There were 1027 pre-2014 specification cars produced, according to the numbers for the one and only M3W recall, to date. That was for brake pedal weld failure but was also supposed to incorporate the chassis crack checks at the same time as the brake pedal inspection, although no mention was made of this in the recall information. There were far fewer cars fitted with the suspect brake pedal box than 1027. DVSA were of the opinion that these chassis cracks would not result in a catastrophic failure, you would get an indication something was wrong, so would not issue a recall for the chassis cracking.
As you say, a good proportion of those cars have had chassis replaced for cracks in the tubes near the upper wishbone rear mount, along with some accident replacements. I couldn't even guess at a number though. The cracks do tend to start from a weld, so the quality (on the day?) of weld may have something to do with it? The specification of the chassis tube was also changed in 2014. From personal experience of fitting an Empire suspension kit to both pre and post 2014 chassis, the later chassis was made to better tolerances than the early one, which required lots of shims to correct the geometry from side to side.
Some chassis don't crack for many miles and others crack at fairly low mileages, there is no set usage for failures. When they do crack propagation of the crack can be quick. I have checked a chassis closely for cracks and found nothing, then found the tube fractured within a thousand miles. You can be fairly sure that a pre 2014 spec chassis will crack at some point, sooner or later. The bolt in brace seems to make little difference, the welded in version that some early cars got seems better. All M3Ws, right up to the last ones made, can crack in the vertical tube at the lower wishbone front mount and can be relatively easily and cheaply fixed with a welded repair bracket.
For info, MMC have recently changed their policy with chassis cracks on early M3Ws, no longer will they replace them free of charge. A "customer contribution" in the region of £10,000 is now required, plus payment for any other work found during the chassis change. There are alternatives, repairs from M3W Services for example. Sorry for the long answer 😊.
Interesting post addressing a big problem
