Odd flywheel marks:Where is TDC?

[John Gelfer]

I'm trying to find the TDC mark on my flywheel. I was expecting a small punch mark, arrow, or something like that. What I see is a bunch of numbers that don't seem to correspond to degrees. Some are odd, and some are even. So far I have seen 15, 17, 18, 27, 36, and 45. It looks like the 45 is TDC to me, but I'm far from certain. All the numbers have a vertical line between the two digits that is taller then the digits. I'm viewing these numbers through a little peep hole. Since it's an L-head engine, it is pretty hard to see when the valves are closed and the piston is at TDC.

Any ideas on this numbering system? BTW, it is an 8 cylinder Lycoming 226 cu. in. engine in a 1928 Gardner.

[nickelroadster]

All you need to do is remove #1 spark plug and put your finger in the hole. When you are on compression stroke, you will be able to feel the pressure building and trying to push your finger out of the way. When you do this you should be able to tell when you are at the top of the stroke. This will allow you to be able to identify the mark on the flywheel that is TDC.

[Rusty_OToole]

All you need to do is remove #1 spark plug and put your finger in the hole. When you are on compression stroke, you will be able to feel the pressure building and trying to push your finger out of the way. When you do this you should be able to tell when you are at the top of the stroke. This will allow you to be able to identify the mark on the flywheel that is TDC.

A more accurate way to do the same thing is to make a pipe to screw into the spark plug hole and make a soap bubble on top. You can tell when you reach TDC when the soap bubble stops growing. This is surprisingly accurate.

[John Gelfer]

Thanks for the help! The "bubble method" worked very well to find the exact TDC. I attached a hose to the fitting in #1 cylinder, and dipped it in some soapy water. My wife watched the bubble grow while I cranked over the engine with my McGyvered starting handle. After several tries, we were certain where TDC was. The number on the flywheel in the peep hole was 18. I don't understand the number, but that's what it is. Maybe they meant 1 of 8?. That still doesn't explain the other numbers.

[Guest trainguy]

The 18 means cyl 1 and cyl 8 are at the top of their stroke,which of these is on compression is determined by the position of the camshaft.

[Rusty_OToole]

So does that mean the flywheel is marked for the position of the following pairs of pistons, 1&8, 2&7, 3&6, 4&5? What about 15 and 17?

[Guest trainguy]

Good question.the marks on the flywheel are used for setting the valves.I will have to give this more thought.The numbers are often accompnied with IO[intake open]ic[intake close,EO&EC for the exaust.

[Rusty_OToole]

Wow cool. This would make accurate valve adjustments a cinch.