The 1st known overclocking on 80x86 was made by People on 8086 in 1980, upgrading from a 4.77Mhz to above 6Mhz ... It was requiring to unsolder the Quartz on the motherboard, and replace it.(I was part of this wagon)
It went from then to the 486DX33Mhz, Where the OC got it up to an incredible 40Mhz ...
The 486sx was overclocking badly, as it was the 1st Microprocessor with Integrated memory controller.
The 1st overclockable CPU with Frequency multiplier was the 486DX2 66Mhz, that got overclocked to 100Mhz ... wow! (since then, Frequency multiplier is very often the limiter)
Internall