I didn't use standard length heli-coils, I sourced longer than standard to get full thread engagement. Time-serts, IMHO, would be a better way to go. It will be difficult to seal the heli-coil to prevent water from migrating up the bolt and into the oil.
The best way to get the insert straight is to use a drill jig. I had to insert a few holes in a block once. Drill a two holes truly perpendicular to a flat surface in a block of steel. The first hole should be the tap drill size. The second hole should be drilled and tapped. The purpose of this block is to ensure that the tap drill hole is drilled perpendicular to the deck surface. The second (tapped) hole will support the tap while the it is starting in. It is crucial that the repaired hole be perpendicular to the deck surface and located properly or you might not be able to get the head bolt in.
When you are drilling and tapping the block, use a shop vac to remove the chips. Cast iron pulverizes into an abrasive dust instead of clean chips. If you use tap fluid, you will create an abrasive paste that will quickly dull drill bits and taps.
I think I have some of those long heli-coils along with the appropriate drill and tap left over from when I did mine. If you would like some help with any of this, call/text me, I'm in Colton.
