The mark is suspicious Paul, but not definitive by itself. It looks as if the gasket has delaminated somewhat when you took the head off the first time, and that does usually cause leakage when you put it back together. You should always scrape the surfaces of head and block so they are smooth and shiny, then inspect the gasket and if one of its surfaces is damaged, you need to replace it. I don't think the gasket was blown when you got the engine, but it may be now. You did take the head off at the beginning of the project, didn't you? That could be where the problem came from.
The first think to do is look closely at the surfaces of the gasket that correspond with the mark, and see if they show carbon, or are eroded by gas flow. Look carefully around the carbon ridge around the cylinder bore side of the gasket, and see if it is missing in places where there might have been leakage. Then see if you can get the layer of delaminated gasket material to flake off the surfaces of both head and block, without actually scraping it, by sliding a sharp blade underneath it on both sides of the suspected blow-mark. If you succeed, see if the blow mark runs across the surface where the gasket was. If you find a mark across the gasket area on either head or block, it was blown. Similarly, if you find carbon deposits on the split gasket surfaces, it was blown.
Last edited by grumpy; 20/01/15 01:52 AM.
