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Re: Now a question on Cylinder 1 being at TDC... Posted by CMyles [Email]  (#1126) [Profile/Gallery]
(more from CMyles) on Fri, 7 Apr 2006 08:28:48
In Reply to: Now a question on Cylinder 1 being at TDC..., Jesse Chamberlain, Fri, 7 Apr 2006 05:59:04 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Jesse, Your assumption is correct. The relationship between the crankshaft (pistons) and the cams is the same for all four stroke engines. That is that the crankshaft makes two rev's for each single rev of the cam(s), precisely. The relationship is exactly 1:2. That's because each piston must hit TDC once for a compression stroke, when all of that cylinders' valves are closed and its' spark plug sparks, and once again for an exhaust stroke, when the exhaust valve(s) are open and the rising piston pushes out the exhaust. (The exaust valves close as the piston hits TDC but there is no spark on the exhaust stroke). Creating the proper relationship between the cam(s) and crankshaft is just a matter of having the crankshaft at TDC (cylinder # 1) and locating the cams as marked (with careful precision, it is not an approximate thing. One tooth off is bad). If you're off the marks you may cause interference between the valves and pistons and bend your valves when the engine turns over. Once you have the cams timed (with the chain tensioner in place of course) roll the engine (the right direction) several times carefully and ensure that the cam marks line up exactly right on EVERY SECOND revolution of the flywheel. You'll see those marks 180 degrees out (on bottom of the cams) at each alternate flywheel rev. (The cams are turning half as fast as the flywheel).
The tricky bit concerns the fact that the ignition spark must occur when the piston is at TDC on its' compression stroke and not on the exhaust stroke TDC. This is where people time their ignition "180 out". Another common mistake is not knowing which cylinder is #1. On a straight 4 it's the piston farthest from the flywheel and the rotation of the crankshaft is clockwise when viewed from the front (the end opposite the flywheel). When checking the cam timing roll the engine that way to put any chain slack on the correct side of the sprockets or you may think that your cam timing is slightly off when it's not. The final ignition (distributor) timing is actually not right at TDC of course but for assembly purposes that is the way to think of it. Hope this helps.
posted by 206.123.221...
, Fri, 7 Apr 2006 08:28:48 <-- Viewing This Message
, Fri, 7 Apr 2006 07:44:50
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