head bolts

[fireguy12117]

so i'm in the process of swapping heads and i was wondering what input people here might have on this whole torque to yield (or however its supposed to be worded) head bolt design on these engines.

the way i see it is that bolt is pretty large and the torque spec would never come close to the bolts max torque value, so why is it that the book says never reuse the head bolts? i have to schools of thought i am uncovering. the first is just as i stated, reuse em, you'll never stretch em, the other is never reuse them, if you stretch and bottom out, you aren't getting the holding power needed on that part of the head.

as it is, i am getting my hands on a quality S&K torque wrench and i picked up a new set of bolts, but this is just driving me nuts- i cant see ever causing a bolt to stretch...but the book says never reuse, so, there goes $40 for head bolts...

 

[Big Jim M]

Ahaa Bolt stretch

so i'm in the process of swapping heads and i was wondering what input people here might have on this whole torque to yield (or however its supposed to be worded) head bolt design on these engines.

the way i see it is that bolt is pretty large and the torque spec would never come close to the bolts max torque value, so why is it that the book says never reuse the head bolts? i have to schools of thought i am uncovering. the first is just as i stated, reuse em, you'll never stretch em, the other is never reuse them, if you stretch and bottom out, you aren't getting the holding power needed on that part of the head.

as it is, i am getting my hands on a quality S&K torque wrench and i picked up a new set of bolts, but this is just driving me nuts- i cant see ever causing a bolt to stretch...but the book says never reuse, so, there goes $40 for head bolts...

It's like this. There is a bare part with no threads on it, on the bolts where they are in the head. When the bolt is torqued to specs the bolt DOES stretch between the head and the threads. This stretch acts like a rubber band when it is stretched. It applies pressure on the head even if the head gasket shrinks some during it's lifetime. Most times when the bolt is reused it will stretch again...but not always! Many times I have reused bolts with great success.. However when building an engine that will have high compression and turn high revs I always use new bolts.
If you do reuse them you will be taking a chance...but many do take that chance.
BTW when torqueing a bolt that has threads up to the head of the bolt.. be careful as there is no area for that bolt to stretch and the head will pop off if over torqued.
Big Jim

 

[fireguy12117]

...as displayed, for example on exhaust bolts where i have broken many in that spot.

well, i have the bolts, i dont want to go in there for a 3 time working on the heads, so the new ones are going in.

that was the best description on how the stretch actually functions. i had a hard time picturing where on the bolt and how it could stretch. in my head you were applying force all the way through the bolt as it cinched down on thread to thread contact, but until you pointed out the unthreaded part it didnt make sense, thanks!

 

[AllanD]

All bolts stretch when tightened.

Torque to yield bolts are given with a torque specification that stretches them to the point where they don't actually spring back.

if re-used they will stretch further and that's when "bad shit" starts to happen

Like having the heads snap off.

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[JP02XLT]

Head bolt set for a 3.0 were approx $35 shipped, not a big investment compared to gaskets / time etc. 4.0 might be a little more? not worth the chance for that kind of money. TTY also lose clamping force if they are re-used & continue to stretch & not re-bound as Jim stated. Almost impossible to do on most head bolts but a "stretch" gage is the proper method for checking applied torque. Manufacturers switched to TTY bolts as there are many factors involved in torquing bolts to a given spec, Grease, Washers, thread prep, on & on, so the manufaturers went to the TTY setup to help elimante some of these variables.

JP02XLT