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Old 09-12-2003, 10:35 AM
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Bolt Strength Question

Does anyone know how much torque a Grade 8, 7-16" X 20 Bolt can take? Is this the basically the same grade that GM used on the Small Block Crank bolts?

2 Days ago I broke off an ARP crank bolt in my brand new Eagle 4340 Stroker crank, I got it out & now I don't have another ARP bolt & don't know if I had another ARP bolt if I would use it for this application. At the speed shop by my house they had Moroso Crank bolt kits for like $12.00. It looked like nothing but a crank washer & and Grade 8 - 7/16" X 20 bolt. Instead I went to Fastenall a bolt specialty store and got 8 Grade 8 bolts just like the Moroso ones for $3.00. I also have a used GM crank bolt. But would rather use a new bolt.

I also want to thank everyone that gave me the ideas on how to safely remove the bolt. I took pix of the whole process that I went through and will post them up in the original thread after I finish up firing up the new 383 today.

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Old 09-12-2003, 03:43 PM
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Rule of thumb for a Grade 8x20UNC is 15 000 psi of tensile strenght. So that means a dry torque of around 80 Ft./Lbs, olied would be around 60 Ft./Lbs

Good luck

Sorry i made a mistake, i meant 150 000 P.S.I of tensile strenght.

Hey what do you know, my first mistake! lmao, just kidding.
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Old 09-12-2003, 06:09 PM
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I use a head stud on my motor for the balancer, just a thought it's Grade 8x20UNC also.
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Old 09-12-2003, 06:56 PM
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How can the bolt break? If you got the crank bolt tight and you go too far the crank moves?
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Old 09-12-2003, 07:27 PM
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This is not the first time I have heard of one breaking and wont be the last.
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Old 09-12-2003, 08:31 PM
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I watched a guy break an 1 1/4" grade 8 bolt with a wrench. He was pretty proud of himself, showing his strength and all. He just wouldn't admit that sometimes you get a bad bolt in the bunch. I routinely have to torque the same type bolt to 1,200 ft.lbs. Unless you were going crazy with the breaker bar, it was most likely a bad bolt. It happens.
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Old 09-12-2003, 09:05 PM
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Stainless steel is bad for 'grabbing' and 'jamming' if that happened to be the material of the bolt. Oiling them up or even Loctite helps a great deal. I swore so many times bolting a SS bolt to an Aluminium bracket.....and the corrosion after just a short while, lets not even go there
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Old 09-13-2003, 07:26 AM
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Quote:
Originally posted by KeithB123
How can the bolt break? If you got the crank bolt tight and you go too far the crank moves?
Yes and that works most of the time, except for when you have a brand new engine with almost 11 to 1 comp and all the plugs installed. When I snapped the ARP bolt that was in there I was just trying to turn the motor about 10 degrees & the bolt snapped.


Remember it's always best to turn your motor over with a crank socket and not the crank bolt. I knew this, but didn't follow the procedure, but now after knowing this for 20 years I will finally put it into practice.
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Old 09-13-2003, 11:31 AM
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Go to the junkyard and stick one in your pocket.Your talkin bout the harmonic balancer bolt right???
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Old 09-13-2003, 11:45 AM
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Hello.

I build a lot of small block chevys and some as high as 14 to 1 compression ratio. I use a lot of ARP bolts, and have found the to be as good as it gets. If you call their 800 number and tell them about that bolt just breaking I bet they will want it back and replace it if not more. They have great products and are great people. I buy the balancer bolts for most of the milder motors in bulk ( grade 8) at fastenall as well. They work fine, the washer i have to buy from GM.

Im sure ARP will be shocked to see this failure.

Thanks
Jeff
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Old 03-29-2005, 10:04 AM
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I have a complete ARP bolt set on my engine and had three timing chain cover bolts snap off right under the head. They hadn't even begun to bottom out and there was no force on the bolt. The heads just fell off.
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Old 03-29-2005, 07:48 PM
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Quote:
Originally Posted by crshrmn3
I watched a guy break an 1 1/4" grade 8 bolt with a wrench. He was pretty proud of himself, showing his strength and all. He just wouldn't admit that sometimes you get a bad bolt in the bunch. I routinely have to torque the same type bolt to 1,200 ft.lbs. Unless you were going crazy with the breaker bar, it was most likely a bad bolt. It happens.
how big a bolt can take 1200 ftlbs of torque? id like to see your torque wrench!!
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Old 03-29-2005, 08:29 PM
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A 1 1/4" bolt , and, I would guess, a torque multiplier was used, however, plese correct me if I am wrong.
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Old 03-29-2005, 08:54 PM
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Just to comment on the possibility of a bad bolt, My brother is a machinist for a company that makes specialty hardware. They make everything from hardware store stuff to aerospace hardware. They are having all kinds of problems getting repeatable quality steels from the american steel mills. One batch will be dead on in all aspects. The next batch might meet the hardness spec but be extremely brittle. The problem has gotten so bad that they have had to start getting Japanese steel in order to meet the specs on their high end hardware.
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Old 03-29-2005, 09:12 PM
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Guess who is buying all our steel here,,,the Japanese. Dang middle men!

BTW;SAE bolt strengths.
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Last edited by 4 Jaw Chuck; 03-29-2005 at 09:21 PM.
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