how many rockwell's in a Benz lug bolt?
 

I need to get some MB bolts threaded further up the shaft (don't ask ... or if you're curious, see my post over on the main page.) Local machine shop says it can be done with their single-point carbide bit, but not if the lug bolts are too hard. Anyone care to guess the "rockwell" hardness for '99 E320 standard bolts? Thanks in advance.

Norman Rockwell?

actualy I don't think any bolts use that scale as hardness is not the only factor in the strength of any alloyed steel. I was a partener in a small machine shop way back and learned that there are well over a thosand different alloys used in steel that controlled abrasion resistance,ability to take impact, shear strengh and so on. metric bolts have a number stamped on the end denoteing hardness or tuffness as american bolts are stamped with varering numbers of crossing lines like a star for classing them.

You can find fastener shops in any larger city that will have a selection of bolt and fastners from bridge sized to tiny.........

William Rogers........

Most wheel bolts have a bulk hardness between 32-35 Rockwell "C"....... BUT, most of them are cased with a thin (0.003") case with at least 77 HR15N (don't have the conversion at my fingertips). So, even if you succeed in cutting threads farther up the shank, you'll lose the benefit of the hardened case, which imparts a lot of fatigue resistance. The fatigue resistance is not just from the extra hardness, but from the residual compressive stresses in the case. Plus, all the wheel bolts I'm familiar with have a corrosion-resistant coating. The cut threads would not.

The bolts are purposely designed with tight metallurgy, and are a critical safety component on the vehicle. They are so critical that the Transportation Ministry of Canada has a zero tolerance policy on defective wheel bolts. I urge you not to mess with them. There are many different size wheel bolts available on the market. Try to find a size that fits your project.

William, hardness has a nearly one-for-one correlation with strength. But as you can see from the above information (and as you suggested later) hardness isn't the only criterion for a part that won't fail during use.

rockwell
 

funny you should ask. i just had 12 valve stems tested at a lab for their r scale hardness. anyway....

the threaded stud is likely in usa terms a B7 or called grade 8. very hard because they've been treated. in metric terms 8.8 is equivilent to our grade 5 tough but not case hardened like B7. from there they go something like 10.2 , 12.2 etc:.

frankly if you cut thru the case hardening you've got untreated steel that won't be that tough. in general carbide cutters treated or with special coatings can machine M2 HHS steel. so in real time cutting this b7 type material is easy.

if you want a hard finished stud i would suggest having a machine shop thread some 1145 grade bar stock then have that heat treated. that way you can work the steel to what you want without changing the integrity of the piece.

if you know someone with a warner swasey turret lathe this would be a piece of cake.

I forgot one thing - wheel studs have roll-formed threads. Roll-formed threads have higher strength than cut threads. Again, you really shouldn't alter an engineered product unless you know what you're doing. BTW, part of my job is qualifying wheel studs for automotive production.

I can't add to the discussion, but I just had to say:

What a cool job Kestas has...

Thanks csnow
 

I like my job. I work in a lab and do a lot of forensic metallurgy on automotive parts (kind of like what I do when I'm wrenching, only I get paid!). I like it because I produce facts and results that are rarely disputed.

If anybody from industry wants to talk shop, I'll be at the North American Testing Expo October 29-31 in Novi, Michigan. I'll be manning the SKF booth hawking our testing services.

Yeah, I break software for a living. Getting paid to destroy stuff is the best!
Something more inherently satisfying about destroying things in the physical world, however...

Loosely related shop question for you:
Is there any significant difference in corrosion resistance among the various classes of hardened fasteners? 8.8, 10.9, 12.9...

No real difference. The difference is in the coatings. Phosphated + oil on the surface helps a little.

corrosion factor
 

typically no better in fighting corrosion.

fasteners in both us and metric are coated with zinc dichromate. zd can be in most any color of clear (silver) yellow and they also have green, red i believe black as well.

clear (silver) is used on grades 2 and 5 us and 8.8 in metric.

yellow is used on grade 8. you will also find black oxide color on b7 rod and some bolts and socket head cap screws.

for corrosion resistence you need to go to an 18-8 material which is stainless steel. the next step up from 18-8 is 316 stainless for corrosion. the only down side to stainless is the strength of the bolt. if you need to use "BRUTE FORCE & IGNORANCE" on the bolt stainless isn't as tough as the treated rod or bolt.

on my cars as i replace fasteners i use stainless steel on the metal type (like a wood screw) on the metric machine screw i have to use the clear zinc coating because that's all i can get locally.

good luck
craig