12-28 Heli-coil insert???

[Schurkey]

I'm working on a '77 Rochester 2-bbl (2GV)

The zinc main body attaches to the cast-iron throttle body with three screws.  One hole in the zinc main body is buggered--the threads are there but they're ready to pull out.

Best I can tell, the screw size is #12.  It measures .207 which is just a bit small for #12, but it's within tolerance.  It's definitely 28 tpi as verified with a thread-pitch gauge.

HeliCoil and Permathread don't make a 12-28 thread repair kit.  In fact, no #12 fine thread at all.  Does someone else make a thread repair kit for this size???

Anyone had success drilling it out to 1/4-28 or 1/4-20?  I've got heaps and piles of the 1/4-20 inserts, but I could probably just drill 'n' tap the parent metal to 1/4-20 which would preserve as much of the casting as possible.

[firefox3]

Apparently a Holley pump squirter screw is also a 12-28 thread.  A quick Google search turned these up:

http://www.blindrivetsupply.com/Recoil-Thread-Repair-Kit-34628-12-28-10pcs_p_2885.html

http://www.sri-supplies.com/recoil-thread-repair-kit-1228-621-34620.html

Holley bowl screws are 12-24 thread. I know that size Helicoil kit and inserts should be easy to find. You could always use one 12-24 screw and two 12-28 screws.
Several options available instead of drilling it out and tapping it for a 1/4" screw.

[Schurkey]

Appreciated.  I think I'll order one of those kits for "the future".  These carb castings aren't getting any younger.

I went ahead and re-tapped for 1/4-20.  (I didn't have to drill the casting before tapping, I pulled the coil of damaged threads out of the casting, and without the threads, the hole was already slightly larger than the tap drill size.)  Drilled the hole in the throttle body and intermediate gasket to 17/64.  Bought a 1/4-20 X 3/4 socket head cap screw at the local hardware store.  Assured that the hole is tapped deep enough that the screw won't bottom-out.  Works like a champ.