took the valve covers off and found some broken up small pieces of plastic in there! Looks like something has exploded but can't work out what it could be!

any ideas?

 

Maybe petrified and then shattered valve guide seals.

 

That was also my first thought. They are the only plastic parts in the area. Fortunately, the seals can be changed without removing the heads from the engine.

 

Had a quick look on Jegs and yes the Stem Seals do look like they could be the reason for the particles. They are black and would probably form a cylindrical shape about the size of a skateboard bearing.

It seems like they may have been there for some time, however the remains suggest that it's just one that's exploded. Are there likely to be any catastrophic consequences having run the engine or is it a minor problem?

 

The worst problem is that you may see excessive oil consumption and some oil smoke in the exhaust.