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Last time I had the head off I was thinking about the hassle of lining up all the different pieces of the the headers, crossover, downpipe.. I was talking to an former 951 guy and told him I was considering pulling the exhaust mani studs and going with bolts to facilitate some wiggle room. He told me that the bolts used in the intake manifold are a perfect fit. Yep, full thread engagement with the stock exhaust manifold washers in place.
I wanted to make a comment about the selection of stud alloys. I know many of us have had seized studs, then replace them with stainless and still end up with seizing. Often we look at the yield or hardness and think that is a good number. What is often missed is what is called the galling resistance or galling stress. Galling is not something that happens to all materials so for instance a steel fastener it can be above the tensile, in this way you would never see it. In stainless steels particularly in the austinetic stainless but as well in the martinsitic, 300 and 400 series alloys. The galling resistance for instance of 316 ss regardless of condition is 3ksi.. the tensile is as low as 30ksi. So you can see how little stress uou need for the screw to gall. What are the options, use a dissimilar metal nut, in this case the stainless will still fail, it will change the number slightly but not above say 15ksi, if the nut is softer then you will still be able to remove it, we call this sacraficial. You can also coat with moly or tungsten or.. antisieze to help, copper nuts are good. The real solution is nitronic 60, we did some research on this at applied years ago and at that time had helicoil make special helicoils for us. For a long time they where not availible but now you can find then as galling resistant helicoils. To that end I have always wondered why people use 316 as a fastener without paying very special attention to dealing with the galling issue. So when tightening a stainless fastener be aware to go to the proper torque it is often set below the galling stress. The issue with heat cycling is that you can easily exceed the torque to to the differential thermal expansion.
Oh wow lots to consider. But as DIY, it's hard to locally source specific fasteners; I have a hard enough time finding metric size selections, never mind galling-resistence specs. I got exxhaust studs from Porsche. What are they made of?
Often an simple alloy steel. That isnt a bad choice. Low coefficient of thermal expansion means it will keeps its tension through heat cycling and very high galling resistance. I guess in the end what I am just saying if you use stainless, at a min use antisieze, add using copper nuts is even better. Dont be surprised when they gall. I use common stainless fasteners on my exhaust, but always use antisieze to help.
Ahh got it. I have heard that stainless is soft and really not suited for our purposes as many think; I use anti-seize now on everything but have recently learned it may cause torque problems
FYI here is what 30 year old factory spec fasteners I took off my exhaust look like after at least 10 years in my climate; not sure what could prevent that!!
10-29-2018, 11:11 PM
Header bolt option
