There was no 40MHz 286. It maxed out at 12MHz (Intel), 20MHz (AMD) or 25MHz (Harris).
Depending on your mainboard/chipset, CPU, and used ISA-cards, HDD-controller and VGA mostly, you could overclock.
Whoops, typo, I had an AMD 386/40 and my friend’s 386/33 kicked its ass. I _did_ actually also have an AMD 286, but it was 13MHz & had a whole 1MB of RAM.