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  • Gasket/oring replacement is generally optional/situational in this application in my opinion. The gaskets are metal and don’t usually fail or leak, but if you’ve lost or damaged yours during disassembly or if they look rusty or corroded, you might want to change them anyway. Otherwise, I usually wipe them clean and reinstall, or if you are worried about a leak, you could finger-coat a light amount of sealant on each side. The intake manifold orange o-rings are usually pliable enough to reuse without issues but again if you feel the need to replace them then they are also included in the gasket kit. Copper crush washers  and fuel injector lower grommets/o-rings are reusable as is but they are also included. The install gasket kit for this application costs about $90 and is available from any oem mazda part retailer/dealer. For 04-05 4 port automatic models the part number is n3z1-10-s50b and for 04-08 6 port models is n3z3-10-s50c . The biggest concern is that the oil pickup tube in the oil pan get a new o-ring, as this one is always super hardened. Most engines come supplied with this o-ring even if you did not buy the complete gasket kit, but the gasket kit also includes this one. 
  • if the old engine died of overheating/burning coolant you should change the thermostat and water pump. 
  • if the old engine died of a bearing/oiling failure, extensive cleaning of the oil pan and it's baffles/tanks as well as the oil pickup tube is required. sometimes it may be better to simply replace it with a new one rather than risk contamination of the new engine. 
  • there is no factory oil pan gasket. you use RTV sealant like the factory did. Ultra black permatex or "the right stuff" permatex is best. Clean the block mating surface with a clean towel and brake cleaner etc. Use a wire brush mounted on a power tool to clean old sealant off the oil pan, and thoroughly flush with solvent etc. then blow dry and clean the oil pan flange with the towel and brake cleaner as well. I suggest installing the pan from the bottom with the engine hanging, because if you flip the pan upside down, leftover solvent will run down and get in the seal before you can get it tightened down. 
  • before installing OMP oil injectors into the rotor housing holes test them to see if the check valves are still good inside. Blow into the top nipple, air should pass through easily. Suck backward on the nipple, no air whatsoever should pass back, and it should hold vacuum for a few seconds. IF air leaks backward the check valve is bad and the injector should be replaced. 
  • There is a plastic vacuum “insulator” 4 way splitter which has 4 hoses that go to each of the 4 OMP oil injectors/nozzles. Most owners are tempted to replace the vacuum hoses, and go to twist or pull them off. DO NOT DO THIS or you will break the plastic nipples off the insulator block. Even though the vacuum hoses may be a little stiff, they should be able to be reused in many cases, but if you feel like you absolutely need to replace them, don’t try to twist or pull them off. Use a straight razor blade or knife and cut a groove into the hose long ways, then use a pick or small screwdriver to peel the hose off of the nipple like a banana. 
  • when installing the primary fuel injectors (red) into the engine block, ensure that the two rubber grommets/insulators are clean and installed on the bottom of the injectors. Sometimes owners leave these in the holes of the block when taking the injectors out during disassembly. 
  • use a can of carb/brake cleaner and compressed air to blow out the old oil and sludge from the OMP oil pump before installing. There are two small holes inside the o-ring groove where oil flows and this internal cavity should be cleaned out. 
  • when installing the oil pressure switch/sending unit on the rear iron housing under the oil filter DO NOT overtighten it. This is a tapered thread sensor and you WILL crack the iron housing casting if you install it too far. It is not meant to bottom out in the hole. It does not need much torque to seal up. Look at the threads before you install it. You can see how deep it was inserted before. This is exactly how deep you insert it this time, about halfway. IF you crack the hole that this thread into it is not a warranty situation and you will have to buy a replacement iron housing plus pay me to swap it out. 
  • thoroughly clean and inspect the lower intake manifold valves before installation. There are two sets of valves on 04-05 4 port engines and 3 sets on 04-08 6 port engines. These valves should be inspected for free movement (the VDI and SSV valves move by hand, the APV valves can be rotated by hand after removing the APV motor from the LIM and turning the gear), cleaned of carbon, and lubricated with thin motor oil or penetrating oil. It may be necessary to remove the SSV and/or APV valves from the LIM to clean them well enough. A couple cans of brake/carb cleaner sprayed down the intake manifold runners can also help clean carbon out of them. If for some reason you disassemble the entire APV gearing system inside the LIM gasket, be aware that it must be specifically reassembled and timed for the system to work. If you reassemble it out of time the system will not work, the engine will be down on power above 6000rpm, and you will get a check engine light/code for port valve failure. 
  • The wiring harness and it's mounting bracket have several ground points on the top and side of the engine block, these points should be sanded or filed to bare metal before installation of the harness and grounds to ensure clean contact. 
  • When installing fuel injector clips onto each injector, verify that you are installing them correctly by checking wire colors vs positions. On 04-08 6 port manual models the injector plugs can easily get mixed up. In fact there are two injectors that share almost the same (green/red) wire colors. IF you plug in the injectors in the wrong order the engine may not start, or if it does it will run poorly. 
  • test all 3 of the electrical vacuum solenoid valves on the back of the oil filler neck assembly with 12v before installing it. The top of the 3 commonly fails. You need to simply use test wires and a battery to the two pins of each solenoid and listen for a click. IF you want to be thorough you can check that the path of airflow changes by blowing into the vacuum hose leading to each solenoid as you trigger it. It is far easier to change this solenoid now than once the engine is back in the car and you find a code for the secondary air injection system. 
  • Reuse original spark plugs instead of putting in new ones which may get contaminated with oil or fuel during the startup process. Use a bench grinder-mounted wire wheel to gently clean the plug threads and tips, not bending the electrodes. Spray the plug ends off with brake/carb cleaner, then use a propane torch to burn the plugs clean and dry. Put a few drops of oil on the threads and install them. There are 2) leading/lower plugs and 2) trailing/top plugs, the holes in the block and the plugs themselves are marked. 
  • wipe off the crank angle sensor/eccentric shaft position sensor to remove dirt, dust, and metal shavings. 
  • there are several blue and red quick disconnect clips on the fuel lines. These are meant to be replaced with new ones after each use since they stretch slightly and may not fully grip after reuse. IF you are reusing them I strongly suggest using a large zip tie around each one, pulled tightly, so that it can never come off on it's own. Push each hose connection down on the rail/fitting very firmly then pull up a few times to ensure it is properly seated, they can fool you sometimes. 
  • when installing the upper intake manifold ensure that the vacuum hose on the drivers side of the engine gets connected. People often leave this single short hose loose or disconnected and it causes idle learning/stalling problems once the engine is started. 
  • use a rag soaked in carb/brake cleaner, gently push the throttle body blade open by hand and hold it fully open, and wipe the top and bottom bore and blade tips to remove built up carbon.