Finding Top Dead Center (TDC)

Method #1 - use a dial indicator - To locate the TDC mark on the timing cover, you have to understand that TDC means "top dead center"...that is, #1 piston is at the very top of it's travel. So, when #1 is at the top of it's travel, the pointer should be pointing to the TDC timing line on the vibration damper.

The tricky part is finding exactly where tdc is...because the piston is very near the top of it's travel for several crankshaft degrees of rotation. One way to find tdc is to use a dial indicator, and note the pointer reading at about .050" before tdc, and .050" beyond tdc, measured in piston travel. Then the pointer position exactly halfway between these readings is exact TDC.

The main problem with method #1 is that you must have the heads removed to measure the piston movement which leads us to method #2.


Method #2 - use a homemade manometer - I found this method while reading a back issue of an old car magazine from the late 60s. I don't remember which one otherwise I would give complete credit as it is an easy and accurate method. The best part is that you only have to remove the sparkplugs.

Basically, a 3/16" clear plastic tube filled with some oil is connected to the sparkplug hole. As the crank is rotated around TDC, the oil level will rise then fall as it passes through TDC. The peak indicates TDC.

A manometer is a meter that works by measuring the level of a liquid in a tube. In this case, we're going to monitor the level of air in a plastic tube that is going to push a little bit of oil. The air level is going to be controlled by the piston going up and down in the cylinder. The basic concept is that the piston is big and will displace lots of air, the plastic tube with oil in it is small. Any minor movement of the piston will move the oil in the tube a large distance.

I did a rough calculation based on the bore versus the cross sectional area of the 3/16" plastic tube and came up with a 0.001" movement of the piston will correspond to a 0.455" movement of the oil in the plastic tube. With a 3 foot length of tubing, I found that I could effectively rotate the crank about 20 degrees.

the pieces

A sliced and diced compression gauge, quick disconnect fittings and some 3/16" plastic tubing

You take a compression gauge and cut the hose in half. Since you don't want to destroy the compression gauge, you add quick disconnect fittings like you use on your air hose. This way you can put the pieces back together and have a working compression gauge.

Next you remove the valve from the inside of the sparkplug thread. It is basically the same as the valve stem on your tires. This allows the air to flow without impedement through the sparkplug thread. I added a female quick disconnect to the sparkplug side.

On a 3 foot length of 3/16" clear plastic tubing, I installed a male quick disconnect fitting. I also installed a male quick disconnect fitting to the compression gauge. Now I can easily switch back and forth from a manometer to a compression gauge.

in action

Here's the manometer in action

The sparkplugs were removed so that the engine would turn easier. Oil was squirted into all the cylinders so that they would slide up and down without too much effort and to seal cylinder #1. The engine was rotated to what I thought was TDC.

The gauge was then screwed into cylinder #1 and and the tubing was looped around the master cylinder as it was handy place to loop things. A little bit of oil was squirted into the plastic tubing for about 4 inches of length and the oil was allowed to settle into one puddle at the bottom of the loop.

A neat trick that my buddy Mark showed me was how to rotate the engine using the power steering pump instead of the balancer bolt. You tighten up the power steering belt so it doesn't slip. Put a proper fitting allen key in the power steering pulley nut and use a pipe to turn the allen key. The pulley turns the belt which turns the crank and there is a lever action cause the power steering pulley is smaller than the crank pulley. Works very slick!

I slowly rotated the crank from 10 deg ATDC (After TDC) to 10 deg BTDC (Before TDC). You have to turn it slowly otherwise the oil will spray out. While turning, I watched the oil level and you could see it rise as the piston reached TDC, peak and then fall as it passed TDC. I tested it several times in both directions and found that my pointer was out by 3 degrees. It showed that TDC was at 3 deg BTDC. A quick bend and it was dead nuts on.

pointer

Here's the timing pointer blinded by the flash but the general shape is outlined

pointing to the right spot

Here's the timing pointer correctly aligned to TDC

If this page has helped you, please consider donating $1.00 to support the cost of hosting this site, thanks.

Return to



Copyright May 2011 Eugene Blanchard