Not Ranked
IF there was 3 heli coil threads in the bottom of the hole-----------then the STUD wasn't proper depth and most likely the cam lobes at that location were up and there was an excessive strain put on the threads as the rocker arm shaft was tightened down opening the valves-------
And if setting up in a Bridgeport milling machine you would just use the rocker boss es for the location of the hole set up and then remove the assy while drilling the head for the new threads----
If your not using a mill, then a machined drill guide tool piece would be preferred over punting the operation----------
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