I am rebuilding a 2002 XR100 for my daughter and am having a problem setting the timing. I bought it used, never ran and I suspect that the wrong piston was installed. With the time set properly the piston hits the exhaust valve preventing the engine from turning over. I´ve seen photo´s of pistons for these bikes with TWO indentations, one for each valve it appears. The piston in my bike only has one indentation. What am I missing? This is a simple engine. Thanks