retorquing the head gasket...

[coise]

It maybe a stupid question but...
At what mileage after changing a head gasket do I need to retorque it?
What is the correct procedure?
Thanks

[vandor]

Around 500 miles. Makes sure you do it with the engine stone cold, like when it has sat overnight.
In the same pattern as when tightening the headbolts, undo each bolt in turn by 1/4 turn, then toque it to 61 ft-lb.
One bolt at a time.

[coise]

Thanks Csaba,
Somebody told me I didn´t need to do it... good thing I checked.
I probably did 650 Miles already, do you think that matters?

Thanks again for the advices this is greatly appreciated.

[timinator]

Can't say that I agree with Csaba exactly on his advice. I assume your car has the 1977 engine in it and not an early engine that might have either a 54 or 58 ft-lb bolt torque. Did you put any thing on the bolt threads or under the bolt head before you installed them? Is "stone cold" in Canada this time of year 70deg. F or the equivalent deg. C? These things make a difference. Depending on those answers if you are ready to torque your bolts it is not necessary to back them off first. Just torque them to what value they need. If any bolts needed to be tightened then plan on checking them again at a later date. It might take several times to get them to stay tight. Driving 650 miles is not the problem. After one heat cycle is the best time to check torque. Then 500 miles would be a reasonable distance to check them again for a street driven car. Some people torque the head on the engine and never worry about it because "somebody" said it is OK.

[coise]

The temperature isn´t a problem since I will do it in the summer (around 25 degree Celcius).
Good to know that 650 miles isn´t a problem to check it.
I was thinking like csaba 1/4 off before retorquing everything, but maybe I should think again.

[So Cal Mark]

I wouldn't loosen them, just check the torque and tighten if needed

[131]

I wouldn't loosen them, just check the torque and tighten if needed

But isn't the point of backing them off to make sure the proper torque is achieved while tightening? It's more likely to take more torque to break the friction to get a bolt moving than the original torque setting.

[So Cal Mark]

the issue of thread and washer lubrication come into question along with bolt stretch. The last thing I want to do is have the gasket break seal.

[vandor]

if you are ready to torque your bolts it is not necessary to back them off first.

Remember that static friction is always more than sliding friction. Meaning that if the bolts are torqued to 55ft-lb, then a torque wrench will have to be set to a higher torque setting (say 60 ft-lb) to break it loose. Once it is moving, the friction decreases.
That is why it is important not to quit turning the bolt when it is almost torqued, since if you stop just short and try and torque it again, the torque wrench will click, because of the higher static friction that has to be to overcome to start turning the bolt.

The difference between static and sliding friction is why a car with a good ABS can stop faster (static friction) that a car sliding with locked wheels (sliding friction).

[coise]

Csaba your ABS example make total sens to me. I agree with you that 1/4 turn back won´t affect the gasket. I will also open my radiator just to get rid of any remaining (sucking or pushing) pressure. Also my brother who is a mecanic told me that in the old car that is what he usually does.
Thanks everybody for all your input.

[timinator]

There are several ways to address the sliding torque question. Say one tightened a head bolt to 55ft-lbs and walked away. If you came back the next day and put a torque wrench on the bolt, and the wrench is set at 55ft-lbs, if it goes click or the digital reading says 55ft-lbs one would assume it is torqued. If instead that bolt started to turn, it is now sliding, and if it then went click or read 55ft-lbs on the digital screen what would be the difference what torque it started at because the result is the same. If you mean that checking torque with a torque wrench is only valid if the bolt is turning first then when do you stop re-torquing? Or do you mean you should somehow determine the breakaway torque of a bolt that has 55ft-lbs loading on the head and set your wrench to let's say 60ft-lbs and if it starts to turn then it must have been at 55ft-lbs. I usually check head bolt torque several times before I think maybe it is tight. It doesn't cost anything to check.

[124JOE]

i think one would creep up on it
50 to 53 to 55.

[vandor]

> If you mean that checking torque with a torque wrench is only valid if the bolt is turning first

Yes, that is what I mean.

>then when do you stop re-torquing?

On a Spider, at 61 ft-lb, after the bolt has turned
That is why I said to back it off first. Lets say you torqued the bolts to 61 ft-lb, and then 500 miles later they are
are 58 ft-lb. If you set your torque wrench to 61 ft-lb, you will never even get to turn the bolts before the torque wrench clicks, because the static friction requires a lot higher torque than the 58 ft-lb to break it loose. Say 65. But if you set your wrench at 65, then after you brake it loose and start turning it, you would over torque it (to 65).

>Or do you mean you should somehow determine the breakaway torque of a bolt

I think that would be nearly impossible to do, unless you are a physicist

[pastaroni34]

> >Or do you mean you should somehow determine the breakaway torque of a bolt

I think that would be nearly impossible to do, unless you are a physicist

Not impossible The key is to have a consistent system and specifications for every aspect of the system. This means consistent threads and consistent lubrication, not to mention accurate measuring tools. In this regard I am a big fan of ARP's Ultra Torque Lube, it really is as good as they say it is, I've done the testing!

[timinator]

Yes Jason me too. Also a fan of Detroit Diesel International lube No.2. You probably saw the Speed Channel program where they showed how torquing bolts on a destructive tester proved that most thread lubes never even come close to achieving the torque necessary to reach the desired clamping force. ARP lube usually took two times to reach the desired clamping force while motor oil never did. Their advice was to keep checking torque until the bolt did not turn when using oil. Seems that the friction force developed when using oil will relax as the engine runs. So one will sooner or later reach desired torque with oil.