NOTE: These instructions apply only to the original MakerBot Replicator. Do not try to replace the nozzle on your MakerBot Replicator 2 or 2X without consulting a support agent.
To replace your MK8 nozzle, first unbolt your Stepstruder. You'll need easy access to the nozzles and thermal cores.
Your extruder needs to be hot when you remove the nozzle -- otherwise the nozzle can break when you try to remove it. Use The Replicator's LCD interface to preheat your extruder.
Once it's hot, insert something between the thermal cores on the two extruders to keep them oriented correctly -- we used the flat end of a utility knife -- and use a wrench to unscrew the nozzle.
Be careful of the hot extruder. Don't touch the nozzle or any part of the extruder with your bare hands, make sure the Stepstruder is on a stable surface and can't slip, and work quickly -- metal tools conduct heat.
Put aside the old nozzle and thread the new one into the thermal core.
The extruder will be very hot, so be careful when you're adding the new nozzle. If you like, you can stop heating the extruder and wait for it to cool down before adding the new nozzle. If you do that, remember to heat it up again for the final tightening of the nozzle.
Still holding the thermal cores in place, start to tighten the new nozzle into the hot extruder. When the nozzle is all the way on, it will hit the thermal barrier inside the thermal core. Be careful: it can be hard to tell when you've hit the thermal barrier, but over-tightening can strip the threads on the nozzle.
Once you've tightened on the nozzle, bolt the Stepstruder back into place.
There's one last thing you should check before you finish: your nozzle heights. Bring the platform as far up as it will go and check that the nozzles are at the same height relative to the platform. If they're not, you may have to correct them with a shim. Otherwise, you're done. Congratulations, and have fun printing with your new nozzle!