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Cracked chassis with bolt in A frame

That’s horrible. I’m pretty sure that my car came out of the factory with the bolt in A frame fitted (and it does seem to be well fitted) and other typical fixes of the period. However the front wishbone mounts do not have fillets on the welds, so it’s a muddle and not easy to work out exactly where I sit in the overall chassis evolution given the 2015 registration. At this time after 2000 miles I don’t have any visible frame cracks and strangely my original Brooklands exhaust system is thus far crack free. Guess I’ll just have to keep checking and take the pain if it ever comes. Though I’m wondering if it ever develops cracks, what MMCs future stance would be on frame replacement?

My theory is that cracks initiated quite quickly in the original chassis (ie micro cracks that are not visible) - without the bolt in brace, they also propagated quite quickly and became visible (we shared this experience with many early owners and had a rebuild). The bolt-in brace has at least slowed propagation for those with the early chassis but with the number of cars with the brace now showing cracks, it looks like not being enough. The good news so far is that the replacement chassis with welded in brace (and other changes?) appears not to crack...

I agree JMD, a bolted in brace (from scratch) should be as good as a welded brace of similar geometry if fitted properly. As mine appears to be fitted in the factory before the vehicle was sold I’m reasonably hopeful that there were no cracks already present, my main area of concern is the lower wishbone brackets which don’t have the welded fillets so I’m going to ask MMC about that.

IMHO

One of my (late-2016) lower wishbone bracket bushes failed a couple of years ago, taking me into a tree! A couple of fabricators have suspected weakness in the HAZ, but I have not heard of other instances.

Scary. I guess the one advantage of a timely bolt in frame is that there’s no HAZ.

Has anyone had an in situ frame modified by welding e.g wishbone brackets?

This is getting more worrisome now MMC stopped the production !! What will happen when e.g. you own an older M3W and gets a cracked chassis in the upper part ? Most (perhaps all ?) previous cases did got a new chassis at MMC. But how will it look like in two or three years (or even more) when you get a cracked chassis ? Does MMC still have a chassis for you in stock ? Do they still change your chassis ? Will it be still guaranty ? This questions is hanging as a dark cloud above the heads of owners with a pre 2014 M3W.

My experience is that every chassis crack I have seen (I have only seen lower ones by the wishbone attachment) Starts in the heat affected zone (HAZ) and propagates round the tube from there. The stresses in this welded area are enormous.

If you want to check the chassis for cracks then, as planenut says, familiarise yourself with where the cracks usually appear and do the following :-

1/. Wipe the area clean and spray the area with penetrating oil ( WD40 for example)

2/ Wait for 5 minutes and then wipe away all of the surface spray residue.

3/ take some powdered French chalk - the sort of thing you use on an inner tube before fitting a tyre or rub any piece of chalk on sandpaper to produce a powder - Blow the powder over the area where the cracks appear and if a crack its present t will magically develop as a stain in the chalk.

This is only a DIY version of professional crack detection but much better than the naked eye inspection and usually shows up cracks you otherwise won't see.

One of our problems is that, in the UK, repairs to motor chassis and body shells must be done by welding, if the vehicle is to pass an MOT test. A well designed brazed repair is much stronger and leaves no heat affected zone or localised stresses. A motor manufacturer obviously has to abide by the rules. I cheat!

The picture of the crack/break that i have posted is just behind the rear upper wishbone mount, there was also a crack on the other side, one year previously i had the fillets fitted to the lower wishbone mounts.