You know that feeling when you are about to be done with a job and then right at the very end everything goes pear-shaped? Well I was putting the cam covers back on the glorious 4.2 and can almost taste the victory drink I plan to enjoy when a bolt I'm torquing down snaps. Once the profanity stopped I looked into the hole to find the bolt snapped cleanly flush with the head. Two questions:

1). Is this a common thing? Like....do these bolts get weak over time or something? I noticed several of them were badly corroded on the tops (looked like the previous owner had not cleaned up after a bad coolant leak). I was using a torque wrench, but I suppose it is possible it malfunctioned. I suppose what I'm really asking is should I be looking at buying new cam cover bolts, a new torque wrench, or maybe both?

2). What is my next move? I've been in all sorts of mechanical scrapes before, but I've never had to remove a bolt from something that I really wanted to avoid drilling into. Is there some clever trick for this or is it as bad as I think it is?

 

https://www.wikihow.com/Remove-a-Broken-Bolt

Not that difficult so long as you have easy access. The trick is centering the drill bit accurately so as not to damage threads.

If the left handed drill bit does not work then you use a bolt extractor in the hole you drilled.

Since the bolt is not rusted in place, as you were torqueing it down, it should not be difficult to get out.

 

The only time I've heard such bolts snap they'd been over-torqued.

 

It probably shouldn't be too hard to get moving.

What torque were you using? The cam cover bolts should be 12Nm

 

My thoughts on screw extractors:


Put your screw extractors in a lockable metal box. Put the key on a string and suspend it into the middle of a gallon milk jug filled with water. Put the jug in the freezer and leave it there. Any time you are considering using a screw extractor, it will take at least an hour to melt the ice and retrieve the key. This will give you plenty of time to determine if you should be using a screw extractor for the problem at hand.

I don't mean to be adding insult to injury, but screw extractors are only appropriate for removing a free-running fastener. If a bolt was seized bad enough to break off the head, you won't be able to apply enough torque with a screw extractor. You'll end up with a broken screw extractor in the stuck bolt. Don't go from bad to worse.

Since the bolt head snapped off during installation, the remaining shank MIGHT not be jammed in the hole. If so, buy a lottery ticket as you are very lucky. Some left-handed drill bits and a reversible drill may be all you need. With any luck, the bit will grab the broken shank and spin it out.

If the shank does not easily unthread, please re-read my previous comments about screw extractors. This is SO important, I'll repeat myself, with caps and italics and all sorts of gimmicks to get your attention:

SCREW EXTRACTORS ARE ONLY APPROPRIATE FOR REMOVING FREE-RUNNING FASTENERS.

A screw extractor can only apply a limited amount of torque before snapping off. If more torque is needed than a screw extractor can apply, STOP! I cannot express strongly enough how much of a nightmare you will create by breaking one off. You'd be much better off to just drill out the broken bolt and rethread the hole, versus trying to drill out the remnant of an ultra-hardened screw extractor. I do not care to discuss how I learned this...



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When corroded bolts are encountered, it's best to replace them. If they can be cleaned up with a wire brush, remember to use a correctly sized tap to 'chase' the bolt hole threads. Use anti-seize compound on the bolt threads to prevent them from becoming stuck, then tighten them to the correct torque.

 

I agree, screw extractors are the tools of the devil.
I was assuming this bolt won't be bottomed, are there different length bolts in the covers?

 

I have to comment that I have never broken a good quality screw extractor, even helping to remove heavily rusted exhaust studs in manifold.
THEY ARE NOT EVIL AS STATED ABOVE!!!! (Extractors have been my friend for many many years.)
The issue is that M6 / 1/4" extractors are small relative to their diameter/strength/brittleness. Many users apply too much torque by using too long of a wrench / lever arm on them. For small extractors I use my in/lb torque wrench. For an M6 / 1.4" bolts I try to stay around 20 inch lbs. (If you go much over that they could break, or strip out the hole.)

As commented in other posts, if the bolt is cross threaded, galled, or put in a really dirty hole, the extractor might not work, but it is worth a try. (As a side note, I have broken many drill bits, both LH and RH trying to drill the pilot hole into a sheared off bolt.)

From my perspective, your biggest challenge will be to drill a concentric pilot hole in the broken off M6 bolt. Drilling a pilot hole can be challenging when it is in the engine compartment.
(Before you do any drilling, grinding or extracting, protect/close off the top of the head and other areas so that you don't get metal particle/pieces into your engine.)
1. I always look at the top of the sheared off bolt to see how flat the surface is. Most of the time it has a twisted look where it is nearly impossible to use a center punch to locate the exact center of the bolt.
2. I use my Dremel tool with a flat topped bur grinder bit that I carefully grind flat the top of the broken bolt.
3. I try to center punch as close as possible to the center. (Don't always seem to be able to punch it centered)
4. I use my Dremel tool with the smallest round tipped grinder I have (approx. a 2mm round head) and carefully grind the center-punched indention so it is as close to center as possible.
5. I take a brand new "high quality" M4 (or US equivalent) and slowly drill a hole down the center of the the broken bolt about 6mm - 1/4". (M4s can brake especially using a hand drill.)
6. I will take my extractor and test to see if it will grab and allow me to remove the bolt. (sometimes you luck out, often this doesn't work.)
7. On some of the broken bolts, I know that the threaded portion is less than 1/2" into the threaded hole. If this is that case, I will drill the hole all the way through the bolt and then carefully drip thread cutter or some other rust penetrating oil into the hole.
(This may not work for your head cover bolt, because I believe that it is 18+mm threaded into the head with a 4mm long reduced diameter non-threaded tip on the end.)
8. If you determine that the bolt is "really stuck" you will likely have to plan on drilling out the bolt and either chasing the threads, or inserting a threaded insert.
(Might plan ahead and order a M6 threaded inset and drill bit, just in case getting the bolt out goes south.)
8. My next step is to enlarge the hole using a new M5 drill bit.
9. I drill the hole far enough into the bolt to thread the extractor further into hole and see if it grabs and comes out. (If not you need to plan on carefully drilling all the way through the bolt.)
(Unless you are really careful and lucked out getting your hole centered, when you step up to a M6 drill bit, you will be cutting into the aluminum threads in the head.)
10. If you were able to drill the M6 concentric the majority of the bolt will be gone. Sometimes steel in the threads will be all that is left. If I've done a good job, I have a set of small picks that I can pull many of the steel threads out. often enough to get a M6 tap into the hole. - If this goes well, then you are done!
11. If your pilot hole is too offset, you may loose too much of the threaded hole and have to install a threaded insert. (Once you get all the old bolt out, you can usually drill the insert hole concentric in the old hole.)
Another recommendation: Don't heat up the bolt too hot trying to drill with a dull bit. You may harden the bolt and make it even more difficult to drill through.

In engine compartments and other hard to get to areas, it has taken me up to 2 hours to get the bolt removed and threads properly repaired. Sometimes back breaking -with lots of cussing...

I have extracted and thread repaired over 100 galled stainless-on-stainless and titanium-in-titanium broken off bolts in past careers. Galled Ti and galled stainless steel bolts are a bitch to drill out and remove. I don't even try to use extractors,

Feel lucky that you are working with steel in aluminum...

 

I restore old cars for a hobby. Have all sorts of screw and bolt extractors, easy-out, bolt-grip, left hand bits, etc. A whole shelf full of every penetrating oil on the market. Love all your stories here as at least I am not alone. Seems like 95 times out of 100 tho, I wind up drilling out the offending fastener and if I'm lucky, just chasing the threads, but probably 88 times out of 100, I wind up drilling things oversize and using a helicoil or threaded insert.

Last weekend after breaking one of my best 6pt sockets on a cylinder head bolt, I wound up drilling off the hex head as after twisting a broken socket around it was too buggered up for any further attempts with another socket. The trick I used to get the hole concentric in the hex head in the first place was to put a 1/4 inch drive socket over the hex head and just score it first in the center with 1/4" drill bit through the square drive hole. Once the center was marked, I drilled with 5/32nds bit through the hex head and down into the shank. Then I enlarged the hole through the hex head with a 3/8ths bit until it popped off. Once the tension was released, the remaining shank of the head bolt unscrewed quite easily with a #3 extractor.

The OP obviously has a different situation as once a bolt has broken off flush, its hard to get the drill started concentric but here are some tips for others who may stumble upon this thread. If I happen to get a bolt backed part way out before the head breaks off, what I do is drill a pilot hole through a piece of scrap metal and then enlarge that hole to the diameter of the exposed shank partway through the piece. That then becomes my centering guide for starting a concentric hole in an exposed shank. If its broken off deeper inside the hole, I happen to have a small lathe and can make a reverse centering guide similarly, but that rarely happens. I have yet to come up with a reliable tool to center the drill when its broken off flush and generally do as Joe does above. Good luck everyone and its been therapeutic commiserating with you.

 

Agreed about easi-outs. If the threaded part is in tight the easi-outs just snap.
Without seeing the immediate area and knowing if it safe or clear which is for you to judge, the other suggestion is to try and cut a straight slot in the exposed end of the thread using a Dremmel with a small cutting wheel. This would mean cutting slightly into the head metal around the bolt which might not be safe.
But then again there should be at least 6mm of metal around the bolt hole.
Then try a good flat blade screw driver on it and if it will not come out then use an impact driver with a flat blade.
Its a method I have used before with success. But in your situation it will depend on a lot of factors such as whether you can actually get a Dremmel and bit to the right location to cut a decent slot. Also whether you can get a screwdriver or impact driver on it.
I'm just throwing this in as an idea.

 

 

Ok I read what the op stated in the start of this thread., Some others seem not to have read it or understood what was said.

The important points are: Bolt was being torqued IN not being undone. It was snapped clean at the top of the head.

So it is not oxidised in place.

If using a reverse thread drill does not remove the offending part then, in this specific instance given the above facts, I would have no hesitation in using the correct size of extractor to try and remove the stuck broken end of the bolt with liberal use of penetrating oil.

Clumsy and inappropriate use of an extractor will likely result in pain. Such as using the incorrect sized extractor and twisting it with some side angle pressure when a broken bolt is oxidised in place.

If you drill a hole that is too large using an extractor will expand the thin wall remaining of the bolt and even make it more difficult to extract. The wall of remaining drilled out bolt has to be thick enough to not expand when under pressure of the extractor.

It is MUCH more preferable to do what I suggest before attempting to completely drill out the broken bolt!

 

I've done this trick many times, but with one slight change. Fabricate a bushing that will guide a smaller drill bit, versus a larger bit that fills the hole completely. With the larger drill bit, you run the risk of damaging the hole in the valve cover.

If you don't have the ability to fabricate a bushing, in a pinch I've used a medium sized bit and wrapped it with foil tape to act as a makeshift bushing. Make sure to wrap the tape so the loose end stays down. You only need to drill enough to start the dimple, and then switch to a smaller bit.

I'm not sure what tools and facilities the OP has. If not up to the task, consider reinstalling the cover (less the one bolt) and limping the car to a nearby shop.

 

Ok...sorry I took so long to respond and thanks for all the replies! So it looks like there are strong feelings on bolt extractors. As several have pointed out the bolt snapped on the way in and was not particularly tight at the moment it snapped (wrench was set for 7 Ft.-LBS, but may be malfunctioning) so I'm thinking that a reverse thread bit may be a good starting point. Fortunately the bolt that broke is the front bolt in the center of the cover on the passenger side so it is fairly easy to access. I'll keep y'all posted on what ends up happening.

I've ordered new bolts just to be safe moving forward. Can't imagine what could have happened other than the possibility that the bolt had been over-torqued previously and already had a weak spot that had not failed yet. I don't 100% trust my torque wrench, but it did not feel overly tight and I would think it would have to be off by quite a bit to snap a bolt. Either way will be replacing all of the bolts with new and purchasing a new torque wrench just to be safe. Any obvious thing I'm missing?

 

Thank you for the update. Please keep us posted.

 

I took a careful look at a valve cover bolt assembly. The upper non-threaded portion has an OD of 5.74 to 5.75mm. The minor thread diameter is 4.5mm to 4.9mm .
The sealing grommet has a metal sheath around it that has 2 flats pinched in the sides to hold the bolt into the seal housing.
When threaded into the head some of the bolt end protrude out the bottom of the threaded bosses on the head, except where the cam sensor is.
This means to me;
-The threads are a little more self cleaning when a clean bolt is threaded into a dirty thread.
-If the bottom threads and/or the tip of the bolt become heavily corroded, the threads could get messed up when the bolt was removed.
- If the bolt broke because it was over torqued sometime in the past, then there is hope that the bolt will easily be removable.

I'l wager that this will be a straight forward extraction.

 

The bolt broke right where the sheath meets the threads. Upon closer inspection of some of the other bolts there are a couple with visible damage in that same area. Will definitely be replacing all of the bolts with new.

 

That is a good indication that the bolts have been over torqued at some point in their life. Where the threads transition to the non-threaded portion is a high stress concentration area.

 

I'm betting that's pretty common as most shade-tree mechanics likely don't understand how this gasket/sealing system works. They'll have an oil leak and try tightening the bolts before doing anything else, like reading the manual or doing research.

But if there is corrosion in the threads, I keep a bunch of wire tube brushes of various sizes on hand. Rifle bore cleaning brushes can be used too, but more expensive. I'll chuck em up in a drill and spin them forwards and back through the hole, often watching it make billowing clouds of dust. It also works for removing/cleaning out old thread lock compounds like on the crank pulley bolt. Need to get all the old stuff out before you can torque it accurately. (Which is another thought, probably not, but did someone previously put some thread lock compound in the hole? It sure makes putting a bolt back in a lot tougher if you don't clean out the old, hardened goo.)

 

Out of curiosity, I took a look at all my AJ-V8 based cars and spare engines valve covers and heads to see if I could find any corroded, gunked up or dirty threads. (That's 3 STRs, 1 XK8, and 6 other spare head sets.)
4 of the engines have their valve covers bolted to the heads, so I looked around the covers, and at the part of the bolt protruding out the bottom. (per image in earlier post)
I didn't see any oil, or corrosion that would mess up the thread. On all of the spare heads, all of the threads look to be in great shape - clean and free of debris - some oil in some of the holes. (Most of the loose heads have been purchased off of Ebay over the past 6-7 years.)
(I am a fan of spraying compressed air down the threaded hole and if the bolt doesn't freely thread into the hole, I'll often chase it with a "thread-chasing tap", then blow it out with air again. (Assuming you have an air compressor, otherwise that Phil wrote works too.)

I will still wager that as Phil stated, some "shade-tree mechanic" over torqued the broken bolt. This leads me down the path that more than one bolt has been over torqued on his covers. Good idea to replace as many as possible.
Another option is to, use the existing ones and incrementally torque them down, 5 to 10 inch pound per torque pattern, until the torque is reached. Often the bolt will start to give and not reach the designated specification and can be removed/replaced before it snaps off. I just had this happen installing a camshaft gear using a used M12 x 2 cam bolt. I posted the image in another thread. you can clearly see the bolt necking down and stretched. I was being lazy and torquing to the full amount. After that experience, I started torquing my bolts again in increments until the specified torque is reached.