Just scored on a lot of grade 12.9 crank pulley bolts
 

I'm in the middle of tearing apart my engine to replace the timing chain guides, and I've read on the forum that sometimes the crank pulley hex socket screws can shear.

so I decided to go ahead and replace the six of them with new screws. this began a journey into learning about hex socket screws.

I went down to my local ace hardware, and comparing my original screw it turns out these are M8 (8mm thread) x 30mm (length) and 1.25mm pitch.

http://i.imgur.com/HSuNn.jpg

http://i.imgur.com/Krhmq.jpg



They were listed as "grade 8.8". huh?

turns out metrics strength grades are different. standard fasteners range from grade 2 to grade 8, whereas metric grades range from 8.8 to 12.9. from Frequently Asked Questions on Bolting Matters

http://i.imgur.com/Zypwo.png




then I realized they had stainless screws as well. why not baby my baby for a few dollars more?

http://i.imgur.com/ATRtH.jpg




however, after reading about it, it turns out stainless fasteners aren't as strong as the strongest steel fasteners (mostly because they are low carbon so they can't be heat treated as effectively?).

so then I started searching online for hex screws, and I stumbled upon this:

http://i.imgur.com/U8uhx.png



score! the size I need in 12.9 strength, at $0.30 a piece!

I only need 6 of them, so if anyone else is planning on having their crank pulley off in the near future, PM me and I'll ship some to you.

more on the metric strength grades from Metric Bolt Properties, Grades, and Strength

so 12.9 means 1,200 mega pascals ultimate tensile strength, and the minimum yield tensile strength is 90% of that.

this is sweet!
I may put them in a sticky in the parts forum.
help ya get em all sold.
please PM me when they are gone to remove the sticky!

thanks vstech!

came across these torque specs from US & Metric Bolt Torque by Grade

I wonder how these compare to the service manual's values?

http://i.imgur.com/dOR7T.png

12.9 might be really brittle and shear-off easier than a 10.9 or 8.8.......

what Hardness do OE bolts use?

They are usually 8.8.....the new bolts I put on one of our diesels from the dealer were 8.8

ahh. then, I'd not recommend using them in a high stress area... if OE used 8.8, that's what I'd stick with!!!

d'oh, hadn't thought about that.

FYI
 

In the mid 1970's MB delivered the 616 and early 617 with grade 10.9 pulley bolts.

Failure was embarrassingly quick, the bolts where superseded to grade 8.8, and mandated NEW fasteners for every remove/install = the issue vanished.



.

thanks for the confirmation whunter.

vstech, you can remove my sticky :)

FYI: The bolts now provided by Mercedes-Benz are grade 10.9, not 8.8 like the originals. Let's hope they hold up.

https://lh4.googleusercontent.com/-U...0/DSC07858.JPG

I'm confused... higher grade should equal higher tensile strength, right? Maybe the 10.9 that failed so bad were poorly manufactured?

Ok, I'm thinking now that lower grade actually flexes more and would not shear the heads off.