Aftercooler cap bolt broke

[Brad Arrington]

Reinstalled the bronze aftercooler caps after servicing the my QSB 6.7 480 aftercooler and using plenty of Alco Metalube. Per the user manual I used a a torque wrench set to 33 ft lbs. A number of the bolts kept turning and didn’t hit the spec’d torque value. I felt one of them pop. Luckily it was only partially broken and the entire bolt came out easily.

The bolt was deformed and stretched. It immediately snapped in half in my hand. What’s going on here? Are the bolts intended for a single use? I have a new set of cap bolts on order. How do I prevent this from happening again?

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[Gene Fuller]

Those same bolts could be used in cast iron or steel, where the 33 ft-lb spec would be close to appropriate. I would check the calibration of the torque wrench. It sounds like a lot more torque may have been applied.

I recently did the whole aftercooler thing on my QSB5.9. I used the spec in the Cummins manual (32 ft-lb) with no difficulty at all. I have done a bit of “wrenching” in my time, and this felt fairly tight, but not ridiculously so.

[Paul Berkshire]

Following up on Rob’s comment, the strain in the bolt creates the clamping force across the joint. But every force has to have an equal and opposite resisting force and that comes from shear across the threads. So as Tony eluded to, the bolt needs to be long enough to spread out the load. All else being equal you might need 2-4 times the engaged thread length tapping into aluminum vs. steel to get the same clamping force without stripping out the hole.

[Brad Arrington]

Rob Schepis wrote:

For sure 33 lb-ft is too high for this application. Maybe that was specified for the aftercooler mounting bolts.

Aftercooler caps is definitely one where feel is best and tightening in increments in a pattern. My preference is a stubby ratchet and go tight but this is just a personal thing.

When reading torque charts or specs it is important to understand if you are reading dry or lubed specs and the grade of the bolt. A torque spec is based on multiple factors including friction under the bolt head, friction between the male and female threads and the stretch of the bolt. Once you add any form of lube you are reducing the frictions so you need less torque to get the same tension load stretch on the bolt. Even the plating of the bolt reduces the required torque. And a higher grade (increased tensile strength) bolt needs a higher force to get the tension load stretch. You can use google to find a never ending array of torque charts, just keep in mind all of the variables — bolt class, bolt finish, lube type, receiving material (alum vs steel), fine vs coarse threads, etc etc

The perspective on the variables of torque values is helpful. I double checked the values from the latest maintenance manual on QuickServe; I confirmed that it specs 33 ft-lb for the cover cap screws and 57-ft-lbs for the aftercooler mounting bolts. The manual makes no mention of adding any grease or not, or any variable in the torque value.

The cover bolts were definitely not installed dry at the factory; all had a type of sticky black grease. Given that, and not wanting them to seize, I applied a dab of Alco Metalube to each hole. That might have reduced the friction to where the bolts would not hit the torque value. I did tighten all the bolts incrementally in a cross pattern.

I understand the experienced way is by “feel”. That, however, has limits. Notably when its the first time doing a particular job and have no experience on which to rely. When that happens you have to fall back on the official service manual and rely on the manufacturer’s guidance. It’s supposed to make the job perfectly repeatable when you haven’t developed the “feel” approach yet.

Anyway, the FedEx man delivered a dozen new bolts this morning. I’ll be replacing all the cap bolts soon with just a regular ratchet.

[Rob Schepis]

For sure 33 lb-ft is too high for this application. Maybe that was specified for the aftercooler mounting bolts.

Aftercooler caps is definitely one where feel is best and tightening in increments in a pattern. My preference is a stubby ratchet and go tight but this is just a personal thing.

When reading torque charts or specs it is important to understand if you are reading dry or lubed specs and the grade of the bolt. A torque spec is based on multiple factors including friction under the bolt head, friction between the male and female threads and the stretch of the bolt. Once you add any form of lube you are reducing the frictions so you need less torque to get the same tension load stretch on the bolt. Even the plating of the bolt reduces the required torque. And a higher grade (increased tensile strength) bolt needs a higher force to get the tension load stretch. You can use google to find a never ending array of torque charts, just keep in mind all of the variables — bolt class, bolt finish, lube type, receiving material (alum vs steel), fine vs coarse threads, etc etc

1 user thanked author for this post.

Brad Arrington

[Tony Athens]

Cummins factory coolers are assembled dry including the bolts–IMO, Just bad marine practice in general.. In your case–I’m not sure what was going on.. Cummins has never been guilty from using poor quality bolts– Maybe too short in some cases though..

Check the depth on the threads in the housing before do this again–Common sense applies here as to the proper length.. As to using a torque wrench on these 8MM bolts going into that housing? IMO, a questionable practice would be my answer– “FEEL” & experience as a wrench turner seems a much safer to me…..33ft lbs sound crazy tight into an aluminum housing to me, and is most certainly an error …

1 user thanked author for this post.

Brad Arrington

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