So l did a stupid thing and lost the bottom thermostat cover bolt somewhere in the engine bay while changing my thermostat. After 4 hours of searching I gave up looking for it - wherever it went, it’s well hidden and stuck, and I don’t want to bother disassembling everything in there to find it. The car didn’t explode after I started it with the thermostat held down only with the other two bolts, and doesn’t even leak after having run it on and off for a combined couple of hours, so at least there’s that. I ordered a new bolt from RPD - they gave me part number 7703002838, which matches the part number I see on catcar.info for Megane III DZ21 (which is the code I get based on my VIN). The problem is that this new bolt that I got is M6, while the ones that are currently on there are M5, but no part list that I can find lists any M5 bolts anywhere around the thermostat, which confuses me to no end. Interestingly, if I try and google the part number, I can find a single image on some Russian website that shows a bolt exactly like the one that I lost (you can tell by the little bit of thread-less body at the end of the bolt which the bolt I lost has, whereas the one I bought doesn’t). Here’s what the bolt I lost looks like: It matches the photo and part number seen here - https://euroauto.ru/part/used/51153385/ While this is the one that I received: You can see that the part number on the bag is the same one, so it’s not as if I was sent the wrong thing (and I ordered two, and both are the same, so it’s also not a case of a wrong bolt being bagged at the factory). Furthermore, all parts lists that I can find clearly indicate that the bolts in that location are M6. At the same time, most references to people replacing their thermostats on this forum suggest people using 8mm spanners, which tells me that I’m not the only one who has M5 bolts there. While it shouldn’t be a problem for me to just get a non-branded flanged M5 20mm bolt to replace the one I lost, and I don’t think it would cause any problems, I’m just mostly curious how it’s possible for all parts lists to show a different size bolt to what a lot of people seem to have. Has anyone ever had to replace those, and run into the same issue? If so, did you manage to figure out the discrepancy?