The mechanic that is working on my ped discovered that the decompression valve in the cylinder head was badly mangled and that's what was in fact hitting the piston and causing that awful noise. He showed me and I can't believe I thought that's how it was supposed to be, it honestly never occured to me that it was the problem. It somehow came loose i guess and it jammed right into the side of the combustion chamber. I didn't notice because I think it was the first time I removed the head so I figured that's how it was supposed to be. I've ordered the valve but until then he explained over the phone that he could somehow fix this by making a new head gasket with a few changes. The problem is he was all out of gasket material, so I need to find some. I think I remember a while ago this being discussed and somebody mentioning going to a hobby store. I'd like to know what thickness I need, what material's best to use etc. I've attached a picture if it helps, the piece that caused the trouble is that one that looks like a big nail or a valve on a trumpet. It's a 1975 Motobecane Mobylette. Thanks a lot everyone, and keep an eye on those valves, they can cause a lot of trouble !!!!!