Reply 20 of 24, by sliderider
80287XL was only ever rated by intel to run up at speeds of up to 12.5MHz....but this may have been because intel itself never made 286s faster than that (AMD and Harris did). A heatsink is probably a pretty good idea though, otherwise you should get a Cyrix or IIT FPU rated for 20MHz.
Both of those can be pricey. The only ones I have seen cost me more than it did for me to put together the rest of the system. lzf70000 has a Cyrix for $85 IIRC and there's a boxed IIT on ebay now for $149.