Thread: Undoing recalcitrant head bolt...

Originally Posted by uninformed

Isn't heating the bolt going to make it expand and bind even more......personally I wouldn't

correct heating the bolt will make it to expand and quite the opposite the bolt head will unbind.

heating the head of the bolt bolt causes it to expand the shank also expands, as the shank expands it gets longer, releasing the tension on the threads allowing you to undo the bolt. In addition to this having the bolt expand makes it "push" at the hole its in which can be enough to break any binding once its started to cool off again and the best way to achieve this is with a long soak heat up (a hot air gun) giving the object metal plenty of time to draw heat away from the bolt before quenching the bolt with a lubricant to cause it to contract away from the hole initially and then grow again as the object transfers heat back to the bolt causing it to expand once more.


(for the purpose of this post Quenching is not done from temperatures high enough to cause any annealing but from temperatures in the range of about 300-350 back down to 40-50 degrees)

Typically the material of the bolt and what its fastening will have different expansion rates causing any corrosion thats holding them to crack apart as the metals "creep" over each other releasing any hold thats been estabilished between the object and the underside of the bolthead. That holds true even if the metals of the bolt and the object are the same as you';ll normally be trying to heat the bolt faster than the object.

if you dont succeed straight up with heat multiple reheats and quenches will help as the metals "creep" over each other penetrating fluids used to quench the bolt will work their way in a lot better. You also get the effect of heating the object (not going to work in this case as youd have to heat the block not the head) which makes the diameter of the hole expand fractionally which can be just enough to loosen the grip that the object has on the threads.

going to a dull red then letting it cool off for a few minutes before heating it back up to a bit shy of the softening temp (about 300 deg C) works really well in a cast iron object with steel bolts but dont try it on any thing ally, bad things happen.

You might also try tightening it first.Sometimes this is enough to break the bind and then allow you to undo the object.Also,if you start to get it to undo,make sure that as you undo it,you then do it back up,do this all the way out,and if needed,add a form of lubricant under the bolt head,and allow it to work its way down the bolt shaft,doing this can help to limit the possibility of the bolt breaking off with the thread seized.

as LowRanger has said tighten it up a tad and then undo it, i found out this accidentally once when trying to get the alloy angled head off a Dolimite block, broke 2 bolts first but got the rest out as said

oooh and plenty of INOX will help

Many thanks for the info !

Yes, I tried tightening it..Today I bought a 10mm impact socket/Allen key, as well as a hand impact driver. - Always wanted to try one.. and this TENG-TOOL was $19.95 on special... (socket was $15.....)

Going down to #1 Son on week-end, which will give me a few days to find my hot-air blower. - Somehere in The Shed...