Last weekend I changed out all my shocks for Bilstein HDs. Only problem I encountered was on the left rear bottom shock bolt. It is supposed to be torqued to 162ft-lbs. I did the right rear first - no problem. When I got to the left side, I started tightening the nut on the bolt (with my recently re-calibrated Snap-On Clicker Torque Wrench) and I never got a click from the wrench. I kept tightening it one rachet-knotch at a time and kept saying to myself "that is a lot more than the other side took" finally the bolt broke (felt like butter when it went) twisted off right behind the nut - torque wrench never clicked? Since no place in town carries that bolt/nut combination I had to go to the hardware store and purchase a Grade-8 5/8" x 4" bolt/washer/nut (24mm head) to get things back together until I can order the right one from GM. When installing the Grade-8 parts, I started getting into the same issue with no click from the wrench and I was sure it was tight enough, so just for the heck of it I switched the torque wrench to the bolt-head side (instead of the nut) and without touching the wrench settings I instantly got a torque click that it was at 162ft-lbs?

So two questions:

(1) Why would torquing the bolt vs the nut give me a "click"?
(2) The stock bolt head says M 9.8 and I temporarily replaced with a hardware store Grade-8 (exact same size). Am I in any danger if I take it easy until the replacement comes in? I guess the stock bolt is a Grade 9.8? Is that right?

 

Fastener torque is a measure of preload, which in turn by standard practice is a function of friction. To get an accurate measurement, you need to have a repeatable method of obtained a consistent friction during the tightening sequence. This is why ARP uses their own lubricant on the threads, they know the friction characteristics so that anyone can obtain the correct preload by measuring torque.

When you switched from the nut to the bolt, you added in all the friction of turning the bolt inside the shock mount and the torque wrench clicked. However, it doesn't mean the bolts is properly preloaded since all of the friction you measured with your wrench was not from the threads.

The 9.8 on the bolt head is a standard for the bolt material strength. I would need to dig out the values to compare to the Grade 8. Are you sure you installed a metric bolt, they are not usually marked as Grade 8.

 

Thanks for the response. What you say makes sense that by adding the additional friction factors at the bolt head end, the torque wrench would click sooner than on the nut end - especially if the bushings and shock were exerting additional force, but I am not sure it explains why when I tried to apply 162ft-lbs to the nut side, the wrench never clicked and the bolt broke? In addition to the wrench not clicking it was physically obvious that I was applying way more manual force than it took to get a click on the bolt side?

I guess what you are saying is that in this situation that torquing the nut side may be slightly more accurate? But so far I have not been able to get the wrench to click on the nut side of either bolt set before bolt failure?

I am not certain that I purchased a "metric" boltset at the hardwrae store, but I am certain that it is Grade 8.

I guess the big question is that when GM states a torque value of 162ft-lbs in this case, what is the best procedure to guarantee that I am hitting the required value as they intended? Should I add a particular lubricant to the shaft and threads? It also seems to me that at such a high torque value it would practically overcome any preload friction being applied perpendicular to the bolt.

 

metric grade 9.8 slightly stronger but roughly equivalent to standard grade 5....the grade 8 you replaced is superior & equivalent to a metric grade 10.9.

what size bolt is that anyways? i'm offshore right now, so i can't look it up in my service manual...

there are different torque values for lubricated & unlubricated fasteners & coated vs. uncoated fasteners...lubricated fasteners require less torque than unlubricated...it's all about the coeff of friction...

the 'butter' feel is after the bolt has been torqued past it's yield point..

as torque increases, friction increases...

 

One option might be to put the bolt through some rigid structure and try tightening the nut. For example, if you put the bolt through the jaws of a vice endwise and then tightened the nut the bolt obviously won't squeeze the solid metal of the vice and should come up to torque.

GM doesn't specify any lubricant. If you put some on, and use the same torque value, you also increase the preload in the bolt. Depending on GM's design parameters, say 75% of material yield (I don't know what they used), the use of lubricant might put you over the yield point trying to reach the torque value specified and the bolt/nut will break. You definitely yielded the original bolt based on your description. Lubricant or possibly a pre-existing damage condition might have been the original cause.

A bolt calculation could be done to determine if the specified torque value is near or over yield, in which case a new value could be determined.

 

Grade 8 is an inch size bolt.

M9.8 is metric and is about 5% stronger than a SAE Gr 5 bolt

I'm guessing the original is 16mm as the temporary replacement is 5/8" (.629" Vs. .625")

162 ft-lb is about 80% of proof for a grade 5 or a little higher for M9.8, and should should not have broken a 5/8" or 16 mm bolt unless the bolt was bad or the threads were lubed. The torque values are normally with dry threads.

 

I read some torquing instructions on the Internet last night and they support all your advice. I should be torquing with "dry" threads (which is all I have done so far). I will get a new GM part numbered bolt in and try again.

 

I happened to download the ARP catalog the other day and read it on the plane yesterday. They explain a lot of technical information on fasteners in the first 20 pages or so, if you are not familiar with bolted joints it might be worth downloading and reading for background information.

 

Thanks - will do.

 

This EXACT same thing happened to me 20 minutes ago... I'm on the forum searching for answers as well.

Rear drivers side bolt, torqued the bolt right through the threads (Torque wrench on nut side, no lube...dry). Never even got to 150 ft/lbs. The passenger side went on just fine.

That's a pretty odd coincidence...

 

Good info here

 

Here was my replacement:

Grade 8 5/8" X 4" with 11 Coarse Thread, Locking nut and washer. Thats stronger than what GM had on there and was nearly identical (had about 1/2" more thread towards the head).

Grade 8 (left) Broken GM (Middle) Stock GM (Right) Replaced both sides with grade 8s