GM Upping torque on M12x1.75 class 10.9 bolt? 4

[95yukon]

The cam bolt in GM 6.5 Turbo Diesel engines is an M12x1.75 grade 10.9. GM changed the torque spec from 65 Lbf-ft to 125 lbf-ft without changing the bolt.

All torque charts I read show approximately 73 Lbf-ft as being the maximum torque for this bolt. I simply can't find justification anywhere why this would work.

I know mechanics who have used the 125 Lbf-ft torque and were successful, but it seems this would be a failure waiting to happen.

Can anyone explain why this might work? I the fact that the bolt is so short? - it is about 45-50mm long, threaded right to the head.

 

[rhodie]

When you say the bolt is unchanged, are you sure of it? Could GM be heat-treating the bolt differently to alloe for the increased torque rating?

 

[TVP]

There are a number of reasons why this is a reasonable torque specification. First, generic torque charts are very conservative, and not used by the Materials & Fastening Engineering Dept. at GM when developing torque specifications. Second, friction plays a huge role in converting torque into bolt preload, which is why generic torque charts are not to be used as a standard by which to judge a particular joint.

The key concept is preload, which can be easily converted into stress in the fastener. An M12 x 1.75 bolt has a proof load (90% of the force needed to cause yielding) of 70 kN, and an ultimate load of 87.7 kN minimum. Using a friction coefficient of 0.2, even a simple calculation shows that 88 Nm produces ~ 37 kN, 99 Nm produces ~ 41 kN, and 169.5 Nm produces ~ 70 kN. All of these values are less than or equal to the proof load of an M12 x 1.75, hence no yielding will occur. CoryPad has frequently contributed to this forum (and others) the necessary engineering calculations for performing a suitable analysis. I'm sure he will follow-up with a more detailed response.

 

[rhodie]

Before I sound like an idiot, I understand that the bolt has been rated 10.9, but could the process of heat treating have been changed to allow for 12.9-like values?

 

[95yukon]

Thanks for the responses - they are helpful getting an insight to what GM is doing.

I have no idea about what GM is doing with the bolt nor any way of knowing the properties, just that the new bolt I bought is also stamped 10.9. The standard reference charts is what I have been looking at including torque charts from CAT, referencing the 10.9 class info.

 

[CoryPad]

95yukon,

What is your source for the 125 torque number? If it is from GM directly, it is probably 125 N m since GM has used SI units for more than 10 years. 125 N m (93 lbf-ft) for an M12 screw with property class 10.9 is an appropriate torque level. For "normal" joints, fastener length does not have a large influence on torque-tension performance. Check the FAQ section of this forum for information regarding how to calculate torque-tension behavior.

TVP,

Your analysis used a friction coefficient that was a little high - for any current GM applications, the max is likely to be ~ 0.16, while the mean would be ~ 0.13, and the min would be ~ 0.10. Also, the min friction coefficient is important to consider at high torque levels - high torque plus low friction can lead to preload that exceeds the fastener's capacity.

rhodie,

There is no possibility of higher strength than property class 10.9. Strength levels above this lead to excessive sensitivity to hydrogen-induced cracking (either from processing or corrosion during use). This led SAE to remove 12.9 from its specifications, and the automobile manufacturers followed suit.





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[rhodie]

Thanks, CoryPad!

 

[95yukon]

CoryPad
Thanks for the response. The 125 is in lbf-ft, not Nm. I questioned this too. This is covered in the GM service manuals for 6.5 diesel engines. I have a 1995 GM manual and from others I have spoke to the same is true through 2000 model years.

The previous spec was 65 lbf-ft plus use of a thread locker. The bolt holds a gear onto a camshaft which has sometimes came loose and destroyed parts, especially on newer engines with electronic diesel distributor pumps. AERA (American Engine Rebuilders Assn) was supposed to have a GM document on this as well but I am not a member and have not found much there.

The builders I talk to who have done rebuilds state they either use the original torque or limit the max torque to 100 lb-ft instead of pushing the upper limit, using Loctite 271 as well.

The original bolt head has a flanged head, the replacement is identical except for the flanged contact area. Apparently the bolt is used on cams and also to hold brake rotor supports to axles, although I don't know the specific torque used in the brake application.

I just want to ensure I don't leave half of this bolt in the cam before I get the engine back together.

 

[CoryPad]

95yukon,

I worked at GM in Materials and Fastening Engineering until 2002. I am surprised that any GM document would have a torque listed in lbf-ft. Perhaps the Service Group that creates service manuals made an error in copying/converting/etc. the original torque, which would have been specified in N m.

Without knowing more about this particular joint, I would not use 125 lbf-ft (169 N m), as this could cause failure of the internal or external threads. Does this bolt have any special features? Is it a thread forming bolt? Does it have a serrated bearing face? What color is the coating? Does the bolt mate with a tapped hole? If a loose nut is used, does it have a prevailing torque feature?

If this were my problem, I would contact the dealer to verify the proper torque specification, and I would contact the GM assistance center (using the GM website) - perhaps a question through them would go to the Engineering department, and could be answered by the person who designed the joint. Good luck.





Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[95yukon]

CoryPad,
I tend to agree with what you say - it seems strange that GM would list this.

I suspected there was an error but I have the torques dual listed in my factory GM 1995 service manual and calling the local dealer confirmed that the torque is also listed as 125/126 lb-ft or 170 Nm in the year 2000 factory manual.

I would be more comfortable going with 125 Nm as this appears to be a reasonable torque for the 84.3mm^2 effective tensile area of a 12mm class 10.9 bolt with 1.75 threads.

The original bolt does not appear to have anything special on it, no thread locker, black colored like a gun bluing. It simply holds a gear onto the keyed front end of the camshaft. It clamps the gear with a large washer and it has kind of a flanged hex head.

The manual does not mention thread locker but the new bolt I purchased has a yellow colored thread locker supplied on the threads. The new bolt has a zinc color to it and a plain hex head. It has 10.9 stamped on the head.

Both hex heads are 18mm.

I have a 2 page document full of GM contacts that I'll try as well as the GM web page. I worked at Cat for 5 years and discussed this with some of the people I worked with and they don't understand it either.

Thanks.