I remember overclocking the 486DX2-66 in the early 90's. I got the idea after reading my brother's Intel data book and noticed that while the max clock speed was speced at 66 MHz, all of the timing diagrams implied it could run to 80. I borrowed a variable speed clock generator and sure enough it was stable at 80, and started to crash at around 82MHz.

When I started to help friends overclock theirs, I quickly realized the "silicon lottery" variance. Some would only run reliably at 78 or 76 MHz. I bought a bunch of fixed frequency clock generators (that were drop-in replacements for the original on the motherboard) in 2MHz increments due to the variance.

This was back before CPU's had heat sinks or fans, so we quickly figured out that adding those gave better margins. We even made some 10-LED bar temperature display that had a thermocouple glued to the CPU case and indicated 10 degree C increments (green=0-60c, yellow=70-80c, red=90-100c).