assesment that a sharp edge that can gouge the surface of the aluminum tubing is asking for structural failure eventually.
It is well known that a gouge, deep scrape or damage to the surface can cause aluminum to crack. Even saw blade markes on the edge of aluminum will start a crack if that item is bent at a minimum radius. To reduce the potential of cracking the edges should be smoothed, even polished, to remove surface irregularities. Loose or over driven rivets that cause gouging on the surface can start a crack in the aluminum skin of an aircraft. It's something you look for during a pre-flight inspection.
Steel is a little more forgiving but damage caused by a loose clamp can start the weakening process. And to add to this; any mount, except welded, if not tight enough will eventually cause surface damage to either steel or an alloy. If the mount is loose, or doesn't have full face contact, eventually will 'wear' against the metal it is clamped to.
Some will say to wrap the mount area with tape or some other material to "protect the frame from vibration". I maintain that if the clamp fits right and is tight, it can't/won't move so therefore no vibration damage will occure.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"