This is by far the most frequently asked question. People assume "more bearings equals better". This is not necessarily the case.

First off, yes, any Mark V can be upgraded. If you have a poly v drive, a 2 bearing quill can directly replace the single bearing quill. If you have a gilmer drive headstock, then you need to replace 3 other parts to allow for a 2 bearing quill. The drive sleeve needs to be converted from a gilmer configuration to a poly v one. The fixed sheave on the idler shaft has to be similarly changed out. And also the gilmer belt needs to be changed to a poly v belt. This typically adds $100 to the cost of your repairs using used parts.

Now, should you convert? If your machine is 1985 or newer you have the 2 bearing quill. You need to think about what type of woodworking you do. The only users I recommend the 2 bearing quill for are bowl turners. All others I think do fine with the single bearing quill. Mark V's used single bearing quills from 1953 until 1985. Why the switch then? A competitor went to a 2 bearing style and SS simply followed suit. It's mostly hype that 2 bearing is better.

You see, the back of the quill is supported by the drive sleeve (see photo above). Thus a single bearing quill is really a 3 bearing quill. Placing an extra bearing in the drive train merely adds more friction. If your bearings and components are all fine, I don't believe you will see any difference between the single and the double bearing performance. In fact, in a 2 bearing quill, the inner bearing is the first to go in my experience. So I believe that unless you are a bowl turner, there is no need to go to a 2 bearing quill. Save some money.