Probably a water cooled manifold and the bolt is near a water jacket,or bolt threads are into a water jacket in the cyl.head,anyway bolt will have to come out and maybe manifold to inspect,manifold is probably alloy so could be subject to corrosion if a/freeze is a weak mixture,check for cracks etc near bolt hole and if no fault found try a little loctite on bolt thread when assembling
wickham, April 2010
