I wouldn't go any lower than a grade 8.8 (maximum torque very approx. ~25 Nm), as although a grade 5.8 would probably be fine up to ~15 Nm, I'd prefer to over-engineer as there's enough other issues to worry about.



I've been enjoying this discussion too, so got in contact with the Pro-Bolt company I mentioned before. A member of their 'Business Support & Product Development' team responded with:

Quote from: Pro-Bolt
Whilst your OEM torque settings fall within our guide, I would not recommend aluminium for replacement exhaust headers bolts. We have not tested this material in this application and my concern is that prolonged use at high temperatures may cause fatigue. I think you would be better using a Stainless steel or Titanium equivalent replacement with copper grease applied to prevent galling.   

Quote from: max on August 28, 2018, 01:02:39 PM
I wouldn't go any lower than a grade 8.8 (maximum torque very approx. ~25 Nm), as although a grade 5.8 would probably be fine up to ~15 Nm, I'd prefer to over-engineer as there's enough other issues to worry about.



I've been enjoying this discussion too, so got in contact with the Pro-Bolt company I mentioned before. A member of their 'Business Support & Product Development' team responded with:

Quote from: Pro-Bolt
Whilst your OEM torque settings fall within our guide, I would not recommend aluminium for replacement exhaust headers bolts. We have not tested this material in this application and my concern is that prolonged use at high temperatures may cause fatigue. I think you would be better using a Stainless steel or Titanium equivalent replacement with copper grease applied to prevent galling.   

Oh yea aluminum bolts will not work. You really want titanium, but an even better idea would be those hollow bolts they seem to have slapped on sport bikes to reduce weight. My SV is full of em. The best feature of those is 2 fold or even 3 fold.
1. Is that they are extremely high quality.
2. They are predrilled. Easy to put in an extractor.
3. They can be sprayed with pb blaster on a weekly basis when in use, and you would get excellent thread penetration cos you can stick the straw inside and spray it, and then plug it with a rubber plug so the pb blaster cane all leak out.

Cool.
Buddha.

Max, thanks for the info on Pro Bolt.  The fact that the engine is aluminum is proof positive that the right alloy of aluminum can take the heat, but of course it is true that a particular bolt may not be made of the best alloy for high heat.  Basically that's why I am wary of just general purpose zinc plated steel bolts--they may have thinner zinc and not be metallurgically suitable for high heat.   

Buddha, titanium is used a lot on aluminum racing engines that have short life and a lot of maintenance, but galvanically it is even harder on aluminum than stainless steel. 

Here is an enlightening professional website page for exhaust system fasteners from a large manufacturer specializing in high performance alloys:
https://www.cartech.com/en/alloy-techzone/technical-information/alloy-selection/selecting-high-temperature-alloys-for-fasteners-in-automotive-exhaust-systems

At the bottom the authors of the page are listed--two professional metallurgists.  From this page: "Note that CarTech A-286 is used for a wide variety of bolting applications.  The alloy is commonly used for fasteners requiring high strength and corrosion resistance in temperatures up to 1300ºF (700ºC).  It combines high strength at elevated temperatures with an intermediate Ni level, between CarTech 431 stainless and CarTech 80A alloy.  This combination makes it an appealing alloy for a variety of exhaust bolts."

This page has a lot of alloys listed in both table and figure form for these high heat exhaust applications.  There is an excellent summary figure I tried to copy and paste, but apparently I don't know the trick to paste the image.

Seems pretty clear from this page that it is probably not a good idea to throw just any old bolt in there--these parts are engineered to this task.  I know that thinking much about it seems like overkill.  But, outfits like this are not paying dedicated pros just for fun--history has proven to them they need to make that investment.  It would be very interesting to see what alloy the OEM Suzuki bolts are and compare it to the alloys listed here.   

Quote from: MaxD on August 28, 2018, 11:26:07 AM
72, I personally will probably be laying in a stock of the zinc plated Suzuki OEM bolts, even though they are overpriced at $3 each.

Yeah I wasn't thinking of exhaust bolts, my originals are fine. But I did replace ALL of my engine side cover bolts with SS about a year ago because several of the originals broke when re-installing them after doing a gasket replacement and I was concerned about them breaking again. I didn't consider the galvanic interaction. But we don't have salt water here, I'm not really worried, but if we get a week or two of constant rain and I'm looking for something to do, I might swap those bolts back.

There's no torque on those case bolts. They are barely above finger tight. The big reason to prefer a higher-grade bolt is not for holding torque but in the event of corrosion the torque required to REMOVE the bolt may be too much to keep from snapping the head off of a lower-grade bolt.

72, like the valve shim situation, a quality bolt engineered to the task with heavy zinc plating and made with high temperature alloy is probably out there for half what Suzuki charges.  I doubt Suzuki makes this bolt themselves.  They probably buy them from several sub-contractors whom they force to compete for lowest price to Suzuki.  Those manufacturers probably also provide them to other sales channels who can't get the "OEM mark-up" and must settle for less margin.  I just don't know who carries it. 

Exhuming this thread as I've been thinking lately and want opinions.  If a voltage source were applied across the corroded junction between a zinc plated bolt and aluminum head, could the galvanic reaction be reversed at least somewhat?  Perhaps enough to allow clean removal of a bolt that's been cold-welded by nature. 

Not sure which polarity you would want or even how to get the current to flow where you want it unless the bolt is already broken off.  Nor do I have any idea how much current would be needed or for how long.  But this idea keeps rattling around in my head and the noise is getting to me.

Kilted, the use of a deliberately applied voltage is used in marine environments to reduce galvanic corrosion.  But, it is in a situation where current flow is very limited by the resistance of salt water between the electrodes.  It conducts, but pretty poorly.  If you have a bolt in threads that are in galvanic contact, then you don't have high resistance.