When changing the head they recommend changing
the head bolts as torquing them stretches them and
when torquing them again they could separate and
then you have a bolt in hole not very easy to remove!
Check the length of your bolts as well—if your bolts have been too stretched, they will have lengthened. They will likely display some visible distortions because of this. You should also always check the length of new bolts, as depending on your engine model, different bolt holes require different length bolts. The wrong length bolts won't clamp down with the correct force, which affects the seal and allows for leaks.
The bolts and bolt holes need to be completely cleaned before reinstallation to ensure proper clamp and torque measurements. Blow out the bolt holes with an air tool. Lubricate the bolts in engine oil as part of the cleaning process.
willy