The length and angle of the stem make a big difference to your comfort and sense of well-being, as well as to how well the bike steers.
Longer stems have the same effect as big steering wheels on cars: when they are too long the steering feels lazy, which is great for cruising but hard work for fast singletrack. Very short stems make the bike twitchy; the smallest hand movement translates into movement of your wheel, which is great for technical stuff but tiring for longer rides. The right stem length depends on your top tube length, your riding style and your body shape. Women are often more comfortable with a slightly shorter stem than men of the same size. Almost all stems are now the Ahead type. One advantage is you get two differentshaped stems in one: take it off, turn it over and refit it for a higher or lower position.
First check how the stem fits to the handlebar. It will either be a front-loader with two or four bolts or it will be a single-bolt type. Front-loaders are the easiest to deal with: the front of the stem can be completely removed, allowing you to change or flip the stem without too much fuss. With older single-bolt stems, the handlebars can only be removed if you strip all the controls off one side of the bars.
Single bolt/quill stems
Remove the grip on one side by sliding a chopstick between grip and bar, lubricate with a squirt of light oil and twist to slide off. Loosen the bolts on the shifters and brake levers and slide them off without scratching the bars. If they‘re a tight fit, lever the clamps gently open with a screwdriver, without bending the clamps. The cables will often be too short to allow you to slide the controls off the end of the bar. Loosen the bolt that holds the stem to the handlebars and slide the bar sideways in the stem so that you can remove the controls without kinking the cables. Undo the bolt at the top of the stem four turns and knock it back into the stem with a rubber mallet or block of wood. This releases the wedge nut at the bottom of the stem. Twist and pull to remove it. Clean carefully inside the steerer tube. Clean the central part of the handlebars, then smear a little grease on the part that will be trapped between bars and stem. Fit the new stem to your bars. Grease the inside of the steerer tube generously and fit the bars. Make sure they‘re pointing directly forward, then tighten the bolt at the top of the stem firmly. Gripping your front wheel between your knees, twist the bars to check that the stem bolt is tight. If the bars rotate out of line, retighten the stem bolt. Slide the shifters and brake levers back onto your bars, then twist the grips back onto bars, lubricating with hot water if necessary. Slide the controls back up to the end of the grip and tighten the fixing bolts.
Removing front-loader stems
These clamp onto the handlebars with two bolts, or a bolt and a hinge. When you‘ve undone and removed the bolt(s), you can take the front of the stem off or fold it out of the way, releasing the handlebars completely.
Refitting front-loader stems
Step 1: Clean both the stem and the handlebars where they clamp together. Any dirt left at this interface can cause annoying creaking. Once both are clean, spread a thin layer of grease or carbon prep on the part of the handlebar that‘s to be clamped in the stem. Grease the bolt threads with an extra dab of grease under the bolt head. Titanium bars, stems and bolts need a generous dab of copperslip.
Step 2: It‘s important to do bolts up evenly. With two-bolt stems, tighten both bolts until there is an even gap between the main part and the front of the stem, then tighten each bolt one turn at a time until both are firm. With four-bolt types, tighten in a cross pattern as shown above.
Step 3: Once the bolts are tight, check that the gap is even top and bottom and, for four-bolt types, that the gap is also even either side. This matters because the bolts will go in straight and be stronger. If one side has more gap, bolts enter the main part of the stem at an angle, stressing them and making them more likely to snap. If the front of the stem is hinged, fold over the hinge and tighten the bolt firmly.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting new handlebars”