RX8 Engine Reinstall Tips

If the old engine died of overheating/coolant burning, the heater core and radiator should also be flushed, inspected, and the cooling fans tested directly with a battery and test leads to be sure they work. The radiator becomes especially suspect over 100k miles as the top plastic tank rots due to heat/age and the small nipple feeding the hose to the coolant reservoir bottle will crack and snap off.

If the old engine died of a bearing/oiling failure, the front mounted oil cooler(s) and the hoses between cooler(s) and engine must be thoroughly flushed or replaced to ensure they will not contaminate the new engine. The oil coolers have thermostats in them that only send fluid through the coolers when hot and bypass it right back to the engine without flowing through when cold. So to flush the coolers you must remove the thermostats and close the bypass hole inside to force fluid through the cooling rows. This is best done with the coolers removed from the car.
check the catalytic converter (if still equipped) in the exhaust midpipe for clogging, cracks, breakage, etc. A clogged cat can damage the engine and could have contributed to the old engine's failure. Simply shine a flashlight inside the front pipe and look inside to check for the fine honeycomb pattern brick. It should look like the fine screen pattern on a house window. Yours may already have been gutted by a previous owner. If you see areas that look like they are clogged, or if yours is beginning to break apart, you need to remove the pipe and either punch all the chunks out with a pipe or bar so that it is completely hollow, or replace the pipe with an aftermarket straight midpipe or another good factory cat pipe. In either case you will then have a permanent check engine light but that's better than killing the engine.

jack transmission all the way up against the trans tunnel
for manual models put trans in gear
for manual models it is a great idea to replace the clutch release/throwout bearing, clean and lightly lube the spline shaft
for manual models check or replace the pilot bearing in the end of the engine shaft and lubricate it
for auto models it is a great idea to replace the oil seal behind the torque converter in the front of the transmission. You can also drain as much of the old fluid from the converter as possible, and even drop the trans oil pan and change the filter and fluid at this time.
drop engine in slowly keeping watch on the exhaust manifold rear flange vs firewall, transmission, and floor pan exhaust shields. Also be aware not to hit or push on the OMP black positioning sensor on the passenger side front of the engine, this is easily broken and very expensive to replace.
work a/c compressor around the engine and up above the oil cooler lines. Don't let the compressor damage the oil pan level sensor or plug/wiring. I recommend leaving this plug disconnected and pushed up out of the way beside the spark plugs during this phase of the install. Once the compressor is up where it belongs you can connect the sensor plug later.
align engine and trans bellhousing in two planes by adjusting trans height and engine hoist height/angle
use a prybar etc. between sub frame and front of engine to apply moderate pressure and help mate engine and trans.
for manual models put a socket on the front 19mm crank bolt and turn it to help engage the clutch and trans spline shaft. Once it no longer turns the shaft and clutch are engaged and you can slide the engine back.
for auto models ensure that the torque converter is not in a bind against the flex plate during install. if you removed the torque converter and later reinstalled it into the trans it is possible you may not have the TC fully engaged in the trans front pump and it is sitting too far forward. When properly engaged the torque converter's front edge will be aligned with the very back of the slot in the bottom of the trans bellhousing. IF it sits farther forward than that the TC may not be properly installed.
align the two dowel pins on the block with the trans holes, and use these two bolts to start pulling the block onto the trans, alternating between them until they are both tight and the trans/engine are mated.
lower engine fully, gently let it rest on the sub frame, feed the trans harness down and install all the top bell housing bolts. connect front oxygen sensor connector. install auto trans dip stick.
inspect the rubber portion of the motor mounts for cracking at the very top. If you had the car running before disassembly and you noticed significant vibration at idle and low rpm, you may have hardened motor mounts that should be replaced even if not broken to eliminate the vibration (this is especially true of automatic models).
lubricate the threads in the sub frame that the motor mounts attach to, lubricate the hole in the middle of the motor mount and if necessary run a tap through the threads, and clean off the original bolt threads with a bench mount wire brush before install. Lower the engine just so that the aluminum bracket begins to touch one motor mount, center the mount on the bracket hole and start the long bolt that holds the bracket to the mount, and thread it in several turns by hand to be sure you are not cross threading it. Now repeat for the other side. Start and hand thread both bolts BEFORE tightening either down, this makes it easier to line up the mounts and engine. Once both bolts are started you can lower the engine fully and tighten all the fasteners.
wipe out any buildup from the inside of the radiator and heater core hoses before installing them on the engine connections.
it is never a bad idea to flush the radiator, heater core (you must turn the inside hvac knob to heat with power supplied to the car to do this) and engine oil coolers prior to install if possible
double check oil pan, radiator, and engine block drain screws for tightness.
when installing the fuel line hose from the engine onto the firewall fitting pipe, press down hard then pull up on the hose to ensure that the fitting is properly engaged. The red quick disconnect clips are not supposed to be reused because they stretch during removal. IF you are reusing yours it is a good idea to put a thick zip tie around it tightly so that it can never come loose. The blue evap hose connection is not as big of a concern.
when reconnecting the oil cooler hose quick disconnects, push hard on them to seat the c-clip in the groove and then pull hard while twisting on the hose a couple of times to ensure it is engaged properly, sometimes they can feel like they are engaged but still pop loose after startup and you risk destroying the engine if you are not pumping oil back into it at startup due to a loose oil hose.
be sure to tension the belts properly after reinstall. the best advice is to look on the tensioner brackets at where the nut was previously locked down before removal, and adjust it back to that point then lock it back down. IF you lose a belt while driving you can easily overheat the engine and destroy it before you even realize what has happened.
it is a very good idea to inspect the oil coolers for bent fins and other debris that will hinder airflow through them and cooling. It may be necessary to remove the coolers from the car and straighten the fins by hand with picks and small screwdrivers, or send them out to be professionally serviced so that air can flow through them efficiently again. Poorly performing oil coolers can cause damage to the engine by overheating and breaking down the oil supply.
the rest of the install is straightfoward, move on to the startup instructions.