This sort of happened to me danny, new pistons 20 new valves engine fully rebuilt, coolant bleed up etc just running on idle just before i took it out for its first run and then- missfire??? i Thought ******* coil packs, give it a little rev and heard a bad top end rattle, gutted one of the new valves had snaped at the top near the collet, it was a dodgy valve where it had snap it was shiny in the middle like there was an air bubble in the valve when it was made! it didnt really mark the new piston because it was only running for a sec thank god! o well mate chin up it happens!!
snapping off at collet bottom groove is typical of misfitted valve springs
see thisd link as to why and its a common problem with those who dont know the 20v head intimately
link here>> http://badger-5.com/bin/5v-installation-note-supertech.pdf
I've posted this before, but do so again to warm peeps of this risk
happens on idle & is'nt a valve issue, but a fitment issue
beware folks