I finally got a decent job (go me) and have the money to build up a frame I bought a few years ago. It's an early 90s steel frame so I'm building it up with early 90s parts, mostly from eBay. I'm now looking for a stem but I'm not really sure what length to get since I've never ridden the bike. Does it make the most sense to build it up sans stem, then get on it and try and guess an appropriate stem length, or is there a more scientific method I could use?

I was also thinking of just buying as many different length cheap stems with the same drop as possible, and seeing which one was best, then buying a nice one to actually ride with, but this is of course the least economical way.

Anyone have a good method?

You could use the fit calculators at Competitive Cyclist and Wrench Science. I think the Rivendell site has one, too, if you're into that sort of thing.

But these calculators are not perfect because everybody's body proportions can't be reduced to a set of trigonometric relations and everybody's body adapts to the act of sitting on a bicycle a little differently.

The calculator at Competitive Cyclist got me close enough, and I probably would've stuck with its recommendation if I didn't already have four other stems to try.

I should add that, at worst, it recommended just 1 cm different than what's on the bike now. Not much to worry about.

when fully built bikes are sold, what is the normal length stem they tend to come with?

Shorter stems for smaller bikes and longer stems for longer bikes. Might as well just go see what's on a similarly-sized bike at a shop.

Duh, I don't know why I didn't think of this.

Measure the reach to the bars and hoods, saddle height and setback from the bottom bracket of your current bike that fits well. Use that to calculate the stem length for your build.

I've found the Park Road Position chart to help this

If you think you will need a stem shorter than 9 cm I recommend selling the frame and buying the next size down. If you use a 70 or 80 you really affect the handling. Ideally you should be using a 10-12 cm stem. On a 54 cm bike i typically use a 110, on a 55 I use a 100 and i typically won't buy a 56. I had a wonderful Lemond Buenos Aires in a 55cm but the damn thing fit like a 57 so i had to sell it.

If you're looking at early 90's parts I'm assuming you will be buying a quill stem? If you want your handlebars level with your saddle, you might consider the Nitto Technomic Deluxe. Just remember that with quill stems, the higher the bars go the shorter the reach.

IMHO I'll just go to the LBS when I'll have to decide and ask the guys there : Hey I need help to find the right sized Stem on my bike. And they'll be like : Nom Nom Nom CASH!

I've seen new bikes in smaller sizes (I'm talking 52 cm and less) with stems that couldn't have been more than 8 or 9 cm at the most, sometimes even less. Conversely, I've never seen such short stems sold on bigger bikes, either.

If you have another bike that fits, then figuring out a comparable length for the bike to be built is not hard to calculate. If you have no bikes at all and not fit info, that's another story.