Thanks for checking out this thread! Any advice is greatly appreciated! I recently replaced the top end in my 2000 YZ 125 just for routine maintenance. Everything went smoothly and I checked at least 1,000 times that the piston was installed correctly, piston pin circlips were seated, etc. I had no issues at all until I started the bike up for the first time to let it warm up for a moment. The only difference with this top end is : 1.) This time I refilled the cooling system with distilled water in an effort to cycle water through the cooling system in preparation for adding a new type of coolant this time. 2.) The bike sat for two weeks with the motor completely together & torqued, but with the exhaust, fuel tank, etc. off the bike until I got more time to finish the job. One week ago, I added some oil to the cylinder walls through the exhaust port - in hindsight I'm not exactly sure what made me do it, but I just thought it may help with initial startup when I got around to finishing the job. Once it was all back together, the bike started after 3 kicks and then I noticed a relatively small "whirring" noise coming from the bottom end. The noise was actually pretty low and it was slightly more noticeable coming from the left side of the bike than the right. The bike only ran for about 30 seconds total before I shut the bike off just to be safe. Could adding oil to the cylinder have actually caused more harm than good? Has anyone ever experienced such a noise after a top end replacement?