Hi Brian, its Thread size.
The connecting rods which came out were M15, I gave up trying to find a repair set/bolts etc for the damaged rod. So purchased 1 replacement connecting rod, then discovered it is completely different to the rest which was part no. 366 030 35 20 80
The 376-030-71-20-40 Connecting Rod For OM352 HMMH were cheaper to buy as a set from America...
Attached should be piccys of the old and replacement part..