The inside of both a stainless steel and yellow gold crown will, over time, become porous, and will therefore no longer be watertight. Crowns may need to be replaced every 15 years, or so. The same is true of the case tube, which is the male coupling, also made of stainless steel. The truth is, most watchmakers, even highly qualified, master-watchmakers, don’t have access to new OEM replacement Rolex parts, so, in the servicing of your watch, parts that should be replaced, will not be. This means that with a good cleaning, lubrication, and timing calibration, they can get your watch to (seemingly) keep good time. This trick will only last a few months, and, all the work that they’ve already performed (and charged you for) will now need to be completely redone.Not replacing a crown when necessary can expose your movement to moisture. Moisture, even in small amounts, can cause a repair to cost more than $1000 in replacement parts alone. If a crown is broken, or missing, many watchmakers will source a used generic (non-Rolex) crown, or use a used Rolex crown. This can lead to moisture exposure, and ultimately, to oxidation and rust. Do it right the first time and save yourself a lot of money, and frustration, in the future. Use only OEM replacement parts.