Retorquing ALUM heads after install

[Alky V6]

The torque value to start a fastener moving again after it has been tightened to a torque value and stopped, can be higher than the torque value to turn the fastener after it has broke free and started to move again. That is why it is so important to torque fasteners in one smooth pull without letting it stop before the final torque value has been reached. The same goes when doing a retorque of the head fasteners. The fasteners have already been tightened and have set. If the torque wrench is set to 80 ft/lbs looking for a 80 ft/lbs final torque, what if it takes 81 or 82 to break the fastener loose? The torque wrench may tell you the fastener will not break loose at 80, but what would the final position of the fastener have been if it was first broke loose and then steadily pulled back to 80?
You're ultimately trying to obtain a certain amount of fastener stretch. Using a torque value to guesstimate the stretch of a fastener is a rather crude method really. That is why checking rod bolt stretch is such a superior method to just torqueing the rod bolts to a book value.
How much friction there is between the threads and the nut or bolt head contact surfaces will dictate how much stretch you've obtained in relation to the torque spec you're using. If the threads and the nut or bolt contact surfaces have taken a set after awhile, it is very likely they will require more torque than the original installation torque to start movement again. That is why it is recommended to first back off a fastener to a lesser torque value first, and then use a steady pull, without letting it stop, back to the desired torque spec.
The reason why there are different torque values for oil and ARP type lubes is because of the different friction coefficients of the two lubes. Slipperiness. ARP lube causes less friction between the threads and nut or bolt contact surfaces, so it takes less torque to obtain a particular fastener stretch value. That alone should give you a clue as to how important that friction at the fastener surfaces really is.

 

[turbo nasty]

That is why it is recommended to first back off a fastener to a lesser torque value first, and then use a steady pull, without letting it stop, back to the desired torque spec.

Can i get an amen...

 

[A2000RICH]

The torque value to start a fastener moving again after it has been tightened to a torque value and stopped, can be higher than the torque value to turn the fastener after it has broke free and started to move again. That is why it is so important to torque fasteners in one smooth pull without letting it stop before the final torque value has been reached. The same goes when doing a retorque of the head fasteners. The fasteners have already been tightened and have set. If the torque wrench is set to 80 ft/lbs looking for a 80 ft/lbs final torque, what if it takes 81 or 82 to break the fastener loose? The torque wrench may tell you the fastener will not break loose at 80, but what would the final position of the fastener have been if it was first broke loose and then steadily pulled back to 80?
You're ultimately trying to obtain a certain amount of fastener stretch. Using a torque value to guesstimate the stretch of a fastener is a rather crude method really. That is why checking rod bolt stretch is such a superior method to just torqueing the rod bolts to a book value.
How much friction there is between the threads and the nut or bolt head contact surfaces will dictate how much stretch you've obtained in relation to the torque spec you're using. If the threads and the nut or bolt contact surfaces have taken a set after awhile, it is very likely they will require more torque than the original installation torque to start movement again. That is why it is recommended to first back off a fastener to a lesser torque value first, and then use a steady pull, without letting it stop, back to the desired torque spec.
The reason why there are different torque values for oil and ARP type lubes is because of the different friction coefficients of the two lubes. Slipperiness. ARP lube causes less friction between the threads and nut or bolt contact surfaces, so it takes less torque to obtain a particular fastener stretch value. That alone should give you a clue as to how important that friction at the fastener surfaces really is.

the term you describe is known as "breakaway torque" and defines the torque required to start a static fastener turning vs the torque required to turn a fastener that has already ovecome static friction and is in the process of turning------there is clearly a difference in the two processes and i am clear on them------BUT----once the engine has been through a thermal cycle I don't believe that you can compare the before and after heat cycle torque numbers since the assembly lubes used on the threads and under the bolt heads and/or nuts are under different conditions------that said i have to say that i agree with our helicopter mechanic as well as nick as far as technique on TR head bolts-------i have done such on well over 50 engines and have never had a single problem with leaking with any kind of gaskets------i can also say that i have never assembled a single engine with used or new heads, alum or iron, and not found at least one or two fasteners that turn at least some amount------i run every engine i build on a breakin stand before i ever install it in a car just so i can do this important step easily-----i do not run the engine under boost or high coolant pressure until it is retorqued------i only run the engine to test oil pressure and create a complete thermal cycle------and i know i have accurate torque wrenches and use correct procedure-------- i even have a rather expensive wrench calibrator that i use before every engine build just for comfort...........RC

 

[Alky V6]

BUT----once the engine has been through a thermal cycle I don't believe that you can compare the before and after heat cycle torque numbers since the assembly lubes used on the threads and under the bolt heads and/or nuts are under different conditions RC

I'm sure you're correct about the changes after heat cycling. But, what exactly are the changes? Does it make the breakaway torque less on some fasteners while making it higher on others? What testing, if any, have you done to confirm your belief?

Isn't it true that some fasteners need to be stretched and relieved, sometimes more than a few times, before they are set to a final stretch figure and put into service? Certain rod bolts? This being the recommended procedure to ensure the best durability in service.

I for one am not interested in what some have managed to get away with on this subject. I would rather know what procedure will give the best chance of success. And, when it comes to applications where high cylinder pressures are common place, why is it that all high performace gasket manufacturers highly suggest that the heads be retorqued, and even bother to lay out the procedure on exactly how to do it?

 

[Otto J]

what has always worked for me was to torque the heads,let them sit overnight and loosen each bolt or nut a turn or so and retorque the next day then finish assembly.
Never had a leak or a blow out