They are not phillips head screws they are JIS (Japanese Industry standard) head screws and the angles used are close to phillips but not exact which is why they bugger up so easily. The last couple of screws I had to replace on OS engines were 3.5mm. (On some of there smaller engines OS uses 2.6mm.) So check carefully as to size. My LHS used a computer on the internet to get the size for my last screw purchase. He then turned around and picked a small package of Dubro 3.5mm off his wall display and they are socket head screws.

Why we must ask ourselves do they use screws in fractional sizes. Could it be that they are harder to find? . . .but that they can supply them?