A loose head bolt will reduce the clamping force at that point allowing gas to escape. The head has a certain amount of spring so it doesn't need to be warped if it never overheated. Installing a helicoil thread insert shouldn't require engine out. Another thing is to closely examine every bore to see that each piston leaves identical top ring contact marks in the bores. Any bore that has a black area between the first and second ring heights will likely have a broken top ring.
Finally got the thread repair kit and all the gaskets needed. And found some time to work on the D1.
As carefully as I could, tapped the hole, but found the first 20mm or so to be very soft and when I finished the thread insert pretty much drops down the hole without screwing it in. That's not good
Is there anything else I could try on the hole like drilling and tapping it out further then fitting another type of insert? Can it be done with the block still in the bay? Any other options?
The first thing to do is find out if there's any sound alloy there at all, by tapping a larger thread, say 16mm or 5/8". Then you get a low tensile bolt in that size, drill and tap it internally to 7/16 UNC, cut off enough to make an insert and screw it in with some loctite. Making a one off insert would be easier than trying to work out how to torque up one odd bolt. An engineering shop may already have suitable inserts, I've seen the odd few around the place.
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