I'm having trouble getting my bike back together for some reason. It's a 1990 xr250r. To install the cam I turned the engine until the "t" on the rotor lined up with the mark on the cover, then aligned the marks on the cam sprocket with the top of the head, as the service manual specified. However, with the T lined up the engine didn't feel like it was at tdc, and now the piston contacts some of the valves. I'm rather stumped. The bike is a bit of a frankenstein, I believe the bottom end is from a '90 and the top on from an '86. Is there another way to find tdc besides the lining up the T?