The MGA With An Attitude
Finding TOP DEAD CENTER (TDC) More Precisely - CS-111B

Another way to determine TDC fairly accurately is with hydraulic fluid flow. With engine close to TDC on compression you can fill the combustion chamber with oil (or mineral spirits) and connect a length of clear tubing to the spark plug port. The large bore piston pushing oil up the small bore hose will amplify the motion immensely. Suppose the hose is 5/16" bore (like fuel hose), and the piston is 3" diameter (1622cc engine). Cross section (or volume) varies as square of the diameter. So, (3.000/0.3125)^2 = 92.16   This means 0.001" motion of the piston will produce nearly 0.1" variation in height of oil in the hose. How accurate can this be? Going back to the cosine function, if you do the math, 0.5 degrees from TDC moves the piston only 0.000067", but it will move the oil 0.0061" in the tube. If your eye can see 0.006" of motion of the oil in the tube, then you can find TDC within half a degree by this method. If not, then 1 degree of crankshaft motion moves the piston 0.00027" and the oil will move 0.025" (almost 1/32"). This you can surely see, so the oil tube method will be accurate at least to within 1 degree of TDC. You can use this to file a timing mark on the crankshaft pulley when changing timing cover pointers from bottom reading to top reading pointers. It doesn't matter if a small amount of fluid might be sneaking past the piston rings, as you still get instantaneous motion of the fluid when the piston moves. This oil tube tip is demonstrated nicely in a John Twist video.

As John Twist says (after the demonstration), the oil tube technique can be somewhat messy, especially whey you meed to remove the oil from the cylinder. And the engine will smoke like crazy on the next start up until it burns off or blows out all of the oil that is left inside.
Phil Parmenter in East Sussex, UK, offers a similar way to do pretty much the same thing with a lot less mess. He writes: "Just found the following on the internet. Sounds like a lot better than pouring oil down the plug hole".
1. Make an indicator from some clear plastic tubing, a jar of light oil, and an old spark plug.
2. Break up an old spark plug and attach a length of clear plastic tubing to it (make it airtight).
3. Remove all the spark plugs.
4. Stick your thumb OVER the #1 cylinder spark plug hole. Rotate the engine (see note below on tricks for this) until you feel pressure on your thumb. That's the compression stroke. TDC is at the top of this stroke.
5. Screw in the spark plug with plastic tubing attached and insert the other end of the tube into a jar of light oil. Continue rotating the engine. Bubbles will appear until the piston reaches the top of its travel. When it starts down on the next stroke, the bubbles will stop and oil will begin traveling up the tube. Stop at a convenient point and mark the tube. Then mark the crank pulley and the engine body at a convenient spot.
6. Rotate the engine backwards and watch the oil recede into the jar. Continue rotating. As the piston continues past TDC and downward it will again suck oil into the tube. Rotate the engine till the oil again reaches the mark. STOP! Mark the crankshaft pulley where it lines up with the mark you made previously on the engine. You should now have two marks on the crankshaft pulley. The midpoint of these two marks lined up with the mark on the engine is TDC.

For this to work the combustion chamber has to be fairly air tight (engine in good condition) so any loss of air past the valves or piston rings will be minimal. End result should be very accurate, because you are noting position of oil in the tube when the piston is some distance down from TDC and moving more relative to crankshaft rotation. How accurate? We again go to the hydraulic ratio and the cosine rule to do this calculation. Suppose during this process you draw oil up the tube 12 inches. With the aforementioned 92:1 ratio between 5/16" tube bore and 3" piston bore, the piston would have moved 0.130 down from TDC, and piston height is
1.75-0.13 = 1.620". The cosine rule allows calculation of crankshaft angle at that point.
1.75 -1.75(cos@ = 0.130
-1.75(cos@ = 0.130-1.75
1.75(cos@ = 1.75-0.130
cos@ = (1.75-0.130)/1.75
@ = arccos (0.9257) = 22.226 degrees from TDC

Now change the crankshaft angle by just 0.1 degree, and work the calculation the other way around.
Cos(22.326) = 0.9250
0.925x1.75" = 1.6188" piston height
1.620-1.6188 = 0.00125" piston travel
0.00125x92 = 0.115" oil column travel

At this piston height, just 0.1 degree of crankshaft motion will move the oil nearly 1/8 of an inch (3 mm) in the tube. This is dead accurate when comparing left hand and right hand approach at about 20 degrees from TDC. If you are concerned about a little air loss sneaking in or out of the combustion chamber, just do the left and right rotation to same oil height a couple of times to assure repeatability. As noted, TDC will be exactly half way between the two marks you make on the crank pulley, so mark carefully and measure carefully. With a 5 inch diameter crank pulley 1 degree of rotation is 0.044 in (1.1 mm) at the rim of the pulley. You can make marks that accurate if you are careful. Accuracy of this method os only limited by your skill in making the marks. In retrospect, the John Twist oil tube could be used in similar manner, marking height of the oil column on the tube at a point slight shy of TDC (left and right of TDC) to be more accurate.

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2012 Barney Gaylord -- Copyright and reprint information