Warning!

[conaero]

Ok just done a thorough inspection of the above arms and I am please to report that there are zero signs of metal fatigue:



The crack appears from the inside to the outside of the bolt hole then along the casting flange to the other end of the bolt hole.

My feeling is that Marios is correct that the improper replacement of the fanblocks or seized bolts stresses the area and it splits down the casting.

I am not convinced of Marios's technique is not without pending incident also but I would give it a go but have regular inspections for the first few K's miles

 

[mchristyuk]

Matt,

Out of interest when your arms were changed did the bolts come out easily, or were they seized and require cutting?

I am currently visiting the parents over xmas, when I get home I will post some good quality pictures of my cracked, but not failed, 4200 arm so people know what they are looking for.

Mark

 

[conaero]

Mark I took them out myself some 5 years ago and I don't recall there being any issue with the bolts.

 

[mchristyuk]

Matt

We never remember when things go smoothly! If they'd been an absolute (cough) of a job then you'd remember it!

As Greg said all the ones he found cracked had seized bolts. Hopefully avoiding this will just be a case of copper greasing the bolts each service..

Mark

 

[mchristyuk]

Actually I have a 3200 arm off my car which I think had seized bolts. I replaced it with a good 4200 arm. When I get back I will dig it out and see what state the casting is in.

Mark

 

[adam01]

Worth posting up on US site to gauage if there r similar concerns/encounters

Must admit read across most masa forums never come across this

 

[conaero]

Worth posting up on US site to gauage if there r similar concerns/encounters

Must admit read across most masa forums never come across this

I would but they banned me for posting a link back to SM for a similar issue.

I appealed to the Admins but it fell on deaf ears.

 

[conaero]

Here you go, just so there is no confusion as to where the cracks normally appear, see green highlights below:

View attachment 25770

 

[drewf]

Why the lines on the casting? Looks like it's made by someone trying to generate a failure point.

It would be so much less likely to crack if it was finished smooth.

 

[Zep]

I agree, the lack of finishing makes a stress concentration point in the casting.

I'm going to use this to see if any small cracks are forming before they become an issue... http://m.rs-online.com/h5/mobile/uk/catalog?url=/web/p/leak-flaw-detector-sprays/4955027/


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[mchristyuk]

I think the finishing may be a bit of a red herring. If you look at the crack on the 4200 arm in my first post it's about 1/3 of the way down from the sharp crease. If the metal had been bent there I would agree, however this is just a casting line so is ugly rather than a structural weakness. Of course I'm no qualified metallurgist and happy to be proven wrong! Sharp creases are bad, but this is surely just an excess of material from the casting process. I've seen similar on engine crankcases and they've never split there...

 

[Zep]

I see your point, without knowing the exact material and treatments everything bar a lab test will be conjecture. Having said that I know from my materials lectures many moons ago that tight radii increase stress in the vicinity of the surface feature (see the photoelastic picture of a sample under tensile stress). What would be interesting is to section the part and see if there were any inclusions in the casting or machining defects which would have allowed the crack to propagate.

 

[mjheathcote]

When I rebuilt my Triumph Stag engine I ground and polished the con rods to remove the casting marks, just like what's on the wishbones, to strengthen the casting by removing the stress raisers.

 

[mchristyuk]

The casting riser could then accelerate fatigue in the part, but the fact it can be cracked just installing or removing a bush suggests that the part is also over hardened, maybe to make up strength for a lack of material to keep weight down?

I will be home later this afternoon so will get the good camera out on my 3200 wishbone and post some images later...

 

[mchristyuk]

Wowzers.. this is the arm that came off the other side of my car. This arm hasn't had bushes replaced BUT it did have seized bolts which required cutting out...





All 3 damaged arms that I have the rear holes appear fine, it's the front ones that have cracked and given way. I'm guessing looking at the geometry they are under the most load as a small wheel deflection would create a larger movement at the front hole than the rear.

Looks like I was about one journey away from a big accident even if the original arms had been left in place. Gulp.

The new replacement arm is going to have it's bolts coated very well in copper grease I can assure you!

Mark

 

[Zep]

Yikes!! Looking at the surface on the inside it is really rough, do you know if the bush fell out or had to be pressed out? Looks like the bush was turning in the arm...


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[mchristyuk]

The bush fell out... With the remains of the seized bolt inside it.

Mark

 

[CatmanV2]

Once it's cracked though, I'd suspect that there would be sufficient increase in diameter to free the bush.

C

 

[Zep]

I see your point. Looking again the roughness is only on the failed side, it's smooth on the inside. How odd...


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[Geofflyn]

As a matter of interest at what mileage has this occurred?
Geoff