Some particular engines let you souvenirs, like 2Stroke as you said and Wankel too. Decocking even though necessary not always a pleasure!
Technical question: I have to refurbish a 3 rails/4 speed gearbox with synchromesh. The shop manual says we must change the circlips and chose their thickness. But, now we dont have any choice, just fit new ones as they come. So, what if I put the old ones back? At the moment, only one main bearing is noisy and the idle reverse gear is worn (dont ask why please…).