Here's a nice way to preserve the most originality one can, when swapping out tuning machines on a vintage guitar: This 1940s Martin had its original Grover machines, several of which were no longer working. Thankfully Grover makes exact replacements, for just this application! I chose, however, to only use the new machines themselves, and not the new screws or bushings. This keeps maximum originality, while at the same time, reducing the chance of the owner losing the valuable original parts, later. It also reduces modification, as the new screws were smaller, and would have required the holes to be filled and re-drilled. The way I did it, there are zero mods to this lovely original vintage Martin. The old machines are safely in a ziplock bag, for the owner's safe-keeping. In the photo on the left, the original machine is below the modern replacement.