By my own admission I am a little heavy with spanner's but I'd of thought the Ali case would of stripped the thread before this would happen  .

 
 
 

 

It's been many years, but I recall something like this happening.  Quite a surprise!
I would say that this is perhaps a case of progressive metal fatigue, occurring after a number of over-tightenings. 
Or maybe it is a fluke, a mistake in the production of that particular fastener.

story of my life!

i would center punch it as centered as possible and then drill it out starting with a small bit and then tap in an EZOUT and it should come out

I'm thinking fatigue too, these old girls are getting on a bit now, and like you say, probably been over tightened over the years.

It was a surprise though, I assembled the crankcase two days ago and installed the engine into the bike yesterday, then today while I was sweeping up, I noticed the head on the floor, so it must of just popped off. The actual bolt came out easy as it's obviously not under any tension now, just two picks and turned it.

Never broken one before & sort of felt they were un-breakable, but No. Happy it came out though for you.

Solution:  Use that torque wrench!

Here you go.

www.eng-tips.com/viewthread.cfm?qid=446489

 Intresting read, thanks.

 That's my surprise, in all three manuals, including the Yamaha one, I don't see any torque specifications. I see a few for the big internal nuts and head, most manuals give a list at the start of each chapter for the particular section of the vehicle but there very vague with the DT.

I try not to be surprized about such omissions, but its unavoidable.
e.g. My Clymer manual failed to picture or diagram the carb for my DT3; they just presented "generic" carbs and theory.  I photocopied a correct diagram from Partzilla and pasted it in.    Then later found an OEM manual which did the job.