There's a bunch of older threads on this board dedicated to the question: what should I use to replace the horrible original fasteners for the engine covers?
I decided it was time to do some math and metallurgical research to try and get a concrete answer, and get some comments from the old timers on the board.
First off: I bought some 8.8 black oxide coated fasteners from Canadian Tire to redo all the covers on Bessie (1978 CB750F3). I rode her 2 years ago in the Rally, she got rained on, washed, then parked.
The oxide heads rusted badly - corrosion from the environment to the fastener. If I have time I'll post a photo, but the result is that I'm probably going to have to touch-up paint sections of the case and covers just to get it off.
The fasteners themselves did not stick, thanks to copious amounts of copper anti-seize that I applied to the threads.
So that leaves a choice: what now? Aluminum or Stainless Steel fasteners?
Here are the common arguments that get made for both, and my mathematical analysis.
Arguments for Stainless Steel (316/A4 or 304/A2/no markings, see below):
- Honda's original fasteners were steel, so use that
- Stainless means little-to-no corrosion from the elements
- Anti-Seize helps prevent corrosion from fastener-to-engine-block
- The fastener is more likely to gradually stretch instead of break suddenly
Arguments against Stainless Steel:
- Corrosion can be a huge problem even with anti-seize, requiring tapping and rethreading
- Can still have environmental corrosion over time, though less of a problem
- Newer bolts with allen heads can overtorque; the engine will give first
- Aluminum/steel is one of the worst combinations for dissimilar metal corrosion due to galvanic corrosion (Note: titanium is just as bad!)
Arguments for Aluminum (7075T6):
- Lighter (maybe important if you're a racer)
- Aluminum against aluminum means no fastener-to-engine-block corrosion
- Aluminum has virtually no fastener-to-environment corrosion
- No anti-seize required, which allows for proper torquing with a torque wrench/screwdriver
Arguments against Aluminum:
- Junctions of similar metals can gall each other worse than dissimilar metals
- Newer bolts with allen heads can overtorque; the engine will give first
- 7075 aluminum can be brittle; the threaded section will break rather than elongate if the fastener fails
- Thermal expansion may lead to fastener failure
Let's try to get to the bottom of this. First, I investigated whether the hardness of the fasteners might be a problem - that is, would torquing the fastener possibly cause damage to the engine block?
From
these two articles it appears that the large majority of aluminum alloy castings are aluminum-silicon-copper alloys. This would provide a 160MPa yield, which leads to a thermal expansion coefficient of 12.2 (vs. 11.9 for 8.8 steel, 15.5 for stainless steel and 25.6 for 7075T6).
Based on these numbers, the maximum torque values recommend for our M6 and M8 fasteners to prevent stripping the threads, given thermal expansion over a 250C range (25C to 275C), is 7.2 Newton-metres for the M6 fasteners and 21.1N-m for the M8 fasteners.
Based on
medical research and
not-so-scientific analysis, 7.2 N-m is on the very high end of what you can apply by hand with a straight-handled screwdriver. This is good news - unless you're intentionally overtorquing these with a huge lever or torque wrench,
you're unlikely to strip the threads with any of these candidate fasteners. You might want to avoid T-handles as a result.
Next I looked at lengthening of fasteners due to temperature variation. Of the 3, Al will lengthen the most given its coefficient. Over that same 250C range (that's 482F), a 2" Aluminum fastener will become a 2.006" Al fastener, or an increaes of 0.152mm. The Aluminum cast engine block will expand roughly half of that, so the delta between the two is 0.075mm. Again, science shows that
you're unlikely to have problems from lengthening fasteners for any of these products.Now the question of galling. Aluminum will gall more than steel in general, but stainless steel also galls. The rule is that the farther apart the metals are in hardness, the less galling you get. 316SS has a hardness of 215, while the cast Al alloy engine block has a hardness of 80. The 7075 fasteners have a hardness of 170. Given this result, the stainless steel fasteners are somewhat less likely to gall against our engine blocks than the aluminum fasteners, but 215-80 vs. 170-80 is not that much of a difference. If you're really nervous here, you can go with 316 stainless fasteners.
A word on the types of stainless: Pro-Bolt sells 316 fasteners. Alloy Boltz (and presumably many of the other no-name) sells lower-quality fasteners made from 304 stainless steel or "no marking" steel. While 304 SS and "no marking" SS has a lower tensile strength than 316, the differences are a couple of percentage points in commercial fasteners, and as stated above, all of these materials are way stronger than necessary in the SOHC. 316's molybdenum content gives it greater corrosion resistance against harsh environments, especially those with high chlorine content - and since our machines aren't going in the sea any time soon, this is probably not a huge concern.
Finally, a reminder - we are taking about
engine cover fasteners here. The biggest stresses these encounter are thermal shock and some oil pressure against the covers. These stresses should not tax either material to the point of failure.
In short -
aluminum or stainless steel are both fine choices. Stainless will get you less galling but requires anti-seize, and will elongate unlike aluminum. If you go aluminum, the lack of elongation is mostly an issue if you're getting close to the maximal stress - so be careful not to overtorque these. Use a standard screwdriver, or use a torque screwdriver and keep the torque below 5 N-m.
For me, I'll probably go Stainless Steel because they're cheaper. If I was vain and I wanted anodized purple fasteners, I'd go aluminum and I wouldn't be worried at all.
