I can be obsessive/compulsive - exhaust manifold bolt torque

[Ed H.]

Curiosity has gotten the best of me. Why the end bolts at 20'/lbs, and the inners at 30'/lbs? The last time I did this, 40 years ago, I just shrugged and accepted it. But now I just gotta know! Thank you! (And "Because the manual says so." is not an acceptable answer!)

[Gary B.]

[Ed H.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

ROTFL! Just another of my features!

[Richard G.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Exhaust manifold center port.jpg
Maybe this will help explain.
A manifold after surfacing af few thousand off the surface.

[Tom E.]

[Ed S.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Even better....... how do the engineers determine what the correct or ideal torque value for a particular fastener? In the case of the exhaust manifold, why not 35 and 45 or...... whatever? This question applies to any torque fastener, not just the exhaust manifold bolts.

[Gary B.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

[Erich M.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Gentlemen,

Gasket sealing design is based on the amount of compressive stress required to seal the gasketed joint. The design compressive stress required to seal a gasket is a function of the gasket material and thickness. The gasket stress is defined as the force per unit area and the bolt torque determines the amount of force each bolt applies the to parts being joined. If you look at the gasketed areas being sealed in Richard's photo, it appears that the gasket sealing area at the outer part of the exhaust manifold is less than the sealing area at the inner part of the manifold. Hence you need more torque (force) at the inner bolts than at the outer bolts to obtain the same required gasket sealing stress (since the same number of bolts are used to seal each gasketed area).

Erich Meyer
Original Owner 72 LT1 Bowtie & Top Flight

[Tom E.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Even better....... how do the engineers determine what the correct or ideal torque value for a particular fastener? In the case of the exhaust manifold, why not 35 and 45 or...... whatever? This question applies to any torque fastener, not just the exhaust manifold bolts.

from an engineering handbook;


The relation between applied torque and axial force - or load - in a bolt can be calculated in this general equation as
T = K F d (1 - l/100) (1)
where
T = wrench torque (Nm, lb_f ft)

K = constant that depends on the bolt material and size

d = nominal bolt diameter (m, ft)
F = axial bolt force (N, lb_f)
l = lubrication factor (%)
Typical values for K with mild-steel bolts in range 1/4" to 1":

• normal dry: K = 0.2
• nonplated black finish: K = 0.3
• zinc-plated: K = 0.2
• slightly lubricated: K = 0.18
• cadmium-plated: K = 0.16

[Larry M.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Gentlemen,

Gasket sealing design is based on the amount of compressive stress required to seal the gasketed joint. The design compressive stress required to seal a gasket is a function of the gasket material and thickness. The gasket stress is defined as the force per unit area and the bolt torque determines the amount of force each bolt applies the to parts being joined. If you look at the gasketed areas being sealed in Richard's photo, it appears that the gasket sealing area at the outer part of the exhaust manifold is less than the sealing area at the inner part of the manifold. Hence you need more torque (force) at the inner bolts than at the outer bolts to obtain the same required gasket sealing stress (since the same number of bolts are used to seal each gasketed area).

Erich Meyer
Original Owner 72 LT1 Bowtie & Top Flight

Erich:

Except in this case, the exhaust manifolds use no gaskets. Just metal to metal. But I think the gasket torque calculations play a large part of determining the torque value in this case, because it is a safe number that provides reasonably good seating of the parts.

Larry

[Erich M.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Larry,

I forgot that no gaskets are used! But I agree with you. I would think the same sealing stress criteria would apply if a gasket is not used.

Erich Meyer

[Ed S.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

Then what is the criteria or rationale for torque specs for applications that do not have a) gaskets or b) pressure concerns..... such as suspension components or internal engine components, bearing caps? There has to be a sound science that dictates torque specs in general.

[Ed H.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

from an engineering handbook;


The relation between applied torque and axial force - or load - in a bolt can be calculated in this general equation as
T = K F d (1 - l/100) (1)
where

...

d = nominal bolt diameter (m, ft)
...

I would have thought this - but looking at the specs listed in the manual, there is some variation given bolts of same dia, for instance bearing caps, pressure plate, and there's no difference in gasket material or similar in those cases as both are metal to metal.

And, further, for the manifold bolts there is a note in shop manual "inner bolts 30 lb/ft" Does that mean only the extreme ends are 20, or both bolts on either side of 1 and 7 are 20, or the inner for 1 and 7 are 30, along with those on either side of 3 and 5. Only those on either side of 3 and 5 are longer.

I'm told I can be cryptic, LOL.

[Ed S.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

I was curious so I did some basic research on the subject. Bolt torque is not the real issue - based on what I learned, engineers determine the desired or ideal "clamping force" between two surfaces or components. The desired force is what is needed for the two components to function to design criteria or to function safely. That is why some suspension fasteners have very high torque values - the "torque value" is merely a metric that is used to specify the clamping force. Once engineers determine the desired clamping force between two surfaces it is a rather simple task to select a fastener that can exert that amount of force without failing. Apparently the strength of various size and design of fasteners is text book info - so when you see a torque spec of some value - you can correctly assume that the engineers selected a fastener that will apply a clamping force that will allow the components to function correctly. The science is not in the torque value - it is in the determination of the clamping force needed to make the component function. And that is what I learned today!

[Bill M.]

Re: I can be obsessive/compulsive - exhaust manifold bolt torque

I work wit very high water pressure and we always tourqe above the head pressure. I would say the the exhaust gas pressure and the fact that the 2 center cap screws have to do the same job as the 4 outer cap screws calls for a higher tourqe to keep gas sealed between the head and the manifold. My 2 cents