So this morning while converting to a fancy new electronic choke I broke the bolts that hold the choke thermostat housing to the exhaust manifold...

So tomorrow I will be drilling and retapping the holes. Anybody have the correct thread count and bolt size? Or should I tap it will something a bit smaller/larger?

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'66 Dodge Dart

Try soaking the broken stud with thin penetrating oil (not WD40) over night and then use a left hand twist drill or a center drill (try MSC Catalog) to remove the stud. Some times as you drill and the stud gets hot it will un-screw its self, worth a try. About 50/50. Even if it does not work you will end up with the pilot hole in the center to drill out with. win-win.

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50 chrysler,54 Plymouth, 64 Valiant conv 4 speed, 66 Valiant V8 wagon, 70 Challenger R/T 440+6 conv 4 speed,80 Colt, 98 Neon ACR,84 Honda V45 Magna
Taking care of 57 300C conv,48 T&C conv. Missing my 67 GTX and 36 Ply coupe

Those are 1/4 - 20 unc threaded holes.
DD

Thanks so much Doc. Am I right to retap them the same size and thread count? Or should I go slightly larger or smaller?

I'm wondering if retapping with the same specs will damage the original thread and kill any attempt to create new threads? And if I should go bigger, what would be a good size?

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'66 Dodge Dart

Maybe you could use the correct heli-coil insert for a 1/4" bolt. Then you could use the same-sized fastener.

Removing a broken bolt or stud is a pain but if approched right, you can restore the original thread.
The "trick" is to carefully center-punch the broken "stub" and then use a smaller pilot drill to get a well centered hole started.

Once you have a pilot hole, work with progressively larger drills, to open the new hole up to the minor thread diameter.
At this point, you should start to see a "fingerprint" of the original thread showing, then you get a tap and use it to clean-out the remaining material stuck in the "V" of the threads.
Many times, I get the "sleeve" of the old fastener to jam onto the tap, then it un-threads from the hole.
If that process fails, install a thread repair insert.
DD

Thanks everybody, it all worked out and I was able to restore the original threads.

I ended up center punching both bolts and using a smaller drill bit and another bit just smaller than the threads. Almost all the bolt came out, and I got the remaining bits out using a tap as Doc mentioned.

I also got my electric choke in place and it works great, its been a good day. Now if only my magnecor wires would get here...

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'66 Dodge Dart