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i have a cilinder head here which has some drilled through holes for the exhaust manifold , which would result in engine oil leaks. So i need some sort of sealant for this , any advise is appreciated as normal thread sealing loctite would not withstand the heat i assume ?
I would have the head welded to close the holes, drill/grind/re-tap as necessary. If the hole is just drilled / tapped too deep, then another possibility would be to red loctite a short set-screw (aka grub screw) to create a sealed bottom of the hole.
You should be fine using a thread sealant - while the exhaust is certainly going to be hot, the fastener isn't going to be that hot as the cylinder head will be pulling heat out of it.
I'd recommend Loctite 5770 - https://www.new-line.com/sealants-cleaning/lubricants-oil-sealants-tapes-paste-water-finding-leak-detection-systems/thread-sealant-ptfe-tape-and-paste/loctite-5770-hi-temp-thread-sealant
I’ve used Loctite 510 in the past and not had an issue, if this helps.