Hot bolt removal wisdom

[JOEPRO]

Given that metal expands when red hot and this is why it greatly helps when removing seized fasteners, I have a question.

In a nut scenario it makes sense that if the nut expands then you undo the bolt when the nut is red hot.

What about a case like below where only the bolt head is visible and it goes into a large cast iron exhaust manifold.



In this case would I heat the bolt head red hot with an induction heater then immediately try to undo it? Or allow the bolt to cool for 60 seconds or so then undo? Theory that the hot bolt would expand and crush any rust or thread contamination then when it cools a little it's easier to remove.

 

[Morrisman]

I’d definitely leave it to cool, or you risk twisting the head off it if it’s still red hot.

ETA. And as soon as you feel it moving, twist it back the other way, then reverse it, bit by bit until it works loose.

 

[sako243]

I think (but I'm no expert) that in this scenario you're hoping to achieve the latter, basically bust the rust seal.

I'd be tempted to leave it cool down as much as possible, the few times I've tried this (with a plumbers torch not oxy) if the bolt is still warm I've found it snaps bsvahze the metal is softer. I don't know if quenching it would help to rapidly contract it or if that would have a detrimental effect.

In my limited experience I'd be undoing it manually rather than with an impact. The impact is great at breaking the seal too but I've found it tends to shear the bolts that have been heated. At least with a spanner or ratchet you can usually feel it going.

 

[jo1010]

Heat red hot, air cool only, see if it turns gently, If not repeat the cycle - they're only small bolts so will snap easy

Oxy heating the cast iron first would be better but access is your problem

 

[Dozzer]

As the other have said, heat then allow to cool, bit of penetrating oil never hurts, we used to use a product called Freeway, seemed to work well.
Also,the part of the stud that is exposed, clean the threads with a wire brush and remove as much corrosion as you can, many a bolt has come free, only to jam on the rust. you can even run a die down it.

 

[zzr1200]

Heat to red hot, then allow to cool. I've seen people on Y/T use candle wax to add lubricant to heated seized bolts/nuts to ade release, I as yet haven't tried it but will do next time I encounter the problem. Heat cycling the seized fastener helps, with small movements back n forth.

The biggest problem is trying to rush the job....

PS:- Personally, I'd take the manifold off the engine and hold it in a vice (even drill a bit of flat stock to bolt it too), easier to heat less chance of damaging other parts.

 

[Kram]

If you can, heat the exhaust manifold instead, may need a specially shaped coil. You dont need to heat the whole manifold, locally around the bolt will be adequate and yours should have the power for that.

 

[Melvyn Best]

PS:- Personally, I'd take the manifold off the engine and hold it in a vice (even drill a bit of flat stock to bolt it too), easier to heat less chance of damaging other parts.

How do you get to the manifold nuts without removing the heatshield securing bolts in the first place?
Heating the bolts and a few hits with a hammer on the bolt head along with penetrating oil normally works if they are going to come out . As said don’t attempt to just unscrew the bolts , work them back and forth gradually increasing how far to unscrew

 

[JOEPRO]

How do you get to the manifold nuts without removing the heatshield securing bolts in the first place?
Heating the bolts and a few hits with a hammer on the bolt head along with penetrating oil normally works if they are going to come out . As said don’t attempt to just unscrew the bolts , work them back and forth gradually increasing how far to unscrew

Yes I'm looking to remove the heat shield for better access to the tops of the downpipe to manifold bolts ×4. I'm not planning to remove the whole manifold from the engine.

The downpipe bolts can snap or strip so I would like to get access to them and heat them up before attempting removal. Someone cut the cats from this classic w124 merc and I have a whole good used genuine exhaust to fit.

 

[Kram]

Does the engine run? That is the best way of heating for exhaust removal. Get it hot until the fan comes on.

Heat shield bolts I've never had issue with, just undo them cold.

 

[bricol]

Based on my experience of heat shield bolts, save time, effort and money - try to undo them. If they undo, fantastic. If they shear, just drill, retap and use new screws.

 

[My_Teenage_Self]

Just bear in mind, most bolts have been heat treated to some extent, making them harder/tougher. Heating to red and leaving to cool is essentially annealing them - making them softer. you run the risk of the head being easier to brek off *IF* it doesn't un-stuck it.

 

[roofman]

Any nut or bolt i come across these days that might give me trouble i go for the induction and it almost never fails...worst bolts ever are the housing bolts on a turbo, the cast iron hot side which without heat you would never get them out. Any manifold bolts going into an aluminum head are hard as the head has so much heat sink, nuts are fine as they are away on the outside.

 

[Pigeon_Droppings2]

Theory that the hot bolt would expand and crush any rust

Does the heat also break down the iron/oxygen bond?...I don't know but I thought that's partly what it did.

If those bolts are into a cast manifold then it might be difficult...on my Figaro project most snapped and had to be drilled.

 

[Gareth J]

The heating and expansion of one thing relative to the other is enough to crack and free rusted threads. Just heat which ever thing you can that will be most likely to get hottest relative to the other.

 

[JOEPRO]

I heated them up with my induction heater. Not red hot but smoking. Then I used my shake n break on my air hammer too whilst loosening and all 4x bolts came out no issues.