One of the head bolts on my 1997 Mercedes C230 stripped during removal. It is a 12 point star, I think they made be called triple square splines. The others are about 4 inches long stretch bolts. There isn't enough space to get anything in there.

Some suggestions I have heard so far and have tried some are hammering a larger size into it, using an impact driver, and welding a socket onto it to give it something to grab. Keep in mind its an aluminum block.

A mechanic friend of mine offered to do an engine swap and since the engine already had high miles I just opted to do that.

  • If I understand you correctly, the bolt itself is intact and still holding the head on, it's just the start head is stripped? Aug 27, 2014 at 22:12
  • 1
    After answering, I just realized this is a very old question. I am assuming you've got it resolved by now. Let us know how it turned out! Dec 27, 2014 at 19:24

4 Answers 4


If the bolt is so obscured that vise grips and welding are not feasible, and so stripped that impact doesn't make sense (I am guessing it is too stripped), I usually do the following. If it's very stripped, you might want to skip ahead to step 8.

  1. (Look ahead to step four; you'll probably want to take one of your bolts to the auto parts store before starting this process.)

  2. Torque down the other bolts in order (follow manual).

  3. Add about 5 ft*lb to the bolts nearest to the stripped bolt (this is intended to unload the culprit bolt) and don't plan to re-use the head bolts because they're overstretched now.

  4. Get the next size up allen key or torx bit that will almost fit into it and hammer it on. I'd recommend doing this with a bit you don't care about, preferably impact rated due to hardness. I'd recommend 1/2" drive for this bit if space allows. If you're wondering what will fit, see this diagram:

    triple square versus 12 point socket

    That diagram shows the difference between your bolt (left) and a 12 point socket (right). Notably, an allen key or torx bit would fit into this. If your fastener is standard, use a metric (and vise versa) to get a size that's just a little bit off (i.e. a little bit bigger).

  5. When you're done swinging the hammer, soak everything in penetrating oil.

  6. If you're feeling confident after hammering, feel free to try removing it BY HAND now. If you use impact tools with this technique, you will successfully extract the bit from the bolt and continue having a bad day.

    • If you feel it start to strip, stop and apply more penetrating oil and apply some heat.
  7. If that strips out or you don't feel confident in the seating, consider trying a different sacrificial tool.

  8. If the above doesn't work, clean up the bolt with brake cleaner, get a new sacrifical bit, and JB weld the two together following the above process. Wait at least 8 hours and then take it off by hand. Don't forget to apply liberal penetrating oil (after the JB weld is set) and I would personally apply heat to the bolt only at this point.

  9. The bolt will be out. On reassembly, chase the hole gently with a tap and blow it out with an air compressor. If it took your block's threads with it, get a helicoil, install it, and use a lot of oil (maybe even anti-seize) on the coil threads when you re-attach the head if you want to be able to remove it. When you go to try removing the bolt from the helicoil in the future, it will likely take aluminum with it. Helicoils are nightmares in disguise. Seriously consider never removing the head again if it comes to this.


Worst case scenario: Very carefully grind the bolt head off then slide the head up the bolt. Once the head is out of the way, remove with a stud remover or weld a new nut to it.

To do this, first make the head as accessible as possible. You can rotate the cams a bit carefully if a lobe is in the way, just be sure you don't have valve interference (rotate BY HAND, stop on any resistance). Shove rags into the oil return holes to prevent metal shavings going down them. Cover everything else with more rags to minimize metal shavings in the head. You'll still have metal shavings everywhere so plan on flushing everything.

A large drill bit works best for removing the majority of the metal quickly. Drill down through the head of the bolt and slightly into the shank. Use a chisel to knock the rest of the head off and a right angle die grinder to clean up any burrs that will inhibit you from sliding the head up the bolt.


Re- tightening the bolts on either side of the offending bolt worked! It took the load off the bolt and to my surprise it loosend,


If there is some of the bolt sticking out you can use a vise-grip or similiar. Welding can potentially be dangerous, only do this in a well ventilated area and free and clear of any fuel sources. If the bolt is sheared off below the surface then use a EZ-OUT or use a drill bit 2-3 steps smaller than the bolt. Drill down the center of the bolt, tap the hole, then get a reverse thread screw/bolt to go into the drilled bolt and extract it. ( when drilling, be careful of filings so they don't enter the engine at any point. Take paper towels or other items and seal any openings up.) When you remove the bolt, clean the hole, re-tap the newly cleaned bolt hole (prevent further problems). Good luck

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