Some people are tempted to take a too dismissive attitude toward anybody, outside the S&W factory, who works on a S&W revolver. With the factory's own track record in recent years, that is not a very reasonable position to take. Some of us fix that which the factory did not, but should have. I am not a professional gunsmith.
But I've worked on a lot of them over a long period of time. My experience is that, in almost every case, the hammer assembly and trigger, within a series, are indeed drop in parts. Same with the rebound mechanism and, usually, the double action parts.
Fitting the hand, the ejector surfaces which it engages, and the cylinder lock is an entirely different story. It requires careful fitting and an understanding of the mechanism beyond the ability of most (but not all) home craftsmen.
Sideplates made over a span of many decades also fit almost perfectly, most of the time, within a model series. I find this amazing, and think most people would not expect it. It is a tribute to old time S&W quality control.
But I agree with many of Fastbolt's comments. Smithing the inside of a Smith is not something for the amateur to jump into willy nilly. It can be learned, and that can be fun, and working on junked or damaged revolvers is a good way to do it. There are plenty of references available to make that fairly easy, but experience is the best teacher. Just don't try to learn on something like a minty pre-29.
I have to say that there is just about nothing like the fine fit between almost any factory S&W hammer sear engagement surface and the corresponding surface of any S&W trigger made, within a series, over many years. Yes, in almost every case they are indeed "drop in". But this does have to be checked and verified EACH AND EVERY TIME. "Almost" is not the same as "always".
