TDC question

ArtS

Let me know if I have got this right. By chance, the long UDC-1 mark was showing thru the flywheel cover opening so I went ahead and painted it white for future reference. Then I looked at the distributor rotor position and noticed it was pointing to number 6, not number 1. Although I did not do this, I figured if I gave the flywheel one full rotation back to the UDC-1 mark, I would see the rotor at number 1 since for every rotation of the camshaft/distributor, the flywheel rotates twice. I am still learning but understand that number 1 and 6 cylinders will always be in the same position in the 4-stroke engine. Thanks for the help and best wishes. ArtS

Park_W

Art, you've got it right. When the UDC mark is showing, both #1 and #6 are at top dead center. One will be at the top of its exhaust stroke; the other will be at the top of its compression stroke, which is roughly when you want the spark to occur. If you have the distributor out and want to know which stroke which cylinder (1 or 6) is on, put your finger over the spark plug hole on #1 and bump the starter until you feel compression developing. Then you know that at the next appearance of the UDC mark, #1 should be firing. Another technique, if you've got the UDC mark showing and want to confirm which cylinder is where in the cycle, is to put a piece of heater hose to the spark plug hole in either 1 or 6 and blow. If that cylinder's on the compression stroke you'll feel resistance, since the valves are closed. If it's on the exhaust stroke you'll not feel resistance, since the exhaust valve will be slightly open (it doesn't close until a little beyond UDC)

ArtS

Park W wrote:

Art, you've got it right. When the UDC mark is showing, both #1 and #6 are at top dead center. One will be at the top of its exhaust stroke; the other will be at the top of its compression stroke, which is roughly when you want the spark to occur. If you have the distributor out and want to know which stroke which cylinder (1 or 6) is on, put your finger over the spark plug hole on #1 and bump the starter until you feel compression developing. Then you know that at the next appearance of the UDC mark, #1 should be firing. Another technique, if you've got the UDC mark showing and want to confirm which cylinder is where in the cycle, is to put a piece of heater hose to the spark plug hole in either 1 or 6 and blow. If that cylinder's on the compression stroke you'll feel resistance, since the valves are closed. If it's on the exhaust stroke you'll not feel resistance, since the exhaust valve will be slightly open (it doesn't close until a little beyond UDC)

Thanks, Park! As always, clear and informative. ArtS