D1 and D2 type bulbs operate at high temperature with great pressure probably near or over 30 atmospheres. The internal quartz arc tube temperature is probably typically around 800 degrees C (1400-1500F).. The outer bulb is not this hot, but it is definitely burning hot.
You may be able to find a kit with HID bulbs that are plug'n'play for 7440 bulb socket, I don't know, I didn't check, but if you plan to use D2S (or D2R or D2C) bulbs like I did, keep reading below. 5) The D2S bulb collar is slightly larger than the opening for the 7440 bulb in the tail, but not by much.