Here are the steps to put threaded inserts or Helicoils into the transmission case. There are several types of threaded inserts available including Helicoil and Recoil which use an STI (Special Threaded Insert) Tap and KeySert or EzLok which uses a standard tap. I chose the EzLok inserts for this job because I had some 9/16-12 taps on hand and they come with a threadlocker already on the threads. Whichever insert you choose, the steps are all the same, Drill, Tap, Insert. Here is the transmission case with the hole drilled out.
Cast iron machines differently than steel. Instead of getting chips when it is machined, cast iron produces dust/grit. This dust is very abrasive so you will need to take some care that you remove any dust that gets inside the transmission case. You will also need to clear out the hole as you drill it because the dust will fill the pocket and make drilling more difficult. I use a small magnet on the end of a stick to reach in the drilled hole and remove the cast iron dust. If you need to put an insert into a through hole, put a piece of magnet on the back side to collect the cast iron dust. It is a lot easier than cleaning the dust out of the transmission case.
Once the hole is drilled, run the tap in to the appropriate depth. The pockets in the transmission case are between 3/4" and 1" deep. You don't need to run the tap all the way to the bottom since a standard insert is just over 1/2" long. I cut the threads so the insert finished just below the surface. Cutting too many threads will cause the insert to be deeper in the hole than you may want. You can always pull the insert out and cut a couple more threads if needed. Here is the tap in the hole cutting threads.
Much has been made of getting the hole and threads square with the surface of the part. In general, the closer to square you can get, the better you are. But putting helicoils into a carb requires much more precision than putting them into a transmission case. The question to ask is "Can I get the bolt in the hole and does it bear on the flange?" If the answer is yes, then the insert is square enough. Here is the insert I used mounted on a bolt with jam nuts. To install the insert just screw it into the hole until the jam nut stops against the flat surface. The red is the threadlocker that is supposed to keep the insert from turning in the hole.
Here is the insert installed in the tapped hole.
It probably took me an hour to set 3 of these inserts in the transmission case, but that includes setup time like finding the drill, drill motor, and tap. The instructions call for a 31/64" drill but I suspect you could probably get away with using a 1/2" drill. The tap and drill tables say that a 1/2" hole will give a 60% thread for a 9/16-12 threaded hole. And a 31/64" drill gives a 70% thread.
All the way around, this is a very easy repair to make. The EzLok inserts are about $2 each, which is a lot cheaper than trying to find a good transmission case.
Rick (resurrecting old iron) Dulas