Fastener grades esp. near muffler

my understanding is that grade 8.8 fasteners are about as good as you can do for a muffler or corrosion resistance.

however, I see higher grade numbers like 10 - is there a much better grade to get for e.g. the exhaust system? what about copper?

UPDATE: thanks for this interesting response - so the term “Stainless” has to do with the plating which is a whole other question, but is the grade in fact unrelated to the plating in theory? Note that what I am asking leaves possible for certain grades to be found “usually” with some type of plating, but only for some reason unrelated to grade.

another question is what does grade say about the alloy? e.g. are copper nuts still available in any grade? Obviously I need to take the info from this thread and search around a while…

I’m not an expert on fasteners, but I would buy stainless steel hardware for the exhaust.

I always thought of “grade” as strenght-related. Higher grades have higher tensile strength, but tend to be more brittle. In an environment with vibration (like an exhaust), we could be talking fatigue failure.

While exhaust clamps DO begin to rust quickly, I don’t think I’ve ever had one rust through. A little rust, quickly, is a good thing…“redneck loctite,” if you will.

Use exactly what comes with the clamp, hanger, etc. As has been said it’s not a strength issue, and even if one rusted out they’re cheap to replace.

The fastener grade is about strength, not corrosion resistance. Exhausts are not high stress areas so the bolts can be 8.8 or even softer. We used to use brass nuts on exhaust manifolds so the nut could be easily cut off in the unlikely event it seized. Grade 4.6, 4.8 or 5.8 should be all be fine. Stainless bolts in any of the lower grades are more than strong enough for any exhaust duty and will perform well with heat/cool cycles. High allow grade 10.9 or higher will get brittle with repeated heat/cool cycles, but that will make them snap off easily if you need to remove them again.

DIY mechanics (or race mechanics) care more about this than pros. You might actually have to remove those bolts again (and again and again, ect in a race car), a pro will just cut them off.

hey thanks for this interesting response - to help hone the question, I added to the original question at the top.

You use grade 2 hardware on an exhaust system except for the connection between the manifold(s) and the head(s).

The reason? Grade 2 hardware is soft enough to where if the fastener becomes corroded to the point where it can’t be removed, you just tighten the fastener until it snaps off.

Don’t waste your money on expensive hardware for an exhaust/muffler system.


There’s no such thing as a “plated” stainless steel bolt or nut. If the fastener is stainless steel, it’s solid stainless all the way through.

Carbon steel fasteners may have a zinc plating for corrosion resistance, but the plating has absolutely nothing to do with the grade (strength).

As was said earlier, grade refers only to material strength. Different metal alloys have different strengths, so a stainless steel bolt might be a different grade than a carbon steel bolt, for example.

There are many types of grading systems for nuts and bolts. The ones mentioned are DIN or ISO, but there is also SAE and MIL specs. I would suggest a little google search would be time well spent.

UNS30400 stainless steel is a reasonable combination of strength, cost, and malleability. It is also referred to as CRES. Other CRES grades are similar to UNS30400, but have lower residual element levels and are probably more expensive. Remember that this is metal, not plastic. The yield stress is 35,000 psi. Even a quarter inch fastener would require a 7000 pound load to exceed the yield stress.

Just do as Tester says and use low strength Grade 2.

There’s really no advantage to having exhaust hardware significantly outlast the components they retain.

so I presume everyone thinks the local HW store has grade 2, or any, grade of fastener? my go-to store seems to have 8, 8.8, but no 2.

so am I going to have a better time finding grade 2 online, say at Bolt Depot?

UPDATE: it seems that metric bolts do not come in grade 2?.. among others…

Don’t the clamps, hangers, etc. come with the needed hardware? What exactly are you attaching?

@texases yeah sure, I can order the things that don’t come with the hooks or hangers already along with them, … depending on the vendor … and whether the part specified in the manual is actually in use anymore, if not then need the superseded part, … seriously, I can go down that road.

yet the ones that came off look like the ones in the bin at the store, minus the corrosion and what that covered up,…

I am trying to get the discussion back to the general topic of fasteners, rather than my particulars/specifics about the exhaust – @TwinTurbo has a good point as well, I could try this - but also, threadlocker occurred as something that might help -but you can see this is getting to be a separate topic, and this might be dealt with better with a separate thread, though from what I have seen here folks say to just put in in with this thread. either way is fine with me.

@JuniorMint - you might want to read this, and any of the links that look interesting to you:

@ JuniorMint: Metric and SAE have their own respective standards. For SAE, grades 3, 5, and 8 stand out in my memory.

Metric fasteners don’t have a “grade 8” standard…but there may well be a metric standard that (roughly) duplicates grade 8.

Then, there’s AN (Army/Navy) standards for aviation…and I think (don’t hold me to it) seperate standards for auto racing.

And, for a “too much information” bit, bolts are rated as to how much force they can endure before deforming, not how much they can take before snapping in two. A high-grade bolt is strong, but brittle (relatively). For an application where the bolt has to take one huge load without severing, then be replaced, (like a seat anchor bolt in a bad crash), a mid-strength bolt might be better, if it stretches under force, but resists snapping in two.

I’ve rebuilt or restored dozens of cars over the years. Replacing almost every single fastener on some. There are many fasteners that have specific requirements and you have to be careful to meet or exceed those requirements. However, with the exception of the manifold bolts, you can use any type of fastener on the exhaust. Its not critical or particularly demanding in any way. If you have flanges/hangers you might want to disconnect, use something with corrosion protection. Otherwise, just use what you can find.

The ACE hardware stores around here have extensive supply. Even Lowes has good selection.

About 10 years ago I restored a 69 vette that I wanted close to OEM. I bought hundreds of fasteners right from a Chevy dealer parts dept. A buddy worked there and I gave him lists of part numbers from my assembly manuals. There were very few I couldn’t get. Surprisingly they were very reasonably priced too.