Thread: Which bolt material for upper shock mount?

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Hi -

I was suprised when I found a twisted off bolt in my upper shock mount. I had used a grade 8 stainless bolt from Home Depot. I'm usually not one to over tighten bolts - but I could have one this one.

Question - should I be using carbon steel rather than stainless?

Luckily I was able to drill it out and extract using an easy out and heat.
When I drilled it out - it seemed soft - that is - drilled out rather easily????

Use a carbon steel Gr 8

If you really want stainless use a ARP stainless bolt.

Home Depot carries stainless bolts but are around a grade 2-3. I would not trust them on a critical fastener. Use Grade 8 standard steel bolts- either cad or black oxide.
The stainless stuff from HD belongs in the trash.
There are stainless grd 8- go to a specialty fastener store or site for those. I use King Bolt in Covina.

Most of Home Depot bolts, regardless of the grade --- aren't worth much. Most that I've seen say Crown Bolt with the manufacturer stated to be somewhere in the Far East. To bolt something on your lawn mower they will probably work. For a stressed fastener, try a known US, preferable, source.

SS bolts are seldom any better then Gr 2 no matter where you buy them. ARP as stated above (they do have some 180,000 psi SS) and McMaster Carr are good sources. With McMaster, you can choose many grades right up to Gr9!! http://www.mcmaster.com/#cap-screws/=93s0h6

Good subject!! I went and did a little research on this as I was under the impressioin that Stainless Steele was "NOT" rated by grades ie: Grade 8 or Grade 5 etc.

And they are not rated by grade, they are rated by alloy as to there corrosion resistance and it seems that most SS falls way below steele bolts in strength, Being generealy in the Grade 2 equivalent. Even the best SS bolts rarely meet a Grade 5 equivalent. The only exception seems to be ARP which does make a Stainless Steele bolt rated at 180,000 PSI which is much higher than even a grade 8 Steele Bolt rating which is 150,000PSI. However even the ARP bolt is not recommended in Grade 8 applications, it is strong but as with all Stainless it is stretchy and this can lead to serious problems in "SOME" applications.

Interesting stuff though!!

RS

What the other guys said on cheap hardware,,,especially stainless!! Also, anytime I use a stainless fastener I put a bit of anti-seize on the threads and the shank of the bolt....

Agree the normal SS bolts are not good for critical service. A friend used SS button heads on his crank pulley to the balancer because he wanted the polished look, and after a few thousand miles all six sheared off and he was in a pickle. Like Jack said, carbon steel grade 8, US made is best.

As I recall Grade 8 is tensel strength NOT sheer strength. Two entirely different things. As mentioned above S/S bolts are not ment to hold any real load they are used in adverse areas I used to use them on heat treat furnaces so they could be removed to make repairs.

Hash marks on the head of the bolt will tell you the grade. 3 marks-Grade 5, 4 marks- Grade 6, 6 marks-Grade 8. No marks, Grade junk.

Just a heads-up for those of you who are putting something together that demands absolutely the finest hardware available. It's hard to beat Caterpillar bolts and nuts. Not only do they make the finest, but the heads of the bolts are taller so that your wrench will not slip off easily. I know all of you have used those fosdick offshore bolts with the little short head that your box end wrench slips off of and rounds the flats. AAAAARRRRRGGGTGGHHHHHH.

Glad I'm not the only one to use Cat bolts.

I've always used a good quality cad plated grade 5 bolt in suspension situations. Reason being, although grade 8 are much higher in tensile strength, they are brittle in shear. The grade 8 will break under shear and the grade 5 will bend under the same load.

Originally Posted by wingman9

I've always used a good quality cad plated grade 5 bolt in suspension situations. Reason being, although grade 8 are much higher in tensile strength, they are brittle in shear. The grade 8 will break under shear and the grade 5 will bend under the same load.

That is an old wives tale. A grade 5 bolt will bend, and break, before a grade 8 bolt will even bend. A grade 8 bolt is nearly 50 percent stronger in shear than a grade 5 bolt.

Originally Posted by enjenjo

That is an old wives tale. A grade 5 bolt will bend, and break, before a grade 8 bolt will even bend. A grade 8 bolt is nearly 50 percent stronger in shear than a grade 5 bolt.

I stand corrected. Upon further review...

“As an empirical guide, shear strengths of carbon steel fasteners may be assumed to be approximately 60 percent of their specified minimum tensile strengths. For example, an SAE grade 5 hex cap screw has a specified minimum tensile strength of 120,000 psi. Therefore, for design purposes, its shear strength could be reasonably assumed to be 70,000 psi.”
The Industrial Fastener Institute (Inch Fastener Standards, 7th ed. 2003. B-8)

Since the tensile strength of a grade 8 bolt is considerably higher the shear strength must be as well. Consider this chart:

Originally Posted by wingman9

I've always used a good quality cad plated grade 5 bolt in suspension situations. Reason being, although grade 8 are much higher in tensile strength, they are brittle in shear. The grade 8 will break under shear and the grade 5 will bend under the same load.

Wive's tale, indeed, but it is rampant in the hot rod world. It's so widespread that even one of the guys at Pete and Jakes in Peculiar, MO told me that one time when I questioned the bolts on something I bought from them. It was the only piece of questionable advice I ever received from them.