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I have seen this a few times: when screwing and tightening bolts in threaded "blind" holes (those not passing to the other side of the part) sometimes the air trapped inside gets compressed and the part walls, if thin enough or made of soft material, crack. You can hear a quick, short and loud pssst! you know the hole cracked. This commonly happens with tight thread steps, or precision bolts, like 1.0

Question is: why not drill a small hole through one side of the part wall reaching the threaded hole, perhaps as near to the hole bottom as possible? That way we could prevent hole or wall fracture, since the trapped air can get evacuated through the little hole, as we screw the bolts in.

I did some quick schematics for this. Thinking it better, maybe dirt/grease glop in the bolts acts like sealant, making the air in the hole more difficult to escape, uh? I also see that the cracks are more frequent in zones where the casting left those mold thin lines.

Trapped air cracks

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  • Interesting. I've never heard of this. Are there any citations you can provide with additional info/data on this? I'd like to learn more about this condition.
    – DucatiKiller
    Nov 21, 2016 at 3:43
  • I have seen it with "street" mechanics and aluminum blocks and gearboxes; they start to screw in bolts like M6x1.0 and even before reaching 3/4 of the thing's depth, gets a little small crack, either in the bottom, or in the wal. Obviously, it should also depend on the piece, I guess if used/abused there would be more risk to crack it.
    – Aram Alvarez
    Nov 21, 2016 at 4:48
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    I've never heard of it either, perhaps it is due to debris or liquid in the hole? Or a bolt that is too long? But you say the bolt isn't all the way in… Somebody must remember chemistry/physics better than I and know how to calculate the pressure that could be generated with a perfect seal – but it just doesn't make sense to me that a small volume of air could exceed the strength of an aluminum or iron casting (unless it was damaged). If there was oil in the bottom of the hole that would be another matter.
    – dlu
    Nov 21, 2016 at 5:36
  • That said, in parts configured as you've drawn them, I can't see any harm in drilling the hole you suggest.
    – dlu
    Nov 21, 2016 at 5:38
  • This isn't needed. You know how I know? Because it's not done. If it was needed, manufacturers would do it. Air compresses easily. Maybe in a similar situation under water, you'd need a hole like you suggest.
    – cory
    Nov 21, 2016 at 14:49

3 Answers 3

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Air usually will not cause this issue. It will compress quite a bit without the worry of it causing issues. If thread locker is use, it would be trapped. Without it, it will escape past the threads without issue over time. The threads only come in contact on one side (unless there is precision machining going on with both the hole and the fastener) of both the fastener and the hole. There is plenty of room for it to escape. Most automotive parts, while they are machined well, do not have these types of tolerances involved which would cause this issue. I'm wondering if the noise you are talking about is actually the noise of the air escaping past the fastener.

As you suggested, if you have any type of liquid in the bottom of the hole, there is a distinct possibility you can cause a hydraulic fracture due to the pressure build up of the fastener as it is forced down into the hole. I've heard this term called hydraulicing. This will severely cause issues after the fact by destroying the integrity of the part with a hole in it.

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  • I think it could be a combination of many factors: maybe if the bolts have dirty threads acting as a seal, may increase the risk, plus as you say any liquid inside the hole. When you hear the noise, you actually see a kind of "mist" ferociously ejected from where it cracked :)
    – Aram Alvarez
    Dec 5, 2016 at 22:55
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    If it's a blind hole, I have no idea how you are seeing air escape from a fracture. I've never been concerned about air in a blind hole, nor has anyone I've ever talked to about such things. Hydraulicing as I suggested is a real issue, though, and one you need to pay attention to during an assembly process. Air itself will never be an issue. No clue why you'd be tightening a both with or on dirty threads. This is a great way to get improper torque values while tightening.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 6, 2016 at 0:04
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I have seen this happen in Harleys aluminum heads (I think it was the rocker cover bolt). However in the older vintage bikes I have seen they cut a grove down the threads for blind hole stud and bolts. As there are lots of threads the integrity should be fine but the pressure can be relived. Picture is 1956 Royal Enfield blind hole stud pressure relief grove Royal Enfield 1956

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Blind holes are not threaded perfectly to the bottom. There is some taper to the end of the threads. Forcing a bolt too deep could cause the cracking you see. This is why studs are usually used on blind holes instead of bolts. Once the stud bottoms in the blind hole, the nut on top tightens until torqued properly. Again with hydraulicing, the forces against further insertion should stop the stud from rotating and tighten at the nut. If you have to use bolts, user shorter ones than the full depth of the hole.

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