Any metallurgists out there?

[GAmes]

As many know the '97-'99 trucks use a canister type fuel filter. The canister has a SAE bolt fastened by a round nut protruding upward and is held in place on the fuel filter head by a captive nut. Two days ago, for the third time the bolt on mine has broken. The first time was in Alabama on a 2 lane country road about 3 miles from the closest town. The second time in heavy traffic, pulling my boat through Austin, TX. Two days ago I was on I40, about 100 miles east of Nashville. The first broken bolt was replaced by a grade 8 bolt, and it lasted a long time. The second, again a grade 8 bolt, but it hasn't been that long ago. I now have a bolt from Lowes, grade unknown. installed. Apparently a grad 8 bolt is too brittle and won't stand up to the engine vibration. I know I'm not over torquing the nut, so what grade of bolt should I be using?

 

[Wayne M.]

I'm not a metallurgist, but I've read Carroll Smith's fastener book. Where exactly is the bolt breaking? Have a picture? Are they breaking in the same spot each time?
I am curious about the SAE thread in the cannister. I thought for sure everything Cummins was Metric a while before '97.

 

[GAmes]

Each one has broken just below (flush with the bottom) the captive nut, roughly 3/8 inch from the end, which is about 1/8 inch protusion. I dropped the broken off piece and couldn't find it, the bolt itself I could picture if necessary. It didn't break clean, so I had to pry out the captive nut to remove the broken piece from the bottom. I plan to use a longer bolt and grinding the sides flat to accommodate a pair of pliers or a small wrench for the next time, assuming I can reseat the captive nut. I'm glad this one is SAE, it makes finding a replacement a lot easier.

 

[TFucili]

Just thinking out loud here. Is there a mount somewhere that is loose or failing/broken? This could cause excessive vibration and cause repeat failures. I have serviced gen-sets that when the iso-mounts start to get soft or break, all sorts of peripheral stuff starts to crack/break/fail.

 

[GAmes]

I'm not following you. Are you referring to the engine mounts? They are original, but don't show any sign of being broken that I can see. I have a Fluidampr. I have also heard of other bolts breaking, so this isn't something new. IIRC, the first one broke around 350k, the second at about a million. I did just complete a drive up & down the AK highway that resulted in a Pacbrake air bag exploding when I hit a frost heave.

 

[TFucili]

Yes, I was thinking anything in the motor/trans mounts that might cause excess vibration.

 

[GAmes]

I'm aware that gradual changes aren't noticed. My wife rarely is in the cab. I think if the vibration was different she would notice.

 

[Wayne M.]

I'm having a tough time picturing the setup. I don't think I've ever seen one. Is this the housing that leaks, and Genos has a spin on head to replace it?
Is there a bottom support of the canister that's broken or missing? I can't imagine a bolt breaking like this doing a simple job. What size bolt is it?

 

[GAmes]

The bolt head size is 9/16, I don't know the thread size off hand. It is the filter head that cracks and leaks. Genos is a replacement head that accepts spin on filters that came on '94-'96

Pictures of my spare assembly. Top/inside of the canister. You can see the threaded end of the bolt.

The bottom. You can see the bolt head inside the cavity.

This is the filter head with the captive nut.

Filter head bottom side. The bolt goes into the center hole.

 

[Wayne M.]

Yes I recognize the top now. Just how tight do you make the nut? 9/16 head= 3/8 thread. I know a grade 5 3/8 bolt shouldn't be tightened more than, say, 15-25 ft lbs. a super premium 3/8 fine con rod bolt is like 35 ft lbs. if it won't seal without really cranking it, maybe you have a problem there.

 

[TFucili]

Then, I guess I would buy a box of the correct sized bolts, and install a new one at every filter change.

 

[fest3er]

Mine broke years ago while driving, spewed out a ¼ tank real quick. A temporary bolt got me home until I could get a replacement from Cummins Atlantic. It's been fine since.Thinking back at the time, I seem to recall hearing/feeling a *crack* the previous time I'd replaced the filter; it was probably the bolt failing. If the canister isn't inserted carefully and carefully snugged up before tightening, it could become canted which might help break the bolt.I suspect they use a grade 8 bolt because of the dynamic pressures involved (fuel pressure and kinetic motion). A regular bolt might be more prone to stretching over time. The G-8 is probably the proper bolt, even though it is harder.You might've been unlucky and hit a few bolts that barely met the grade. Dunno.

 

[GAmes]

Replacing at every fuel filter change is a bit of overkill I think, but I have considered perhaps every 6th one or so. That is between 90 and 100k. Specified torque is 10 ft/lb. I've never used a tq wrench since it takes a long extension and a universal. I snug it up with a twist of the wrist that I doubt is excessive, but I suppose possible. Today I'll remove the bolt from the spare and see if there are any markings on the head. Thanks fest3er, I think G8 is the best choice too, but thought I'd ask. There isn't anyway to cant it off to the side, it will only fit going straight up.

 

[fest3er]

Watch it closely. It *can* get a degree or three off. That's enough.FWIW, I think a twist of the wrist is too much. Thumb at the bolt/nut, and finger force only at 2-5 inches out is enough. Unless my occasional guitar and piano playing has made my fingers a lot stronger than most.

 

[GAmes]

I use what is essentially a nut driver, so no real leverage. The way the filter seats into the head pretty much squares the bolt, if it wasn't it would leak. Haven't got to the bolt in the base yet.

 

[GAmes]

I removed the bolt and it has 6 marks indicating it is a grade 8, but it also has a u on one side. I can't find a reference to that marking. I did find one little tidbit I found interesting;

[FONT=&quot]Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Far more than you will ever winch or pull on. Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure.

Could it be that to keep from overtorquing the nut I inadvertently caused the failure of the bolt by undertorquing?[/FONT]

 

[DavidC]

The torque on my 99 is 10 ft. lbs that I do religiously. I think it would be very hard to under torque as 10 ft. lbs is not much and I would think sealing the canister under torqued would have problems.

Dave

 

[GAmes]

I guess I need to get an accurate inch pound tq wrench. I'm still going to start replacing the bolts every 100k. I can only flinch when I think about that bolt breaking when I was on the AK highway last month.

 

[Wayne M.]

That statement about under torque relates to a fastener moving around like say a steering box coming loose. I really doubt the can would seal at less than 10 lb. and that's just so far off what that bolt can handle.

 

[GAmes]

I really doubt the can would seal at less than 10 lb. and that's just so far off what that bolt can handle.

I totally agree.