Inside the clock housing of my 66 Rally Pac (C5ZB-10B944-GW, 8cy, 8,000 rpm) was the melted blue plastic illumination bulb diffuser (pic 1 and 2 - on the left). Over time, the bulb left a dark "soot" on the inside of the housing near the duffuser. Fortunately, the blue plastic diffuser is common to several years instrument clusters and I got a replacement (it was also slightly melted but in better condition - pic 1 and 2 - on the right). Note the hole burned in the top. A closer look at the bulb that was used, an 1893, showed that it was too tall and touched the diffuser. Ford Service specifies a 1816 bulb, which is the same physical size as the 1893 but one that puts out a lot more heat. I asked others about this melting, it seems to be common, the bulb to use and got a few good answers, namely - change the bulb. Recommendations included using a 1895 bulb, which is a common 64-66 instrument cluster bulb (and other places) and used to illuminate the tachometer of the Rally Pac; using a 1445 bulb, a smaller diameter globe used in 64-66 floor console glove box; or using a LED bulb. I opted for the 1445 as it is shorter than the 1816/1893 bulb and has a smaller profile than the 1895.
The problem still remains as far as what Ford says to use as a replacement, a 1816 bulb. The clock housing is Ford part number C5ZB-15000 which indicates it was used in 1965 Rally Pacs. There is no Ford part number identification on the clock itself.
Jim
