if youre unwilling to take off the head, and on a bmw flat twin its much easier than a jap four!
That's the bit I don't get. Given that it take only a few minutes to pull the head off and a set of head gaskets is well under $20, if it were my bike and I had just bought it, I'd be pulling the heads off anyway to have a look at the valves and guides (dropped valves being a leading cause of premature airhead injury and/or death) and frankly I'd probably throw rings at it too given their low cost so that I was starting from a known base.
Helicoils are at best a temporary fix for a sparkplug, you will find that combustion products will inevitably migrate their way up the thread and sooner of later you will wind the coil out with the sparkplug. Not to mention that you have to clean break the installation tang once you wind the coil in and a clean break can only be guaranteed (on a genuine helicoil, not so with some other similar systems) by a "downwards" break which potentially leaves you with the problem of fishing for the tang with either a magnet or a wad of grease soaked rag on the end of a flexible rod.
With the head out you have two choices, mediated by your ability/willingness to spend money. (1) have the head welded up and a new Sparkplug hole and set made, or (2) have a hardened steel insert fitted - the type that ever use steel "tangs" to anchor them or the type that use a "crowded thread" to permanently lock them in place.
I have used Helicoils to repair sparkplug holes, but based on experience I can only regard it as a temporary fix.
