Hello, does anyone happen to know the OEM torque specs for the exhaust manifolds. Ive reaf a few different values. One was 22-28ft lbs and the other was 18ft lbs. Looking for dry OEM values, not stud/nut values
Thanks, but They're a metric bolt (m8x1.25) i dont see that on there, and I'm looking for the GM OEM specs, not sure if they would be different from that list?
Sorry the other chart had larger metric sizes at the bottom, here is one for smaller ones. I know it's nice to have the factory torque specs, but I've normally found them to be within the general bolt specs. Manufacturers and the engineers match the bolts to the required torque of the application as a general rule. So, for this table, your torque (M8x1.25 - 10.9 class) would be 27.75 foot pounds. This sounds about right.
[This message has been edited by ZCR1 (edited 01-23-2019).]
Sorry the other chart had larger metric sizes at the bottom, here is one for smaller ones. I know it's nice to have the factory torque specs, but I've normally found them to be within the general bolt specs. Manufacturers and the engineers match the bolts to the required torque of the application as a general rule. So, for this table, your torque (M8x1.25 - 10.9 class) would be 27.75 foot pounds. This sounds about right.
Awesome, thank you! I was just unsure if these charts would be accurate given the fact there is so much heat at the manifold. I'll go by those numbers. Thanks!
The standard bolt charts are set up to stretch a bolt to some predetermined percentage of its strength.
For the Fiero log manifolds, the situation is a bit tricky... the tightest possible without snapping the bolt is not the best. Due to thermal expansion, the flanges need to slide a bit underneath the bolt heads; if the flanges are held too tightly, there is too much stress and the welds crack.
Too loose, and there could be leaks.
The V6's log manifolds are kind of a weak point in the Fiero design, in my opinion.
The bolts are specific for the application. The bolt, when torqued properly will have the proper stretch and clamp load while being within the range of the application.
The charts are not set to stretch the bolt to just below the breaking point, but to the proper working load. If the former were the case any force exerted on a bolt torqued per the chart would snap.
[This message has been edited by ZCR1 (edited 01-24-2019).]
18 ft lbs would indicate a class 8.8 bolt is required. If you use a higher class bolt, thinking it will hold better and under torque it, it will become loose because it won't have the proper stretch to distribute the clamping force to all the threads. So use the right bolt for the application. Likewise, if you use a higher class bolt than the application calls for and torque it to the bolt specs, you can damage the component.
Knowing the size and torque for the application will tell you the class of bolt needed and knowing the class and size will tell you the torque needed. I was assuming the required class was 10.9. If the FSM states the torque should be 18 foot pounds then you want to make sure you are using class 8.8 M8 x 1.25 bolts.
[This message has been edited by ZCR1 (edited 01-24-2019).]
18 ft lbs would indicate a class 8.8 bolt is required. If you use a higher class bolt, thinking it will hold better and under torque it, it will become loose because it won't have the proper stretch to distribute the clamping force to all the threads. So use the right bolt for the application. Likewise, if you use a higher class bolt than the application calls for and torque it to the bolt specs, you can damage the component.
Knowing the size and torque for the application will tell you the class of bolt needed and knowing the class and size will tell you the torque needed. I was assuming the required class was 10.9. If the FSM states the torque should be 18 foot pounds then you want to make sure you are using class 8.8 M8 x 1.25 bolts.
Originally posted by ZCR1: If you use a higher class bolt, thinking it will hold better and under torque it, it will become loose because it won't have the proper stretch to distribute the clamping force to all the threads.
Lets restrict the scope of the discussion to bolts that are: not torqued to yield not welded have no requirement to bend when overloaded
To my knowledge, 10.9 material has the same Young's modulus as that of that of the 8.8 material.
Hence, if you torque a 10.9 bolt to an 8.8 spec, the stretch will be identical to that of an 8.8 bolt torqued to 8.8 spec.
Young's modulus (E) is a function of the modulus of rigidity, (G) or shear modulus, and Poisson’s ratio, (v). So, as these vary within a given metallurgy, E varies. The modulus of elasticity, E, is defined as E=2G(1+v).
G and v are affected by tensile strength which is generally associated with hardness in steels. Strength is a measure of the ability of a material to resist stress, so it follows that as a material is rated higher in strength, the resistance to stress is going to increase, and the corresponding elongation due to a specific stress is going to be less, resulting in a different stress/strain ratio, and hence a different curve, and a different E.
If a stronger 10.9 bolt of this same material is substituted, and torqued to the same value so as to produce the same initial stretch in the original bolt, no elongation occurs at all, because elongation in this material does not begin until a higher amount of stress.
Determining the proper required preload is often a major factor controlling a joint’s fatigue life. In a typical rigid assembly, an external load below that of the preload has little effect on fastener tension. Thus, the fastener generally does not fail in fatigue even if such a load is repeatedly applied. For rigid steel parts, the conservative practice is to tighten the fasteners to 75% of yield. Lower torques should be considered for flexible joints, joints with gaskets, or assemblies subject to high temperatures.
So, using a fastener with a higher tensile strength may not give you the same clamp load after it has been cycled because as the gasket and surfaces thermally expand and contract, the fastener cannot maintain the “spring” tension allowing it to be vibrated loose or suffer other types of failure.
There are formulas to enter the coefficient of friction to the required torque equation, or just use the lubed torque chart. Most of the time the manufacturer will provide this along with the recommended surfaces it's to be used on.
Originally posted by ZCR1: Lol, your forcing me to do work here...
When you share an opinion that is different than the opinion held by the masses, it is normal to expect to do some explaining!
I do agree with the premise that the Young's modulus of a bolt should not be changed willy-nilly without some thought.
However, I disagree that the elastic properties (such as E) of 8.8 and 10.9 are different because they have different tensile strengths. My proposition is that the elastic properties are independent of the tensile strength, or hardness.
…. Essentially, if you tighten a higher grade fastener to a low torque, it may be at the very bottom of the elastic clamping zone giving very little room for joint relaxation before the fastener becomes loose because it has not stretched as far as a lower grade fastner.
This is one reason why people who install ARP universal header bolts have loosening issues when torqued to factory specs. Stage 8 just adds a mechanical lock so they can just turn out one grade of fastener saying that it won't become loose, but can loose clamping force after joint relaxation even though the fastener hasn't turned which can lead to gasket failure.
[This message has been edited by ZCR1 (edited 01-26-2019).]
According to the factory service manual for an 87,
For the V6 it's 18 ft lb For the 4 cyl it's 43 Nm (31 ft lb) for the Outer 4 bolts and 50 Nm (36 ft lb) for the inner 3 bolts)
I'm looking at the Fire Recall TSB and it shows on the V6 that it uses hex-head bolts on the bottom of the flange and studs with nuts on the top of the flange. Bolts and studs torque to 25 Nm. The nuts that hold the heat shield onto the studs torque at 5 Nm. The manifold to crossover flange bolts torque to 30 Nm. I don't know what Nm converts to in Lb Ft.
In this TSB, there are no specs given for the L4 engine.
