3406B Cat breaking manifold studs

Son's truck with a Cat 3406B breaks a couple exhaust manifold studs every 30,000 miles or so. It's apart now for the fourth time. The first time it happened he replaced them himself. The second and third time the studs broke he took it to a reputable truck shop to have them replace the studs. The third time the truck shop put in stainless studs and two of them just broke. We just got done drilling and torching one of them out successfully and the second one comes out tomorrow. Stainless studs are NOT going back in!!!

Do any of you guys who have worked on these engines have any ideas why the studs are breaking? Is this a known problem with this engine setup? I don't think it's in the same location every time. The manifold has not been resurfaced until today. When I took it in to have it surfaced I asked the machine shop for advice and the man who does heads and manifolds said he didn't know why Cat used 3/8 studs instead of 7/16 on that engine but other than that he didn't have any advice. The bolt holes in the manifold are oversized enough to give some wiggle room but the holes aren't way oversized.

The reason he took it apart this time is a couple of manifold gaskets were blown. He found the broken studs when he took the manifold off. So this is where we sit at the present time. The manifold is resurfaced, he has the new studs but we sure don't want to do this again.

Oh yes, these are the studs that go in the head. Maybe you have that figured out. Why can't I edit a post anymore????

Use grade 5 bolts , grade 8 will break. 5's will stretch ,

8's break . That is a problem that Mack also had .

george

Using an insulating standoff of a half inch or so (piece of tubing stud size) and two belleville washers With the inner one bell out and the other bell in. Then tighten so the washers are not quite flattened. Will prevent breaking forces on the stud, while holding well. It may require longer studs. Jim.

Jim I have seen that done and I always wondered why. Come to think of it the machine shop man did mention that.

Does the manifold have a slip
joint in it? If it does, is
it seized up? Could you maybe
brace up the turbo with a
bracket to the block just to
help take some strain off the
studs? Probably not the
issue, but it may help. I
believe the issue is being
caused by manifold expansion
lengthways.

Ross

It is a three piece manifold. We had to tack weld it together to keep it the right orientation before we removed it. This time one of the broken studs was the far rear stud. The other one was a little ahead of center in the middle section. The rear one was a real doozy to get out. It was back into the firewall just a bit. Had to use an angle drill and hope to get centered right. We finally got a hole drilled through and then carefully blew it out with a torch using little short bursts. A tap cleaned it out and we still have a good tight hole with good threads. Stainless blows out pretty easy with a torch. This was my first experience with a broken stainless stud.

Do you have the (about) 1 1/2 inch long
spacers on the studs? It was a pretty
common occurance for them to break. Thats
how I learned the finer points of broken
stud removal. I would only replace with
the proper parts from cat. The last ones
I installed were a special alloy. Monel
maybe?

I always got the broken ones out welding a
washer and nut to them. Mig welder makes
quick work of it, and wont stick to the
cast iron.

Stainless shouldn't blow hardly at all with
a torch, it has a higher melting temp and
also it doesn't oxidize well like steel
when hit with the oxygen blast

How many different types of stainless are there? Do some cut better than others? I'm not an expert on stainless for sure. Before I used the torch on the broken stud I tried it out on one of the other studs that we had removed and are led to believe are stainless and it melted and blew easy. I didn't try to actually cut it, I just heated to the melting point and hit the oxygen and I blew a chunk out of it. When I was torching on the broken stud I heated till a few sparks flew, then I gave it a quick blast and quit, I never did just hold the flame in the hole and try to do a long cut. Tomorrow I will talk this over with him and mentioned the suggestions from you guys. I'm inclined to suggest longer grade 5 studs and use spacers. I did use a mig to weld a washer and nut on twice and it didn't work. I have done this many times on various tractor and military restorations and most of the time it has worked but not this time. It just seemed like the weld didn't want to stick to the stud. This truck is a 1994 glider using a used powertrain and possibly a rebuilt engine. Who knows what done to the manifold in the past!

I believe that the issue is that your slip joints are not slipping. Did you say that you welded the joints? These joints
need to slip to enable the manifold to expand at the same rate as the head. I've seen many solutions, up to and
including machining a groove and installing a small piston ring. Whatever you do, that joint needs to be free.

The welds were ground away after the manifold was resurfaced but tomorrow I will check to be sure the three sections are loose enough.

The studs break because the manifold expands and contracts with exhaust temps.Replace the manifold;make sure the stud holes are larger than the studs by almost 2 sizes-3/8 stud=1/2 hole.Buy Genuine CAT parts!All new studs,gaskets and manifold sections.DON"T weld them together=They HAVE to slip in their expansion joints.
Next problem to consider is why is exhaust breaking the studs=is the manifold getting too hot for a reason?Such as over fueling/lean air mix,hard pulls lugging the engine,not downshifting,too rapid shut down with out allowing the engine to idle and cool down.Is there too much exhaust back pressure=possible clogged muffler? Or was it just the manifold being welded at the slip expansion joints and the stud holes too small? Mark

Interesting. Never considered that.

Thanks for the suggestions Mark. Like I stated the manifold is not welded together during use but we should check it to be sure it is not frozen together. The bolt holes in the manifold appear to be 1/2" for the 3/8" inch studs. We don't know what the horsepower is, it does pull fairly well for an engine this size, though. There is a pyro and I'm sure he watches it because he is a very careful operator. All of the ideas you guys have given us does give us something to work with now.

OK, talked with my son this morning. It does have standoffs on the studs I did not remove this manifold and this is the first time I have worked on it. The holes are 1/2" for the 3/8 studs but it does not have Belleville washers. The previous owner told my son he was having trouble with broken studs too and the truck has straight pipes. My son couldn't stand the noise so we stuck a resonator on it. Still too noisey so we put a muffler on too. Studs are still breaking . He said the pump is mildly turned up and the pyro will occasionally go to 1200 on a hard climb. That pyro temp sends up a red flag to me but I don't know what this setup can withstand. Today we will be checking the manifold slip joints.

Don't overlook the obvious, install them with a torque wrench.
I don't have the torque specs for that engine handy but in the past I have come across applications where if you tightened the manifold fasteners up to what felt right you would have bolts/studs breaking on a regular basis.
Torque them up to proper specs [which always will seem light] and they have that little bit extra they need to deal with expansion.

Well i may as well throw in my 2 cents worth here . Is there a piece of flex pipe somewhere after the down pipe from the turbo?? Or is it solid from the turbo on back ??? How are the engine mounts ??? Where is the probe for the pyro , in the manifold before the turbo or after the turbo in the down pipe . If it is before the turbo then then 1200 on a hard pull is hitting the max , after the turbo she is getting really hot . just where are the studs breaking at out on the end like on 1-2 or 5-6 or on 3-4 If they are breaking on 3-4 holes it may be due to heat and the weight of the turbo and the pipe and a flex pipe that is semi frozen or the fact that there is no flex pipe and you have engine mounts that are working to much . Just some ideas to think about as i am no expert on the 3406 B's . I did 90% of my trucking either with OLD Detroits Cummins and a couple Mack's and one 3406E . Each and everyone of them were wound out to the max . The only engine that i ever had a problem with manifold bolts or studs was on a 460 gas Ford and that was due to it getting way to hot . I have never in my life ever met a truck driver , farmer , or race car driver or equipment operator that told me he had to much power and each and everyone would love to get just a little more .

A newer flex pipe comes off the turbo. i don't know what the mounts are like. I did not see the studs in previous breaks. This time the far back stud on #6 broke and the other one broke on #3. The pyro is right before the turbo. The previous owner complained of broken studs too and after talking to my son again I found out the previous owner bought it from a farmer with 250,000 miles on the clock. Farmer bought it as a new glider (Freightliner) in 1994. I have an inkling the engine was overhauled before it was dropped into the frame. I don't know when the next owner bought it from the farmer but it did sit still a lot since it only had 390,000 miles on the clock when my son bought it a year and a half ago, giving me an inkling the manifold might be stuck. It will be checked out this afternoon. The only reason we tack welded it was to hold it in the exact position it was in the truck just in case it wanted to twist in handling. Maybe our concerns weren't justified if it is indeed stuck. Son called the dealer he got the studs from and they are from cat. Will keep you posted. Thanks again for the suggestions.