why do you have to replace head bolts when rebuilding an engine, wht can't you just use the old ones with some locktite and torque them up?

i think they're designed to stretch when you torque them up, and that locks them against the thread, if you were to reuse them, i *think* if you tried to get the same torque there's a risk of them not being as tight as they should be, or shearing off

just checked haynes manual and it says to tighten the head bolts in 3 stages.

Torx type bolts:
Stage 1 . . . 35 to 40Nm or 26 to 30lbf Ft
Stage 2 . . . 70 to 75Nm or 52 to 55lbf Ft
Stage 3 (after 5 minutes) . . . Tighten through a further 90º

why do you have to tighten them in stages?

[Edited on 10/5/09 by omega0684]

To let them stretch a bit

To make sure you get a good even seal

The head has to pull down exactly straight or the gasket won't seal properly, By tightening them in stages you're making sure that the head is pulled down flat and also it ensures that the bolts are under the correct tension/torque.

'Stage 3 (after 5 minutes) . . . Tighten through a further 90º '

if you did this with used head bolts theres a chance they will snap.

As you mention Torx, I guess you are referring to a (relatively) modern engine? These would be stretch bolts which is why they have a torque value followed by an angle. I think that they were originally developed for ease of the manufacturing process as the clamping force is pretty much the same within about 1/2 turn of the spec. However, they do deform in doing their job and would continue to deform in subsequent uses. This results in a "necking" of the bolt and a potential weak point which is why new ones have to be used each time.

The ones that have a number of degrees to go after the torque is set are stretch bolts/studs.
The first two stages pull it down straight, the final stage stretches the bolt.

I've heard of people re-using them without problems, but you're taking a risk of having to do the headgasket again if you don't get away with it.

For the sake of a fiver for a pinto head bolt set just replace them!

Follow all the guidelines in refitting the head, they are there for a reason. Otherwise you'll soon have to have it off to replace the gasket again.

Dont use loctite! Lube the threads with oil before fitting the bolts. Also make sure there is oil under the head of the bolt.

David

Cylinder head bolts are subject to cyclic stress due to combustion pressure pulses. This causes fatigue of the bolt material and may lead to failure hence precautionary bolt changes. If you increase the combustion pressure with tuning then the situation becomes more critical.

As regards tightening of the head bolts.
Stage tightening in a specific sequence is to reduce the chance of distortion of the head (or cylinder block). It also allows time for the gasket to "settle" and take up its new shape under load.
The reason a specific angle of rotation is used for the final tightening is that at high bolt loads the friction between the bolt threads becomes quite large and can vary considerably from bolt to bolt depending on roughness of the bolt and hole threads. Therefore tightening torques become unreliable as a means of putting a required load into the bolt. If you use an angle of rotation then the bolt load is not affected by the friction.

For what it's worth ....

I was told not to oil wheel nuts as there was a danger of overtightening them due to less friction.

Alex, I thought this was an old carb'd pinto engine you had ?

the later pinto's have the stretch bolts with the torx heads
but the early pinto's had non-stretch bolts with a spline head
https://www.burtonpower.com/dets_product.aspx?App=SO&PGrp=09051&PSet=1

you torque the old ones in a different way
(it won't water which type you have as long as you torque them the right way)

[Edited on 11/5/09 by mcerd1]