Hitchcocks Forum

Just been setting the ignition timing and points gap on my 2000 Classic 500 and needed to accurately find Top Dead Centre (TDC). I found this method on the web and decided to give it a go. After a bit of time taken in setting it up it proved to be an easy and extremely accurate way of finding TDC. I've now completed the work and my bike is running as sweet as a nut. Anyway here it is if anyone is interested........

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 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.

Lot of real dumb stuff found on the web

A lot of real dumb people as well.

And if your valves are not 100% airtight?

Not to mention your rings, which at that low speed will almost definitely not be.

I bought H's Top Dead Centre tool... works for me.

I don't know what's given cause for the snidey remarks but the idea of using a method which measures before and after TDC and then halves the difference is simply attempting to be more accurate. It's pointless (pardon the pun) to try to measure 0.8mm (or 10 degrees) BTDC if you're not bang on TDC in the first place. The problem is that around TDC there is a ‘dead spot’ where the crank can rotate back and forth a few degrees with virtually no change in the piston position. This ‘dead spot’ is a fundamental aspect of the mechanical relationship between the crankpin and connecting rod. This characteristic makes it very difficult to accurately find true TDC using a DTI or a measurement rod. The method outlined above DOES require the combustion chamber to be fairly air tight so any loss of air past the valves or piston will be minimal. BUT the level of oil in the tube only has to be held until the tube is marked, or you could even mark the tube beforehand and simply draw the oil up to the mark. The crank can be marked at leisure as long as you don't move it.

Good idea Rigger, I can see that working. Ignore the snidey remarks, that's why people wont post here because of the responses they get from the resident moaners.

Thanks Lucy! I should also say that the beauty of the method is that all the bits and pieces you need are often kicking around your garage or shed. An old spark plug, a jam jar, a bit of tube, and some oil. I used Araldite to secure the tube to the plug but other methods are available. And to check exactly when the points were beginning to open I used a transistor radio 'tuned' to a blank station. With the ignition 'on' the radio gives a loud 'click' at the precise moment the points open. The only 'techy' thing you need is a timing disc. So very accurate timing achieved without a dial gauge, or a stroboscope, or bulbs wired across points.

I wasn't meaning to be snide but I think my argument is valid.
You are quite right about the difficulty in accurately measuring TDC and for this reason I prefer to not try at all. I can easily get the timing within a few degrees of the correct setting with nothing more than a piece of plastic down the plug hole. I then tune it in by ear and whether it kicks back or pinks while running.
It's interesting that doing it this way I get a setting which is slightly different from the "correct" one but the engine runs and starts spot on.