Rear Exhaust Stud

I tried getting the broken stud out of the rear of the exhaust manifold, drilled it, easy out, torch, penetrating oil, that sucker was in there to stay. I even tried heating it, then spraying it, to cold shock it without any results.

I finally gave up and drilled it out and now have planned to thread it for M12.

My question is, any trick to cutting threads into the cast iron block? I've got a couple of good Taps, the guy at the tool place guarantees to be unbreakable. I've cut threads into cast iron before and it also made me antsy, you really have to crank on that Tap to get it to cut.

I would have just left it broken, I have in the past with no issues. But the gasket blew out right where the stud was broken and the exhaust manifold was spitting exhaust onto the firewall and very near the engine harness.

The proper procedure is to turn a ¼ of a turn only and back up to break the chips.
Use plenty of lube/cutting oil.
It’s also by feel. If difficult, I turn 1/8 of a turn only or adjust based on effort.
You can also remove the tap after few turns and clear the chips with compressed air.
I never broke a tap this way, even in hardened stainless steel.
Use tape to mark depth on the tap to prevent bottoming out.
(FYI There are 3 different types of taps with a different front taper to facilitate tough jobs).
Hope this helps.

Ditto all of that. I had to drill & tap the head on my old XJ for a broken off thermostat housing bolt. It's not too bad if you go slow & carefully, don't try to turn it if you feel it hit a lot of resistance suddenly. Reverse it a lot to clear the chips, and pull the tap out and clean it off often to remove all the chips from it as well. A good tap will cut cast iron no problem but you don't want the chips to build up and get in the way, that's when you can damage your newly cut threads.

Cast iron threads easy. Use tapping fluid and take your time. be sure to use the proper tap drill. An m12 is a large tap so it should not break. One trick is to stop when you feel the tap twisting. Its in a bad spot so turn the tap with a ratchet and square socket on an extension.

8Mud said:

I tried getting the broken stud out of the rear of the exhaust manifold, drilled it, easy out, torch, penetrating oil, that sucker was in there to stay. I even tried heating it, then spraying it, to cold shock it without any results.

I finally gave up and drilled it out and now have planned to thread it for M12.

My question is, any trick to cutting threads into the cast iron block? I've got a couple of good Taps, the guy at the tool place guarantees to be unbreakable. I've cut threads into cast iron before and it also made me antsy, you really have to crank on that Tap to get it to cut.

I would have just left it broken, I have in the past with no issues. But the gasket blew out right where the stud was broken and the exhaust manifold was spitting exhaust onto the firewall and very near the engine harness.

Click to expand...

It's easy to tap iron - just take it slow. It tends to form a small chip, as I recall - I like to use grease and back the tap out every few turns (which clears most of the chips.)

Why M12? Those screws a 3/8"-16, overtap to accept a coil insert in 3/8"-16 and call it good. That's my typical repair - why have one odd-sized screw?

I've also been having EXCELLENT results in cutting new studs out of brass (full hard) or bronze (half hard or full hard) threaded rod as replacements - service gets MUCH easier!

a 2 flute plug tap would be best. less chance of going in crooked with a 2 flute and a good lead. Use oil, even used motor oil will work. Do not force it, if it's real hard to turn, something's wrong, the tap is most likely misaligned. Cast iron makes more of a powder than a chip, so you don't really need to back it out as often, however you'll want to once you get towards the bottom so you can blow it out with compressed air. the only thing that will break that size of a tap is a side load, or if you drilled the stud off-centered and a sliver of it is still there as part of the wall of the hole.

I'm a huge fan of the Irwin taps with the straight lead-in - it's sized to the diameter of the drill required, so you simply drill, then stuff the tap in and start turning it forward/back as usual and it starts straight every time. Then if you need to tap closer to the bottom of the hole switch to a plug tap or bottoming tap.

5-90 said:

It's easy to tap iron - just take it slow. It tends to form a small chip, as I recall - I like to use grease and back the tap out every few turns (which clears most of the chips.)

Why M12? Those screws a 3/8"-16, overtap to accept a coil insert in 3/8"-16 and call it good. That's my typical repair - why have one odd-sized screw?

I've also been having EXCELLENT results in cutting new studs out of brass (full hard) or bronze (half hard or full hard) threaded rod as replacements - service gets MUCH easier!

Click to expand...

An odd size screw doesn't bother me, I typically take the manifold off every five years or so.

Some of it is availability, without special order of the tap or bolt my options are limited. I haven't got every tap, just the more common stuff.

Seems to be plenty of meat back there, I figured to make it bullet proof, more of a why not kind of thing.

I sent my son after some 10.9 bolts, he came home with 8.8. I likely have some 10.9 in the shop. He also came back with some A2 70's in M12, which was a good idea, but I wish he had gotten A4 80's. I imagine with the bigger size it is OK to go with a softer bolt? I have a pretty good selection of bolts in the shop (I likely have a few of everything) if you think it mandatory to go with a SAE 9 equivalent? 8.8 is about a SAE 5 equivalent, A2 70 about an SAE 4 equivalent.

I'll let you guys know how it turned out after I get finished coughing my lungs up with this cold and get back to work on the Jeep.

I've tapped some cast iron in the past, though seldom. It always seemed harder to turn the tap and it made kind of crunching noise instead of the sound steel makes when you are tapping it, always made me nervous.

Thanks all for the input and calming my nerves a little. I'm committed to M12 now that I've drilled, the question is if the A2 70 or the 8.8 is good enough? I've got some 10.9 in the shop, but they all seem to have another pitch to the threads and aren't standard M12 and I may not have the matching Tap.

Paradoxically, the PC8.8 screws will be better - they'll hold more strength after repeated heat-cycling. I'd stick with those.

As far as the "crunching" sound when tapping iron? That's normal - examine the chip.

Cobalt drill bit, a right angle drill and a three stage Tap, wasn't really much of a problem. I actually snapped the front off also, with the torque wrench set 5 pounds under spec. Whoever designed and spec'ed out the end studs on that manifold should be sentenced to spend the rest of his life replacing them.