I've now pull the camshaft and lifters, and while inspecting the lifters have notices that one is different from the rest. The odd one out has INA stamped on its under-side which the others don't. The shape of the underside is also slightly different to the others.I'm guessing this might have been the cause of the metal. Three pistons do have very slight 'kissing' marks on them from valves, but none more so than the others. I said in another post that the head is an exchange unit from VW as it has a stamp on it, but the wierd thing is that the bores on the engine still have all the honing marks to them, leading me to believe the engine has been rebuilt (came out of a car with 175,000 miles). So possibly since rebuild the timing jumped? breaking a lifter, which was then fixed?