tightening torque for exhaust manifold nuts

[pW2]

My engine has an aluminium block and a caste iron exhaust manifold. I´ve had to remove the manifold to get a bit of exhaust pipe off. It´s all back but what torque should I use when tightening up the nuts? The studs are M8 and there´s a thin high temp gasket. Presumably the limiting factor is the amount of torque the studs can stand before the thread strips in the aluminium.
Thanks
Peter

 

[elton]

Presumably the limiting factor is the amount of torque the studs can stand before the thread strips in the aluminium.
Thanks
Peter

No, the stud should be stretched within its elastic limit by tightening the nut to the correct torque. The engine workshop manual will specify the recommended torque.

 

[MrCramp]

No, the stud should be stretched within its elastic limit by tightening the nut to the correct torque. The engine workshop manual will specify the recommended torque.

THAT IS DEFINATELY WRONG. You do not stretch the stud.

You will strip the thread on the aluminium. Elton.I think you have misread the question, he is asking about the exhaust manifold into an aluminium head, not the aluminium cylinder head into a cast iron block.

On a Peugeot 306 diesel engine, similar materials, the torque (from the Haynes manual, and I have done a few of these) is 16Nm, 12 lbf ft.

 

[Deleted member 36384]

MrCramp I agree with the intent of your post but Elton is correct. Any tension applied to a stud will cause it to stretch, even the force caused by human fingers pulling on the stud, albeit a tiny stretch.

Of course, only the correct torque, properly applied, will stretch the stud within the limits that allow the flange to seal without applying such an extensive stress that causes any of the assembly threads to strip.

 

[elton]

THAT IS DEFINATELY WRONG. You do not stretch the stud.

I suggest you read up on some basic mechanical engineering physics before you touch another engine.

 

[Malish]

excessive use of handbags here.....

There will be some subtle stretch but a low torque is advisable. You will get a better answer if you tell us what the engine is...

 

[MrCramp]

MrCramp I agree with the intent of your post but Elton is correct. Any tension applied to a stud will cause it to stretch, even the force caused by human fingers pulling on the stud, albeit a tiny stretch.

Of course, only the correct torque, properly applied, will stretch the stud within the limits that allow the flange to seal without applying such an extensive stress that causes any of the assembly threads to strip.

I was trying to give a practical answer that would enable the job to get completed without there being any possibility of too much torque being applied..

Aoppologies to Elton as I now realise my reply was a bit sharp.

 

[pW2]

Thank you all, I think.
The engine is a 1700cc Seat diesel from 1991