I am wondering what the Q factor is on my Deore XT FC-M730 triple crankset is. This is a 1990-ish crankset. The Shimano catalogue of the day doesn't show the Q factor (maybe it wasn't a thing back then). Does anyone know? Have been pedaling it since 1991. It rocks.

There is probably a way to measure it, but I am a lazy bastid.

Take off one of the arms and mount it pointing the same direction as the other arm, then it's easy to measure across the outer faces with calipers.

On older 3 piece cranksets like the FC M730 the Q factor isn't set in stone like today's 2 piece cranksets. It depends on what length bottom bracket you use; a wider BB used to clear the chainstay will result in a wider Q factor. For what its worth, I like these older 5 arm 110/74 BCD cranksets; lots of chainrings to select from.

Measure from each crank to the centerline of the down tube and add together.

Takes about the same amount of time as it took me to write this.

Eggman, what does that mean, 3 pieces vs. 2 pieces? On the M730, there's a right arm and a left arm. What's the third piece?
In my case, it's an old bike and the BB is what it is. The spindle could be shorter, if that's what you mean, but I suspect it's as short as it can be for a triple. I too like the availability of many chainring sizes and have experimented over the years.

Well, yeah, but...

Good suggestion! I got 159mm. Thanks all.

I've used the "measure to the centerline of the seat or down tube" method, too, but it's easy to introduce error if you don't have a specific point that you can use as a reference.

Make a pencil mark. Does not matter if it is perfectly centered.

The third piece is that part that connects the right and left arm together, the spindle, which is part of the bottom bracket. You can use a cartridge style BB, which is available in various lengths, or a cup and cone style BB with various length spindles. We wont talk about 1 piece ("Ashtabula" ) type crankset. I think the suggestion to reverse one arm and then measure the Q factor on your bike is the only way to answer your question.

I placed a piece of black electrical tape on the downtube, with one edge on the subjective center, and used that as my mark. I might be off a millimeter or two, but I'm close enough for my purposes.

If the spindle is piece three, what is a two-piece crankset?

Several modern designs (Shimano Hollow Tech 2, for one) where the spindle is fused with the drive-side crank arm.

Yikes!