I've snapped a bolt in the head and it's practically welded in, will I have a massive leak if 1 of them isn't screwed in? Its the top right corner one that's snapped

Cheers

It will leak but you shouldn't lose loads of oil, but it will probably look bad as oil runs down the back of the block.

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How slow / fast a leak are we talking? It's only till I can helicoil it

Enough to make a mess, but not enough to actually risk internal engine damage due to running out of oil on a trip.

Thanks