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I'm making continual progress on the three Things I'm working on. However, as soon as I resolve one issue with one vehicle, something else pops up to takes its place.
The best of the two '73's in the inventory went for a road test of about ten miles two evenings ago after adjusting the valves and bleeding the brakes. The moral of this saga is if it's gonna run, it better be able to stop. The owner discovered this on a two mile trip to his farm the weekend before to perform a much needed and long deserved pressure washing to hose down years of oil, road dirt, and crud in the lower engine compartment area. It's about 75% better than it was, but could still use another blast or two before we get things back to reasonable cleanness to perform some basic maintenance.

Anyway, the road trip reinforced some earlier suspicions about the exhaust system noted when setting the valves. Some unusual noise and sound pitches from odd areas of the running engine led me to believe we either had loose or broken exhaust studs/nuts. Since we had planned to replace top side intake gaskets before this exhaust problem reared its head, and the two systems share in the joining of the preheating pipe for the intake manifold, we decided to completely remove the exhaust system (original factory two muffler set-up), and have a look at what the problem(s) might be.

Last night, I accomplished this task with medium difficulty. Results: 3 already broken off studs, 1 badly worn stud (flange stud hole wear), and 1 stud totally half assed with some American self threading bolt which I haven't taken the time to determine the threading. Let's see ...that's a 62.5% problem/replacement issue. Glad I'm not gambling, I'd be losing my butt.
The three broken studs are protruding approximately 1/4-5/16" from the head surface. The worn one should be replaced. The half assed bolted stud hole will likely require a 10 x 8 stud because of the what I would assume is the supremely messed up original 8 mm threading in the head. Additionally, I have two broken bolts in the right rear manifold where the preheat manifold joins the exhaust system. These shouldn't be much of a problem to remove as the piece can be easily taken to the drill press and accessed. The exhaust studs may be an entirely different problem however. We have no plans to remove the engine at this point if we can somehow struggle to do this R & R in place.

Any advice and hints from those of you Thingers who have also likely encountered this sticky problem???
Is the steel exhaust stud mounted to the aluminum head, about the same kind of general issue as the use of chromoly push rods in a aluminum engine? i.e. will the aluminum head expand at a faster rate than the broken stud if the area is heated, thus facilitating or easing removal? I would assume liberal doses of penetrating oil will be required also.
I am a willing student... if there are any tenured exhaust stud R & R professors out there willing to lecture.

ztnoo

Take the engine out. It will make repairing the studs alot easier.

Apply some PB Blaster a number of times to help. Then use a torch and apply heat to the stud and try to work it out.

If you still can't get it out, then you'll need to drill it. Get an "extractor" set that includes a lefthand drill bit and extractor. If you're careful ,then you should be able to remove the stud without damaging the threads.

If the threads are damages, then tapping for a 10mm and using a 10/8 stepstud is better than using a helicoil.

Well, the exhaust stud R & R turned into a real adventure. I was able to remove only one of the four studs (three broken, one badly worn. Heat and lots of PB just wouldn't do the trick. I've been told by local guys they have seen the aluminum block (and head) threading come right out with the studs, if you can actually manage to get the stud removed. They claim there is a bonding that occurs sometimes which is almost impossible to overcome. Seems like they may be correct.

So the only alternative seemed to be drill them out with a numbered drill bit set after tying to carefully establish a center punched drilling point. I admit laying on my back doing the front studs (one broken lower exhaust stud on the left and right banks), wasn't a picnic, but there is room to work above the cv/axles. Believe it or not, as luck would have it, I managed to break a small bit off in the first stud I drilled. Damn. That's about one of the biggest garage/shop no no's there is, and I managed to foul that up from the get go. I was proceeding slowly, but it snagged in the stud and that was that. Frustrated, I moved on to the other front stud. Had better luck on this stud, but it did drift off center a bit, and I had to carefully attempt to correct the drift. I stopped drilling when I approached the shank size of the stud.

Around the back side, because of the bumper, it was a little tougher because of space limitations. I concluded I needed to use a small, short (front to rear in length) Makita electric drill and a Makita cordless right angled drill. This was the flange worn stud and it did break off on the attempt to remove it with heat and PB. So it was ground down near the head surface for center punching and drilling. This stud went the best of the three I drilled, maybe because of better body positioning seated on a five gallon bucket. I carefully retapped the three stud holes I had success drilling out and they should be fine with Loctite Red Threadlocker when installed. Also, with the luck of something beyond me, I was able to also retap the upper hole in #1 cylinder which had be halfassed with an American self threading bolt. I couldn't believe I pulled that off, because I was almost certain I would have to replace this stud with a 10 X 8 mm stud.

What about the first stud with the broke off drill bit? We scratched our heads and theorized at length about possible options. I tried one more time to sneak a drill bit off the side of the broken bit, but to no avail. Huuummmm.
Well let's see what else were we contemplating doing to the vehicle anyway? Well we had planned to replace the badly leaking pushrod tubes and seals at some point in the near future (even though I know they can be done without removing the heads while still in the car). Another thing was there was no rear engine seal in the car...long gone and torn out by some unknown previous wrench. Screw it. Let's drop the engine, pull the head off, and take it to a machine shop for removal....using the welded bolt to broke stud routine and lots of acetylene heat. While we've got the mill out, we'll do the pushrod tubes and seals, install new engine compartment seals, clean the motor up (largely accomplished on the bottom side before removal), take the engine tin off and tune it up in a few places and probably repaint it, too. We can do some general housekeeping in the engine compartment area while its open. Sounds like a plan which makes some sense, rather than tackling these problems piecemeal.

I retrospect, that's what we probably should have done in the first place. Drop the motor and get it up where you can have some real leverage and good body positioning for tackling a tedious job. But the studs can been done in the car...with great patience, great care, a slow gentle touch....and a ton of luck. Amen.

ztnoo