I recently had a screw break off while I was tightening it.
After digging around a bit for a replacement, I discovered that a pan head M6 X 1.00 X 5MM screw will work.
As you can see in the photo, 5MM refers to the length of the screw. I think the 1.00 refers to the head, which is very close to 1cm in diameter.
Initially, I replaced the broken screw with a steel screw from Lowe’s. Despite the fact that the Lowe’s screw was 1cm in length, it still worked. However, I reasoned that HJC uses short, plastic screws, possibly, because they are less likely to penetrate your skull if you’re ever in an accident.
I was forced to order the nylon screws in bulk. So, I have 99 of them left at the moment. If you need one, send me note. I will send you a few for cheap.