20 Watt Halogen Bulb
The bulb is a sealed halogen bulb rated at 20 watts at 12 volts. It is available at hardware stores such as Home Depot or Lowes, and lighting stores. It costs about $5 to $8. You should get the bulb that is a Spot Light (ESX is the designation) and not a flood. This one is an GE model ESX. The bulb and reflector is a sealed unit. The diameter of the unit is 2 inches. Do not confuse this with the larger or smaller diameter models. There are similar models that appear sealed in the blister packs but lack the sealed glass faceplate. These are typically used for yard and garden lighting and are installed in a fixture with a tempered glass faceplate . I do not recommend this type for a flashlight.
Philips Brand under the AccentLine (tm): If you use this bulb instead of the GE brand, consider the following. The 20 watt sealed unit was only available as a flood bulb with a beam of 36 degrees instead of 15 degrees for the GE spot beam. A more serious problem is the lack of an adequate reflector. The two Brands ( with the GE on the left and the Philips on the right) have different density aluminized reflectors. The GE is thickly aluminized, including the bulb neck, while the Philips bulb has a higher degree of transparency. This causes the flashlight head to seriously heat up, and could be a problem. The only way to minimize the heat is to utilize the original Maglite reflector. The bulb will fit in the reflector if you carefully trim the upper 1/16 inch flange off ( to allow the reflector to seat lower) and cut off the bottom extension to allow access to the bulb pins. This will reflect back enough of the bulb light to keep the heat to an acceptable level. Note that solder will not adhere to the power pins of the Philips bulb. You will have to use small crimp terminals, like the ones used to connect wires to the battery. The terminals crimp to the wire, and will slide over the bulb pins if adjusted properly.
Sylvania Brand: This bulb is recommended if you can not find a GE bulb but can find either the Philips or the Sylvania. The Sylvania Halogen Spot (ESX) reflector is aluminized about half way between the GE and the Philips bulbs. It may be able to be used without the extra reflector, but will operate cooler with the extra reflector.
The different bulbs can be identified by their reflector patterns. In this photo they are L-R; GE, Sylvania, and Philips. The GE, on the left, has the bulb neck aluminized, the others are not. The bare glass neck radiates as a black body, transmitting heat internally to the flashlight head, whereas the GE will radate hardly any heat from the neck area. This is why the GE brand is better, in addition to the thicker reflector surface.
These photos show the beam pattern at a distance of about 3 feet. The narrow spot (15 degrees mfg specs) is the best and will give you a beam of about 9 feet wide at 30 feet out. The wider flood bulb will give about 24 feet beam at 30 feet and be a lot dimmer. I do not recommend this bulb for biking but I made a cheap plastic second light for the car that uses the flood bulb and is ideal for that use.
GE Spot Pattern Bulb-----GE Flood Pattern Bulb