More work on my customer's 1911 Model T.
The heli-coils that have shrunk are all close to a water passage. I believe that there was some weeping from the water jacket to the head bolt hole. The head bolt holes all have some corrosion in them. If the cast iron behind the stainless heli-coil gets rusty, the corrosion will "grow" the hole shut and force the heli-coil to become tighter.
I used a small cape chisel to knock the first thread of the insert into the hole, then grabbed it with a pair of pliers and twisted the insert counter-clockwise to wind it out of the hole.
The hole was cleaned out to the bottom, threads were chased with a heli-coil tap and then blown out with compressed air.
A new insert was installed, the drive tang removed and then the head was installed without the gasket to check that the new bolts would thread completely into the block.
The new bolts bottomed in the holed before they came tight with the head so the block or head have been machined at some point which explains why the old head bolts were a bit short.
I machined 0.100" off of each new bolt and re-checked the fit. This time the bolts all fit properly so I'm ready to install the head with a new gasket. #ifixstuffyoucant #sgmechanicalservice #prewarcar