Got the head off this evening (ive done a few 6 engines now and worryingly, the head bolts didnt feel anywhere near tight enough) and it all looks ok. MLS gasket didnt have any obvious signs of damage but i know it only has to be the tiniest bit of damage.
Engine was rebuilt two years ago and this was pretty obvious. Bores had been honed and looked lovely except for number 4. There is a mark down the back of the bore and i cant work out what it is. The valve guides in the head on this cylinder also look burnt compared to the others which look perfect.
Any ideas what this could be? want to get an idea before we bolt it back together!