1991-1998 240sx: Top Dead center tutorial

ONLY for ADVANCED technical discussion about the 240sx!
vancouverbc
Posts: 3197
Joined: Thu Oct 30, 2003 1:30 am
Car: 1991 240sx

Post

Index of all Tutorials

safety : Disconnect battery.

Tools

1.27mm shallow socket for rotating the crankshaft. you can rotate from below or above engine.2. chalk and paint.3. small flat head screwdriver and adjustable pliers to remove relays

Top Dead Center(TDC) occurs when the piston head in cylinder 1 is at it's highest point. If you take the sparkplug out, you can look through the hole and see the top of the piston. The distance from piston head to top of hole is about 147 mm(5.78 in). There are two top dead centers. Once when the piston comes up to compress the gas and once when the piston comes up to push out the exhaust from combustion.

Moving the crankshaft

You can move the crankshaft by turning the ignition key to start for an instant several times until you see the timing marks. You can paint a mark about a foot clockwise past timing marks to warn you that timing marks are close. Always rotate clockwise. You can use chalk to make the timing marks easier to see.( to avoid flooding, remove the egi and egi pump green relays or unplug the 4 injector electrical connectors at the injector)

Transmission in neutral. having the sparkplugs out will make it easier to rotate crankshaft.

Exhaust vs Compression top dead center

The easiest way to know that your top dead center is compression tdc is to observe the postion of the rotor by removing distritbutor cap. It will be pointing close to cylinder one on the compression stroke and close to cylinder 4 on the exhaust stroke. This, of course, only works if you distributor timing is in the normal range. If your timing is really off, you will have to place your hand on the sparkplug hole(sparkplug removed) and feel the pressure build up on the compression stroke as you rotate crankshaft by hand.

alternativelyNote here the position of the cam lobes on #1 cylinder. They are facing outwards which means its at TDC on compression stroke. If the #1 cylinder cams are facing inwards then just rotate your pulley another 360 degrees until they are facing outwards.



Crankshaft goes around twice for every time the rotor goes around once.

If you dont have marks on crankshaft pulley

You will have to place hand over first sparkplug hole to determine when it is the compression stroke. then, rest a screwdriver on piston head and rotate crankshaft until screwdriver sticks out the most. You dont have to have valve cover off to do this. Just stick something down sparkplug hole. Your crankshaft pully may have slipped so it is a good idea to do this evne if you have marks.

Modified by vancouverbc at 10:02 PM 4/18/2009
Modified by vancouverbc at 4:48 AM 5/25/2009


walperstyle
Posts: 29
Joined: Fri Mar 10, 2006 9:21 pm
Car: s13 crap
Contact:

Post

Note: There is a keyway on the crank pulley that will also tell you when you are at TDC. Its kinda hard to see in this picture, but with the Crank Pully Off, its rather obvious its pointing at the top.
Image

Here is a better picture of where the keyway notch on the pulley itself, via a picture I found of the pulley
Image


Return to “240SX Technical Forum”