Hello André,
The initial IBM PC used a single 14.31818 MHz oscillator which was divided by 3 to yield the 4.77 MHz system clock (33.3% duty-cycle), and that divided by 2 to yield a 2.38 MHz signal, which was run through a FF to divide by 2 again to create the timer clock of 1.19 MHz. The 14.31818 MHz clock was used by the Color Graphics Adapter (CGA), which was divided by 4 to provide the color burst signal required for NTSC composite color output. There are two separate signals on the bus connector: Connector pin B30 was marked OSC, which is the 14.31818 MHz and B20 was marked CLK, which is the 4.77 MHz.
When we brought out the PC-AT, things changed slightly. The 14.31818 MHz clock signal was kept, for compatibility with the CGA adapter (and timer clock), but a separate clock generator was added to drive the 80286 CPU, which ran at 6 MHz. As a result, the Tech Ref for the AT noted that the Clock signal was now 6 MHz and was not synchronous with the OSC (14.31818 MHz) signal. Some time after the initial AT announcement, I managed to obtain a couple SkyRocket boards, which were AT based system boards with additional memory and some faster chips... I also obtained a couple early 12.5 MHz 80286 CPUs and was running the system (and the bus Clock signal) at 12.5 MHz.
Needless to say, one can overclock the old ISA bus, provided you have some faster chips. Running it slower shouldn't be a problem, but you would likely have some issues using an old memory expansion board as these were based on DRAMs and the refresh rates would be reduced accordingly and likely fail. However, I doubt you would be looking to run one of the old memory adapters.
You can find out more by downloading the technical reference manuals at the link below. Note that with the early PC releases, we published full schematics and the BIOS listing in these references. As far as connector spacing goes, the integrated 2-piece connector (8/16-bit I/O connector grouping) is 5.50-inches end-to-end of the connector housing.
Manual PDFs can be found here:
http://bitsavers.trailing-edge.com/pdf/ibm/pc/ Hope this helps.
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