For anyone considering rigid fuel lines on their engine, I would sugest that you read this thread as part of your risk trade analysis:
http://www.vansairforce.com/community/showthread.php?t=57122
This is not a rejection of rigid lines - it is a suggestion to look at all aspects of a "minor" change to a critical engine part before making a decision. Nick shared his story a couple years back to make sure that people knew what happened in his case. I don't know if Lycoming uses rigid lines on any of their standard configurations - they very well might - but thre are a lot of flexible lines flying that work well.
We tend to think of the engine as one big monolithic chunk of metal, but I have seen slow motion films of engines on test stands that show quite a bit of flex between the parts - the cylinders can really wiggle around realtive to the case, for instance, and the case flexes.
Then again, the standard prop oil line is a rigid stainless tube - so that clearly is acceptable in that installation. And it probaby was the result of some engineering work to achieve that.
I'm not saying that rigid fuel lines are bad - I am saying that anyone contemplating a change to the "norm" should think about all of the aspects of the change, and that is true in any critical aircraft system or structure.
Vibrational analysys is not simple, and can surprise you. For those who have done the analysis and got the data to back up the design - great! But don't do it just becasue "That Looks About Right".