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How do you bring a motor to top dead center?

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2015-07-15 18:26:50
2015-07-15 18:26:50

Top dead center is when the #1 cylinder piston at the top of its stroke just after the compression stroke. This is when the #1 spark plug will fire. This location is used for setting the timing gears or chain and for locating (clocking) the distributor. Be careful, you can be off by one full rev of the crankshaft because top dead center occurs every other revolution. The #1 piston is also at the top of its stroke just after expelling the exhaust gas and just before air/gas intake for the next spark plug firing. So, if the #1 piston is at the top of its stroke and the exhaust valve just closed and the intake valve is just opening, you are 180 degrees out of phase. Rotate the crankshaft 1 full revolution. If the #1 piston is at the top of its stroke and the exhaust and intake valves have not closed/opened, you are at top dead center (TDC) and the distributor will/should be pointing at the #1 spark plug wire.

To bring the motor to top dead center, disconnect the spark plug wire from the ignition coil, remove all the spark plugs and remove the valve cover from the #1 cylinder valves. Jog the starter (or rotate the crankshaft by hand) until the #1 exhaust valve is almost closed and the #1 intake valve just starts to open. Then rotate the crankshaft 1 full revolution. When you are close, you can measure the height of the #1 piston with a screw driver in the #1 spark plug hole. Be careful not to let the screw driver bind against the piston or the head. Rotate the crankshaft back an forth slightly until you have located the highest point for the #1 piston. This is top dead center.

An alternativeTop Dead Center (TDC) is, indeed, the position of the #1 piston at the very top of its compression stroke. Just as pointed out in the good first post.

Let's review. It's a four cycle engine. On the compression stroke, both the intake and exhaust valves are closed and the piston comes up and compresses the fuel air mixture. The plug fires when the piston is near the top and the mixture is burned rapidly which forces the piston down on this, the power stroke. Both valves are still closed. The exhaust valve opens and the piston comes up on the exhaust stroke, forcing the spent mixture out into the exhaust header. The exhaust valve then closes. As the piston starts down, the intake valve opens and a new charge of fuel/air is "sucked" into the cylinder on this, the intake stroke. Then the cycle repeats. The important point here is that the piston is at the top of its stroke twice in a cycle. Once is at the completion of its exhaust stroke. We want the other one, the TDC of the compression stroke. How do we find it? Easy.

In following the (good) advice of the first post, if one doesn't want to remove the valve cover, don't. Do wear appropriate protective equipment and get help as required. And work safely at all times, being sure to "think through" the steps. Start by pulling the wire from the coil to the distributor cap as suggested. No need to have any high voltage flying around, is there? Pull the plugs. Now either get help to "bump" the engine over (as was correctly and succinctly offered) or to turn the engine over by hand. Here's the trick. Carefully! either block the #1 plug hole with a thumb or stick a finger in the plug hole so that it takes up most of the space. Remember not to shove your finger all the way down through the entire length of the plug's threads. The object is to block up most of the plug hole, not get your finger inside the combustion chamber or get a finger stuck in the hole. Do NOT force a finger into the threads for the plug. It's an invitation to get stuck. Be smart about this, okay? Think it through.

See what's being suggested? "Blocking" or "plugging" the hole will let you "feel" the compression as the piston comes up on the compression stroke. The obstruction does not have to be complete, but only "mostly" complete. You'll be able to feel (and, possibly, hear) the piston force air out past your obstructing digit. All that remains is to have someone "bump" the starter 'til you feel the compression, or turn the engine by hand to feel it. The advice offered as regards using a screwdriver and making sure it doesn't "jam" while you are rolling the engine is critical! The advice was offered by someone with a working brain! Heed it! You don't want to damage your cylinder walls, piston or spark plug threads! Whatever is used as an "indicator" to find TDC should not be so short that it can fall into the cylinder. That's a really dumb position to be caught in; it's a no brainer to avoid it. The thinking and careful person will not have this problem. Consider the alternative to the screwdriver - a nice long chopstick. It isn't going to damage anything, and it's softer than a steel screwdriver. Obviously, it is wood (bamboo) and can break or splinter, but if care is used, it will prove equal to the task of locating TDC without risking damage to any of the aforementioned parts.

With your compression stroke thus discovered, use your indicator to find TDC and you're all set. Good luck wrenching this one to a successful completion.

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