A bad bearing is always noticable any where. It its not smooth then replace it. A lil gritt in some bearings isnt bad but will get wore but in engine thery spin the crank and if feels a lil gritty or rought they need replacement. Also like said aswell the engine will make noises like hissing, and verious noises that are off setting. Normally a bearing replacement is popular after about 2 gallond or so on non ceramics in an engine. Not saying they blow out that fast its just common to replace them. Also since im this far if you dont wanna spend the 100 bucks on a bearing puller youtube how to remove nitro engine bearings and it will show a cheap and easy way to do so. I recommend if you got the money just send it to the speed shop or some where similar and just get a cleaning, check up, and bearing replcement. For the price and tko bearings included and installed is a unbeatable deal.