Exhaust manifold bolt broke off.

[Kestas]

Originally Posted By: onion
Exhaust bolts can be really hard to drill out- the metal can be extremely hard and brittle due to repeated heating and cooling cycles.

This makes no sense. If anything, repeated thermal cycles would soften the steel. Any "hardness" felt during drilling is probably because you're hitting oxide scale while drilling, or someone used a high-grade bolt to begin with, which is ill-advised.

 

[onion]

Originally Posted By: Kestas
Originally Posted By: onion
Exhaust bolts can be really hard to drill out- the metal can be extremely hard and brittle due to repeated heating and cooling cycles.

This makes no sense. If anything, repeated thermal cycles would soften the steel. Any "hardness" felt during drilling is probably because you're hitting oxide scale while drilling, or someone used a high-grade bolt to begin with, which is ill-advised.


Well I'm not qualified to argue theory with you. But any experienced mechanic can tell you that exhaust manifold bolts routinely break off (which is where the 'brittle' assumption comes from)- this is common on engines made by most manufacturers and in all manner of applications... so it isn't due to some one-off odd bolt selection. And once the manifold bolt breaks off, it's usually quite hard to drill (which is where the 'hard' assumption comes from).

This is a common problem- I couldn't possibly tell you how many times I've dealt with the phenomenon. So rather than suggesting it ain't true because it doesn't fit the theory you're applying... maybe you need to take a second look at that theory- and additional factors that you may not be considering. I'm sure it's some combination of corrosion, thermal strain hardening, less than ideal material selection... and probably several other factors.

 

[Kestas]

I've sectioned and studied all kinds of materials and applications that have seen elevated thermal cycling, including exhaust systems, exhaust manifolds, and heat exchanger tubing. They all have the same thing in common - they're soft. What I have seen is rust wedges that form and grow from the surface into the material, but they don't grow that deep. I'm very much in tune with all the factors that can be considered with regards to material changes.

I've personally drilled broken manifold bolts myself. They're soft. But I'll agree that they're not easy to get out. It's no more different than drilling through one inch plate steel by hand. After all, you're not drilling sheet steel.

 

[RH+G]

I agree with Kestas. Most OEM bolts are soft and very easily drillable. The problem with drilling the bolts is getting started and having room. Once started and u can hold the proper angle any drill bit will suffice. I never needed no carbide or cobalt bolt to drill an exhaust bolt.. Drilling grade 8 or 10.9 bolts on the other hand is another story

 

[onion]

I'd rather drill out a grade 8 or 10.9 bolt any day over an exhaust bolt.

Like I said Kestas- you're the expert here. I'm in no position to argue theory. But IMO you're missing something. I've run across plenty of hard exhaust bolts that I had a [censored] of a time removing with an ordinary drill bit.

 

[Texan4Life]

Is this a threaded hole?

You might have to bit the bullet and take the manifold off so you can get it out in the open to work on it.

If threaded I would drill it out with a LEFT hand drill bit. Half the time you will get lucky and have the drill bit snag and unscrew it.

 

[mechanicx]

I have no comment on whether exhaust bolts get annealed or soften or not, but I've never drilled out any bolt, in general and not just exhaust, that I thought was easy. I just look at bolts as harden steel and hard to drill through even with a cobalt bit. I suppose if you heated a bolt with a torch and let it cool slowly it should be softer but I can't really tell.

 

[onion]

Originally Posted By: mechanicx
I have no comment on whether exhaust bolts get annealed or soften or not, but I've never drilled out any bolt, in general and not just exhaust, that I thought was easy. I just look at bolts as harden steel and hard to drill through even with a cobalt bit. I suppose if you heated a bolt with a torch and let it cool slowly it should be softer but I can't really tell.


I've tried that, and in my experience it actually makes the bolt harder. Heat gets conducted away from the bolt pretty quickly... it's hard to make it cool down 'slowly'. I guess with the application of enough heat over enough time, it might be possible- but I've never been able to do it.

For removing broken bolts in general, my preferred method is to remove the part and mount it on a drill-press... then it'll usually come out easily enough. But if removing the part isn't an option, I'll usually square up the end of the bolt with a carbide burr on a die grinder, then center-punch it so I'll get a straight start, and start drilling it out with successively larger left-hand cobalt bits. With luck, that left-hand bit will grab the bolt and spin it out- although that rarely happens with exhaust bolts. Sometimes I keep drilling it bigger until I can remove the remaining threads with a chisel and clean up the hole with a tap. Sometimes that doesn't work and I just go ahead and drill the hole out bigger and fix it with a heli-coil or the like.

For a worse-case scenario where the drill bit gets crooked and off-center, and the bolt hole is mangled beyond repair (hasn't happened to me for years, but I have to clean up other guys' messes on occasion), I have a fix that some of ya'll might find amusing. And old-timer at a Cummins dealership showed me this. I'll go ahead and drill out the hole a little bigger, and tap it for the closest size pipe thread. Then I'll install a solid pipe plug good and tight with some green locktite- a cast iron plug for cast iron, an aluminum plug for aluminum, etc. Then grind off the plug flush with the surface, re-locate, re-drill, and tap the hole. It makes for a nice, functional repair.

 

[Kestas]

If you are heating the bolt red hot and it's cooling quickly, the bolt will get harder.

 

[mechanicx]

Originally Posted By: onion


For a worse-case scenario where the drill bit gets crooked and off-center, and the bolt hole is mangled beyond repair (hasn't happened to me for years, but I have to clean up other guys' messes on occasion), I have a fix that some of ya'll might find amusing. And old-timer at a Cummins dealership showed me this. I'll go ahead and drill out the hole a little bigger, and tap it for the closest size pipe thread. Then I'll install a solid pipe plug good and tight with some green locktite- a cast iron plug for cast iron, an aluminum plug for aluminum, etc. Then grind off the plug flush with the surface, re-locate, re-drill, and tap the hole. It makes for a nice, functional repair.


That sounds pretty slick and I don't recall seeing or hearing about it before. Sometimes on and exhaust stud flange that you have drilled out and maybe the hole is oversized or mangled you can slip a bolt through it and tighten it with a nut. It's a little mickey mouse but it works and its pretty fast and easy but probably works best on the tapered seat exhaust pipes. Probably most guys know about that trick though.