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You need to be careful in judging just what you have and the design that's been used originally in these crankcase. Having a case/casting hole that has a tapped thread right up to the surface is simply a different design to one that uses a clearance bore first with the thread deeper inside the casting (the top threaded type tries to pull the surface up at the gasket, making distortion more likely) with the deeper type design aiming to avoid this.Accuracy in understanding, with input from those on here having relevant experience of that engine, is key to understanding if it's actually damaged or designed like that in the first place.Bolt length is absolutely critical on these, as it is for many tightly designed structures.
I prefer the time-serts over the helicoil. Although quite dear, they're never going to move of snap etc. I had three done on the 400 cases as they were completely shot. They just feel more substantial than helicoils.
Note;- you can buy Helicoil in longer length if that's more suitable to match original orientation of machining, 2.0 X instead of 1.5.