Taken from faqlight.carpassion.info
And why putting H3 HID kit is "useless":H1 HID:
Depth: H1 capsule is 19mm (44-25) from base of filament to the tip. The same distance on an HID capsule is 23.3 mm (52.5-27.1-2.1). You should leave 5mm distance for the HID bulb return wire. So the required depth is now 28mm (23.3+5). This means that you need 9mm (28-19) extra depth. In a reflector housing it can hit the glare box, if it exist. In a projector housing, it will typically hit the cutoff shield.
(Pics are not in the same scale)
2) Diameter: The hole diameter in the reflector is almost equal (1mm difference). However, the hole has to be made bigger so that the return wire gets its 5mm distance to metal. The return wire is 8.7mm away from the center. Add 5mm and you need 14mm of cutout from the center. Even if you have plastic reflectors this has to be taken into consideration. The reflective material is a chrome substance, and is indeed an electrical conductor.
A standard H3 bulb is 55W and is typically found in less critical applications like aux. lighting. H3 bulbs has been found in some low quality Asian aftermarked headlights. H3 has a lateral mount filament, and HID is only fore and aft, so a result of retrofit is that you will end up with garbage.