M4 looks like an upside-down mushroom. The stalk's bonded into an invar cup, which is bolted into a steel cup that's attached to the 3 vanes that make up the M4 spider. The idea was to insert a pin to thoroughly secure the invar to the steel cup to be sure that M4 can not move around At All.
The man for the nerve-wracking assignment of taking a drill to the SAC was Craig. Part of the extensive preparations included reserving a space for him in an undisclosed Northern Cape branch of the Witness Relocation Programme on the off chance that things might go badly.
With M5's cover in place, an optical diaper on M4 & a lot of plastic in place to catch any swarf the vacuum cleaner might miss, the drilling jig was attached.
We decided to make this seem easy by taking the super-structure off after Craig had psyched himself up for drilling with it in place - much better idea!
James on Swarf-Sucker duty...
It was all done before we knew it & Craig could go off for a hard-earned smoke break - great job!