V6 exhaust bracket - modification to prevent failure

[Bolin]

This project aims to design and test improvements to the V6 exhaust bracket, which is prone is failure.

The bracket in question is p/n WCU101000, which is attached to the rear (bulkhead) side of the engine block by 3 set-screws, and is affixed to 2 lugs on the starter catalytic converter (pre-cat) by 2 set-screws which screw into captive nuts on the bracket itself.

These brackets have two common failure points - a fatigue crack across the narrow 'leg' section and the 'feet' with the captive nuts attached can also break off. The photo below shows a new bracket along side one which has lost the 'leg' due to a fatigue crack, and one foot has been twisted. N.B. the photo shows the engine block side of the bracket.

IMG_8647.JPG

It appears that the two lower set-screws can corrode and seize in place, probably worsened due to the heat from the exhaust, leading to the heads snapping off upon removal - stainless steel set-screws should cure this.

Strengthening of the bracket such to prevent the fatigue cracking and breakage of the 'feet' is required. The current proposal is for two strengthening plates to be welded to each side of the point where the stress crack occurs, as well as an extra bead of weld across the approximately 90 degree bend at both 'feet'. The photos below show the proposed extra weld by dotted lines. The solid line is the typical location of the fatigue crack.

IMG_8651.JPG

IMG_8652.JPG

However, I am not an engineer! So advice is wanted with these proposals please, as to whether or not they are adequate enough to prevent the fatigue and breakages, and if not, then advice as to what would be adequate enough.

If you can advise, please let me know.

Many thanks and kind regards, Bolin.

[KDA4G]

An important piece of information is to know what clearances are available around the part in question, with a view to adding additional material. Has it already been cut down to the absolute minimum in order to fit into the space available, and is the bracket made from plain ordinary mild steel that can easily be brazed, welded, or bonded with a structural adhesive? What is the thickness of the sheet material from which the original part has been made?

A better option would be to manufacture a re-designed part, but you'd want to try a couple of made-up prototypes first.

[Zeb]

I would firstly be investigating making up the entire bracket in a thicker gauge of metal...may be much cheaper as an option?

[Mofo]

I would firstly be investigating making up the entire bracket in a thicker gauge of metal...may be much cheaper as an option?

That would tally with my line of thinking, I'd rather buy a modded / improved part than have to worry about buying a replacement and THEN modding it. I don't have this bracket so mark me down for one

[KDA4G]

Yes, but if it is metal fatigue, then it is affected by the shape of the part, not just the thickness. Temperature effects will also come into play. Sometimes, with metal fatigue, a small change to the shape does the trick, and adds less weight and cost than a thicker gauge of metal. Can anyone post up a picture of the fracture surface, to show if it is in fact metal fatigue, or not?

[Bolin]

I'll try to get a pic tomorrow, with the way they fail (crack that grows and then breaks off) it looks like how I understand fatigue to work.

Chris - my thought was that it the modification was cheap enough to do, then to sell the modified brackets, rather than giving folks just the idea of what to do. I suppose many folks won't want to then track down a fabricator who will do such a small job.

Carl - not sure about the cost of making a new bracket, perhaps BenC can help with that? There may be minimum order numbers etc. Thicker metal may also require longer set screws to go into the engine block, which would increase cost further.

Derek - It appears to be ordinary mild steel. There isn't any spare space between the bracket and the pre-cat. There is space above, but the thing is a right pain to fit with the pre-cat in place, so an increase in size might make it virtually impossible to fit without removing the exhaust, which members fitting it may not want to do. There is some room for making the bracket thicker, I would think that with any sensible increase in thickness there should be enough room for it, just a question of how much is needed, and what is the most cost-effective way to get the extra thickness, if indeed that is all that is needed.

I need to get underneath and try removing and refitting the bracket again to see if a 'web' could be added between the leg and the main part, but I think that might make fitting even harder.

[KDA4G]

OK - the lack of space indicates why the part has been designed that way (i.e. poorly).

With fatigue failures, if you add a thickening piece, it can mean that the point of maximum stress is moved along to the end of the thickening piece, and then the failure will occur there instead, and possibly even sooner. This may be avoidable if the stiffener is either much longer than shown in the picture (or not just a plain rectangle, but shaped in some way). Welding can also change the sensitivity to fatigue, so bonding on a stiffener with a structural adhesive, which is a DIY job and may be sufficient, is worth considering. Another option is a transverse web - difficult to describe in words - like a flange all the way across. Or, a couple of straight lengths of 5mm mild steel rod all the way across, one on the front face and one on the hidden face, brazed rather than welded.

It's not obvious from the diagrams what this part actually does - is it only needed during assembly on the production line perhaps?

[Suffolk Boy]

i did a similar project with the egr bracket on my diesel.these snap after a few miles.with no replacement avaiablle i made my own.i used 3mm thick stainless steel plate using the old one as a former i bent it in a vice and fashioned to suit.this has stood the test of time so far.i looked into the inhrent rigid propertys of stainless as opposed to mid steel and they are a lot higher.i would not go higher than 3mm as unless you happen to have a 35 tonne press it is unworkable.in which if you do you bend to 89.9 as opposed to 90% to allow for spring back.having said all this a good engineer should be able to design a replacement and have fun whilst doing so.if you find the right one lol

[Bolin]

OK - the lack of space indicates why the part has been designed that way (i.e. poorly).

With fatigue failures, if you add a thickening piece, it can mean that the point of maximum stress is moved along to the end of the thickening piece, and then the failure will occur there instead, and possibly even sooner. This may be avoidable if the stiffener is either much longer than shown in the picture (or not just a plain rectangle, but shaped in some way). Welding can also change the sensitivity to fatigue, so bonding on a stiffener with a structural adhesive, which is a DIY job and may be sufficient, is worth considering. Another option is a transverse web - difficult to describe in words - like a flange all the way across. Or, a couple of straight lengths of 5mm mild steel rod all the way across, one on the front face and one on the hidden face, brazed rather than welded.

It's not obvious from the diagrams what this part actually does - is it only needed during assembly on the production line perhaps?

Thanks Derek, I hadn't thought about the fatigue point moving, and didn't know that welding could have an effect (as I said I'm not an engineer, so I am very appreciative of the help!)

I think I understand the transverse web idea, but I can see the issue of complexity increasing the costs. The rod sounds like a better idea.

I know that some folks have run their cars for over two years without this bracket, so it doesn't appear to be critical, but without it the front part of the exhaust (downpipes and pre-cat) and only supported by the exhaust manifolds and manifold-to-downpipe clamps. Thee clearly must be some stress in this area, hence the brackets failing.

[Bolin]

i did a similar project with the egr bracket on my diesel.these snap after a few miles.with no replacement avaiablle i made my own.i used 3mm thick stainless steel plate using the old one as a former i bent it in a vice and fashioned to suit.this has stood the test of time so far.i looked into the inhrent rigid propertys of stainless as opposed to mid steel and they are a lot higher.i would not go higher than 3mm as unless you happen to have a 35 tonne press it is unworkable.in which if you do you bend to 89.9 as opposed to 90% to allow for spring back.having said all this a good engineer should be able to design a replacement and have fun whilst doing so.if you find the right one lol

Thanks Phill, starting to think if having new brackets made up would be the easiest option.

[KDA4G]

Temperature cycling due to the location near the exhaust won't help either. This can accelerate corrosion and initiate surface pitting, thereby creating initiation sites for fatigue cracking. A few leaky exhaust gases and road spray will also encourage corrosion. Do the original parts show much corrosion, and do they have any special surface treatment?

New parts made up to the same design in 3mm stainless sounds like a good suggestion as it covers several angles in one go - maybe add a bit of extra material if space allows, by generally smoothing out any sudden changes of shape, if you see what I mean.

http://www.bssa.org.uk/topics.php?article=104

[BenC]

Temperature cycling due to the location near the exhaust won't help either. This can accelerate corrosion and initiate surface pitting, thereby creating initiation sites for fatigue cracking. A few leaky exhaust gases and road spray will also encourage corrosion. Do the original parts show much corrosion, and do they have any special surface treatment?

New parts made up to the same design in 3mm stainless sounds like a good suggestion as it covers several angles in one go - maybe add a bit of extra material if space allows, by generally smoothing out any sudden changes of shape, if you see what I mean.

http://www.bssa.org.uk/topics.php?article=104

Colin kindly sent me a diagram of his replacement bracket, and Jake has also sent me a broken but complete bracket.

I'd agree with you on damage to the surface of the material, i.e rust, burrs causing the cracking were the bracket is prone to fatigue. Polishing the stamped original should reduce the chance of cracking initially, but rust would create an issue soon after I would think.
Laser cutting the blank from stainless, bending the bracket up and welding stainless nuts on would be my initial suggestion, but I would want to find out the original material specs first. Suggestions on strengthening the original profile sound like a good idea. Pressing a 'dimple' into it is another one, but all adds to the complexity of the item, and therefore initial tooling costs etc. Like all these projects, the bigger the batch produced, the cheaper the individual piece part will be.

[KDA4G]

Looking at the way the parts fit together, it's tempting to think that this part is mainly required during assembly, and then to reduce vibration. The 1.8 has a much sturdier looking bracket than the V6, and so does the 1.8 T, but the diesel doesn't seem to have have one at all.

The production line video clips (for a 1.8) suggest it goes like this:
- Engine and transmission on subframe is 'stuffed up' into the vehicle;
- Complete exhaust system is then lifted up, and 'hung' from the underside on flexible hangers;
- This particular bracket is then used to attach, locate and support the downpipe;
- Connection is then made between the exhaust manifold and the downpipe.

Unfortunately the video doesn't show a V6 being fitted, but maybe the bracket is used to attach the complete exhaust system to the block, just for alignment purposes. Once the two downpipes are connected up to the manifolds, the bracket is more or less redundant, hence the flimsy design. The weight of the exhaust system would then be fully supported by the hangers. But, after that, it would help to reduce vibration by fixing the location of the front end of the exhaust system, and some people have indeed reported noisy vibration if the bracket is fractured.

Once the vehicle has been driven for a few years, there will be more movement in the engine mounts, and also in the exhaust system hangers. The bracket then becomes over-stressed and has by then been subjected to many cycles of vibration and pitting due to corrosion, so it fractures. Some people have however reported new brackets failing after just a few weeks - this may be due to soft engine mounts or exhaust hangers increasing the stress on the bracket, but presumably it is not due to corrosion or surface pitting effects. So, one possibility is to check the engine mounts for wear, and then replace all the exhaust hangers at the same time as replacing this bracket.

One would expect the V6 to have the lowest vibration levels of the the three engine types, but once the bracket has failed, then there would be a bit more stress on the manifold joints. Maybe one option could be to get the part made up in stainless steel, by the people who do the stainless steel exhausts for the 75, and see whether they have any suggestions to increase the life of the bracket?

Pressing a dimple into the part sounds like a good idea, though, if there is enough material to do that, or maybe something more like the bracket on the 1.8?

[Bolin]

Ben and Derek - great stuff!!

I need to do a bit more research on the subject, I hadn't heard about them failing so quickly, hope my new one is still OK! Engine mounts are OK I think.

Just a thought, how would stainless steel compare in terms of strength to a 3mm mild steel version? If the latter is thick enough for corrosion not to be a problem (but I guess that it might still be after many years) then I would assume it would be cheaper, and could make this more viable.

I intend to do a bit of research into any reported problems with V6 manifold joints, as this would appear to be the greatest potential concern with a failed bracket.

[Jake]

The part that Ben has came off of a car that's engine mounts were fine, granted midsomer had two dead ones.

On cars that have broken brackets - there is a notable rattle over cars that don't. Haven't seen the state of the Pre-prod car's yet, but given that it's had an exhaust in its life, my guess is that it'll either be broken or missing.