Ive changed my head gasket done timing but engine wont start, I dont know if cat has blocked so I was told to take oxygen sensor out so engine gasses can escape through the hole where it fits on exhaust but it still wont start I would be so gratefull if you could help me thanks. Kevin
Do a compression check on all cylinders. Compression should be at least 120psi.
If compression is low, recheck timing (although you've did it a couple of times already).
Next, perform a cylinder leakage test on the engine (if compression is low) B sure to have the cooling system full while doing this test (if air is escaping through the water jackets it shows up as air bubbles in the cooling system (if you don't have water in the cooling system you can't see the bubbles).