Clay Wyzykowski

Hello all i have been in the process of putting my engine back together for the second time after melting a piston last summer so. It all begins like this i took the engine out last summer and replaced the damaged piston and replaced all piston rings on all pistons while pistons were out all cylinders recieved a light hone as the cylinders didnt look bad at all i replaced all gaskets as well new pan gasket two new head gaskets timing cover gasket etc. i reassembled the engine put it back in the vehicle and fired it up i broke the piston rings in like i have read as not to let the engine idle and vary the rpms till engine reaches operating temp. Engine fired up and ran through the rpms fine after it had cooled i realized the turbo had been puking oil so i took it apart to see if it was leaking oil it was not, so naturally i assumed a piston or two must have extreme blow by and its pushing oil out the exhuast i checked 5 out of the eight cylinders for compression doing a leak down test and they all held at 180-175psi i figured they must all be good next i figures i would start the truck again with the turbo off to see if i saw oil spitting out the exhaust when i turned it over the engine backfired twice and blew the valve cover gaskets out. I then checked the same 3 cylinders put a new one that i had not tested before all had zero compression! Ive taken the timing cover off the timing is dead on i am at a lose right now and have no idea what the problem could be I assume the back fire was because i did a compression test without unplugging the injectors and the fuel must have ignited some how im the exhaust but im still confused how and why i lost all compression in the four or five cylinders i checked. Could it be the reluctor wheel spun? Please help!