Why are the center Head Bolts Tighter than the others?

[JohnM]

I don't know if this has been addressed in the past. If so,  please, lead the way........

 

Why are the specs showing on the 2.5 DOHC head bolts in the center #1,2 at 25 ft lbs and 3,4,5,6 at 11 ft lbs before the 90 degree sequence? Would the gaskets fail

at any point if the bolts weren't tight enough?

This will be my 2nd attempt to get it right.......

 

Thanks

 

[johnceggleston]

i do not know why,

but i know they need to be.

it is specified by the factory engineers.

[Dinky26]

That's just the way they designed them to be.

[forester2002s]

This is a good question.

 

On my EJ251, the head-gasket leaks are typically on the outer-bottom edge.

So I could rephrase the question: Why aren't the outer-bolts torqued slightly higher?

[porcupine73]

I would guess the center bolts may have a higher torque to keep even pressure across the gasket? I.e. the bolts in the center put force effectively over more surface area of the head gasket than the bolts at the edges?

 

>> Why aren't the outer-bolts torqued slightly higher?

Maybe increased torque does not necessarily mean increased leak resistance.

[mikaleda]

I think the brand off gasket you use is more important than second guessing the manufactures tourque specs. From what I hear on the 2.5s you should use only OE head gaskets

[ivans imports]

The heads warp from center out mostly on middle were ex is they were trying to prevent this. The ex part of head gets hotter and more likely to come loose

[Gloyale]

Might be a good place to note that 2.5 1/3/5 (SOHC) engines the center bolts have a larger washer too, in addition to the higher torque.

 

Recently ran into an engine were the bolts had been mixed up during the last install.  The larger washer bolts interfere with the cam carrier frame.

 

Small washer bolts have an orange paint on the heads.  The large washer are plain.

[keith3267]

The head is relatively stiff, that is it doesn't flex very much.  When you initially torque the center bolts, you begin compressing the gasket.  As you torque the outer bolts, you further compress the gasket effectively relieving some of the torque on the center bolts.  You be confident that the engineers have correctly calculated the needed initial torque so that when the process is complete, the gasket has the same clamping force across its surface.  The clamping force is the most important factor, not the tightening torque on the head bolts.