Dropping valves

[isaac]

So this hasn't happened to me but sounds VERY impending based on head noise that's ratcheted up in the last few weeks. I've stopped riding the bike because of it. It seems to be an epidemic with these bikes considering what I've been seeing.

The evidence:

1) A few dropped valve stories on this site, even cases of the same bike doing it a second time later after a total rebuild.

2) A quick search of craigslist or ebay shows quite a few bikes that "need a piston and head" or "blown engine"

3) You just plain can't find a used cylinder head or piston for these things, but you can find used blocks, cranks, connecting rods and everything else all the time.

So what's doing it? I'm already midway into a teardown on my bike to find out. So far I'm not finding any parts that are loose and shaky like the ominous, growing rattling sound I was hearing would indicate, though at 12,500 miles parts of the exhaust cam lobe and cam journals show wear, but not exactly severe. Nothing feels loose to indicate serious wear up top. I do know this bike has had some issues where the head makes noise until the first time I ride it above 5,000rpm or so to bring oil pressure up enough to seemingly feed the head better, and that problem is getting worse.

That said, the bike doesn't burn oil, which would be indicative of serious valve guide wear or failure. It doesn't have leakdown issues from the valve guides when it's sitting, so the valve guides must be getting oil. That said, it'd have to be some serious issue to wear guides in only 12,500 miles.

So what's causing this? Has anybody really looked into it? I've heard of other engines having issues where the valve would literally stretch the stem until it just snapped at some point, sending the valve down into the cylinder and causing a total write off. But I haven't seen any discussion of it here, aside from the occasional story where somebody had it happen, everybody says "that sucks", and they either junk the bike or buy a new head and piston.

Any ideas?