In post #275 of this thread, I shared a theory about the engine of #231 being repaired before for some reason. Whilst I was doing some research in the past few weeks, I discovered that the early E28 M5 and E24 M635CSI's were originally assembled with single valve springs that have an realistic chance of failing. This design problem is discussed in detail in a seperate thread about the M88 valve spring risk. When I removed the cylinder head, I found out that the valve springs are of the double spring design so this issue has been addressed on #231 a long time ago.
Furthermore, I have found the conclusive proof that the 93,45mm bore diameter has been determined during the production process. The proof is a '00' inscription in the top-side off the short-engine. This matches the same coding that the TIS uses for an intermediate bore size. The measurements report from TMR confirms this as well.
With other words, the bore-wear is limited to 30um max that is the result of 155k km. Extrapolated to the max pistin to cylinder wall clearance, this means that the short engine could have lasted 510k km before needing a rebuild. However, the piston to cylinder wall clearance has exceeded the maximum value. This simply contradicts so there has to be another explanation.
It is my theory that #231 suffered from a failing valve spring in the first year(s) of its life and that this failure caused one or more valves that dropped and damaged the pistons. In the repair that followed, it is likely that the mechanic missed (or was not aware) of the '00' code on the short engine and just ordered the standard piston set (for size '0'). TMR's measurement report supports this; all pistons measured 93,35mm (max), matching the requirements for size '0' and not '00' that requires pistons with a diameter of 93,40mm.
This proof is only conclusive if I can find markings on the old pistons that proof this. Unfortunatelly, the piston ident cannot be found on the piston-crown. Instead, the piston ident can be found on its top side, however this is coverd by carbon deposits. I removed these and found M88/3 and 10,5 texts. However, I did not find the full text that refers to the piston diameter, but only the last two digits on teh spot were I expected them. Since these clearly reads 35 (and not 40) I have to assume that TMR's measurements re correct.