Air trapped inside threaded "blind" hole vs. hole integrity

Question

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.

Pᴀᴜʟsᴛᴇʀ2 · Answer

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.

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.

Norm Aylward · Answer

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

Answered by Norm Aylward on January 10, 2021

Air trapped inside threaded "blind" hole vs. hole integrity

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