Modern high-quality aftermarket head gaskets typically require no retorquing because their compressibility is limited and uniform, and they're so labeled. Any gasket that doesn't specifically state "No Retorquing" on its package or instruction sheet, however, must receive this extra step. Run the engine up to normal operating temperature, let it cool down, then tighten the bolts to specs again. It's a good idea to do this a third time after 500 miles or so.
http://www.aa1car.com/library/ic697.htm
Some head gaskets remain resilient and retain torque better than others, so they do not require retorquing. Others, though, can lose as much as 50 to 60% of their original torque after only 100 hours of service!
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Finally, if a head gasket is the type that requires retorquing, run the engine until it reaches normal operating temperature (usually 10 to 15 minutes), then shut it off. Retighten each head bolt in the same sequence as before while the engine is still warm. If the engine has an aluminum cylinder head or block, however, wait to retorque the head bolts until the engine has cooled back down to room temperature.
On some applications with retorque style head gaskets, it may be necessary to retorque the head a third time after a specified time or mileage interval due to the design of the engine. Follow the vehicle or gasket manufacturer's recommendations.
http://www.supras.org.nz/techinfo/bhg.htm
What can I do to prevent another BHG?
If the BHG repair is done correctly the first time, you should never have further problems with it. An engine that BHGs once is not any more likely to BHG in the future unless it has been damaged in some way (eg warped head or block). Continue BHGs are a sign of other significant problems with the engine.
Once the BHG repair is complete, there are a couple of good rules to follow to ensure that the gasket seals correctly:
For the first 1000Kms after the repair:
Use little or preferably no anti-freeze in the cooling system.
Drive the vehicle very gently. A good rule to follow is that if you see your boost gauge going over the zero point you need to back off.
After 1000Kms:
Retorque the engine as per the article mentioned previously. For a stock gasket you should be retorquing the head to 70-75 lbs-ft. The procedure should be carried out with the engine stone cold, or as close as possible to it.
Drain and refill the cooling system, adding anti-freeze in the usual quantities.
Preferably do an oil and oil filter change to ensure that any impurities such as gasket debris are removed from the oil system.
And if you're really in love with your engine:
Retorque again after a few months to ensure all slack has been taken up on the head gasket.
