General question - crack exhaust manifold - consequences?

[Paul S.]

Re: General question - crack exhaust manifold - consequences?

Getting penetrant to the threads of the exhaust manifold bolts is not so easy. I see most people putting the penetrant on the bolt heads, and I would suggest that is counterproductive. Putting the penetrant on the bolt head only will make the socket slip on the head, and it is unlikely unless the engine is lying on its side that the penetrant will reach the thread. The interface between the manifold and head is where the penetrant needs to go, but whether that seal will allow it to pass is another matter. It certainly can't hurt if placed at the interface of manifold to head.

I've always understood that the effect of Aerokroil or other penetrating oil is to penetrate where the bolt head meets the surface not the bolt threads. For most fasteners you would be hard pressed to get penetrating oil on the bolt threads, in any situation. I typically treat (spray) where the bolt meets the mating surface--not tried my exhaust manifold bolts, so I don't know how hard or easy that would be.

[Terry M.]

Re: General question - crack exhaust manifold - consequences?

If the purpose of the penetrant was to reduce the friction between the head of the fastener and the object being clamped, why do fasteners break off at or below the surface they are threaded into? I have rarely had just the head of the fastener break off. When the shaft breaks under torque it would seem to me it is the threads holding the fastener in place and the shaft fails when the torque applied exceeds the strength of the fastener material. My experience is that it is usually corrosion holding the threads tight.
Regardless the mechanism removing the fastener is the goal, and whatever works is OK.