Quote:Then how come everything we do, we set the bike to TDC on the compression stoke?
sillydilly wrote on 05/09/11 at 09:19:53:because the compression stroke sets the CAM to TDC. I believe the piston (crank) rotates 2 for everytime the cam goes once. so if your on the wrong TDC, your cam will be 180 degrees out of rotation which means lobes straight up and valves open. TDC on compression stroke ensures that the pistons at the very top, and more importantly, your valves are both closed with the lobes facing straight down
On a stock cam, at both TDC's the valves are closed. So you could be tempted to adjust the valves at any TDC. But don't. Check this out, won't take long promise. Next time you adjust your valves, before you close up, rotate the engine a bit and reach in and feel the gap on the rocker (lift and press, repeatedly) see as you progress in rotation the gap gets larger, not by much, but it does. The reason behind this is mfg tolerances. Thru the compression stroke you want to guarranty full valve closure, so after TDC on the compression stroke they releave the cam so there's more clearance, just to cover all bets.