I am assuming you are referring to the shaft seal. I have replaced these before and if you have a sanden compressor then it will go better than normal but.............if you do not have good experience with working on compressors than I would highly recommend that you just order one. You will be money and time ahead. Most of all you will be much less frustrated.
Absolutely agree !!! Everything MUST be kept PERFECTLY clean, and you'll still need a little luck. I'd suggest a rebuilt (Air Pro) or new from ackits.com. That's what I've used for the last decade on mine. Don't get a cheapie rebuilt one, you'll likely be sorry. And even if such is warranted, you'll waste the cost of your refrigerant and your time.