Why replace head bolts when replacing head gasket?

Most head bolts are torque to yeild so they are throw aways, if you reuse them, you can warp a head, have coolant leaks, loose compression, all kinds of things. Just replace them to prevent any damage to it; if possible, head studs are better and are usually reuseable.

Ej25 Head Gasket

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Why replace head bolts when replacing head gasket?

when changing the head gasket on a subaru ej25, why would i need to replace the head bolts?

Are you sure he didn't use new bolts? If I were quoting that job, I would not include the cost of new head bolts either. I would consider those consumable goods like washers silicone small vacuum lines that I would replace just because of old age A job that big, the mechanic doesn't sweat the small stuff.

They use bolts designed to stretch when torqued to specifications. The stretch keeps everything tight since the heads and the block expand with engine heat at different rates. Once used, the bolts should be replaced because they will not re-stretch and properly maintain the correct torque.

torque setting you wont get a good seal with stretched out bolts. and make sure you get the correct sequence this is high tolerance, high speed, High heat cycle and high pressure stuff. Your capturing a compressed explosion. How many millions of cycles do you want your engine to do? You may be able to reuse the bolts but its a gamble.

the old bolts are torqued, thus meaning there stretched and no good., is your head warped or cracked? be sure to check that too!

Because the heat and pressure on the bolt causes them to warp... heat makes metal expand... U don't want to put in a damage head gasket bolt, one bad bolt can lead to a exhaust leak... not woth the headache... replace the bolts!

trust me replace them if it calls for it, i tried to get around that one time and one of the bolts broke

Because they only sell them in a kit ?