@09:09 but arent there 3 TDC's on each piston. one for intake, one for compression and one for exhaust. TDC on intake stroke exhaust valves are closed and intake are starting to open. TDC on compression stroke all valves are closed. TDC on exhaust stroke exhaust valves are open and intake valves are closed. in order to do leakdown test you need to ensure you are indeed TDC on the compression stroke and not TDC on intake or exhaust stroke..