Stellenbosch long-suffering little bike shop, Stellenbosch bikes, belong to Herman Geldenhuys.
Herman had a pre-sling many years ago, and for two years he revved the shit out of that machine at every opportunity, usually daily.
One evening he was at it again, on the pavement in front of the old Castle bar, when a valve broke off it's head, which went straight through the piston.
Luckily no fire, but Herman's oil spillage, a line about 5 meters long out the exhaust, left a mark on that pavement that lasted years.
If forced to an answer on the question of the toughest engine ever in a motorcycle, it would be the oilcooled pre-sling GSX-R.