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I am building a bike. The headset is in the head tube, and the next thing is to fit the stem and handlebar to the steerer. At this point, it seems I need some headset spacers in between the headset and the stem.

How many headset spacers am I likely to need? Or rather, how much total height of spacers? Is there any way to work this out other than trial and error?

The frame has quite different geometry to my current frame, although my body's geometry is not changing. Should I try to reproduce the space on my current bike?

Headset spacers are available in all sorts of combinations; would it be prudent to get something like a pack of five 10 mm spacers? Or a set of a 10 mm and two 5 mm? Am I likely to need more space than that? Less?

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Put the stem on and measure whats sticking out. The spacers need to be a little (2-3mm) higher than the top of the steerer. Then follow @Mikes Answer for selecting the height of the stem - it helps to think about this before buying spacers so you have a selection that can be made up to suit various heights, however spacers are cheap, so get a few extra. If weight is a concern, set the bike up using the cheapest you can find, once happy replace with lighter weight ones. –  mattnz Oct 15 '12 at 4:15
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up vote 3 down vote accepted

You can get a close approximation if you have access to a trainer. Install the bike in the trainer with the fork and headset installed. Using a front wheel riser block will make the fit closer to your actual riding position. You can now install the stem at any height you like. When you find something close measure the gap between the bottom of the stem and the top of the headset. I would get a variety set. Most come with a 2mm,5mm,10mm and 20mm. I just did this myself and I got an extra 20mm that I installed above the stem.This allows for some flexibility later. The spacers can be shifted for a different fit for different riders or resale. It is easier to cut the steerer later than try to make it longer.

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I don't have access to a trainer, sadly. Makes sense to get a variety set, though, and very good point about leaving some spare above the stem! –  Tom Anderson Oct 15 '12 at 21:50
    
If you don't have a trainer, get an assistant! (i.e. someone to hold you upright while measuring your fit). By the way, I usually buy this combo: 20mm (1), 10mm(1), 5mm(2-4). It provides a wide range of positions up to 50mm, wich may be even too much. Indeed nothing is wrong puting spacers above the stem also! I buy cheap spacers only and put the money somewhere else on the bike ;) –  Jahaziel Oct 18 '12 at 17:08
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